Eizenberg's General Anatomy

GENERAL ANATOMY

Principles and Applications

Norman Eizenberg, Craig Adams, Christopher Briggs and Gerard Ahern

www.anatomedia.com

http://www.anatomedia.com/

Credits About the authors

Principal Authors Norman Eizenberg (MBBS) Christopher Briggs (PhD) Craig Adams (MBBS MD) Gerard Ahern (MBBS)

Contributing Authors

Priscilla Barker (PhD) Ivica Grkovic (MD) Alex Pitman (FRANZCR) Prototype Contributors Henoh Dolezal (MBBS) Zdenek Dubrava (MBBS)

Content Consultants Marius Fahrer (FRACS) Robert Marshall (FRACS) John Furness (PhD) Jenny Hayes (MBBS)

Educational Consultant:

Cyril Driver (Med) Clinical Consultants Christen Barras (MBBS) Maurice Brygel (FRACS) Erica Fletcher (PhD) Robert Heng (FRANZCR) Justin Kelly (FRACS) Martin Richardson (FRACS) Andrew Rotstein (FRANZCR) Ramin Shayan (MBBS) G. Ian Taylor (FRACS)

Photography Stuart Thyer (BAppSc)

Dissections Priscilla Barker (PhD) Matt Jackson (BSc)

Illustration and Images Priscilla Barker (PhD) Diana Keshtiar (BSc) Prototype Illustrations Quang Minh Phan (MBBS) Yun Fan Lu (MBBS)

Graphic Design Gavin Leys

Design Consultant

Chris Hanger

Design Consultant

Michelle Gough (BAppSc)

Norman Eizenberg (MBBS)

Project Leader of Anatomedia Coordinator and Senior Lecturer, Postgraduate (PG) Surgical Anatomy ACB/University of Melbourne (U of M) Member, Anatomy committee RACS Contributor, RACS and RACDS Fellowship Courses General Practitioner and Member, RACGP Research: Medical Education, Anatomical Variation Universitas 21 Fellowship Award (2000) Meritorious Service Award RACDS (2006)

Craig Adams (BMedSci MBBS MD)

Professor, Clinical Skills (FSMed) Examiner, (Australian Medical Council) Education Consultant, (General Surgeons Australia) Contributor, Emergency/Trauma Courses (RACS) Recipient, Faculty Medicine Excellence in Teaching Awards (1998-2009) Research: Medical Education

Christopher Briggs (PhD)

Deputy Head, ACB/U of M Coordinator & A/Professor, UG Anatomy ACB/U of M Contributor, Diploma of Surgical Anatomy Course Contributor, PG Surgical Workshops Contributor, PG Physiotherapy & Forensic Anthropology Consultant Forensic Anthropologist, VIFM Research: Applied Anatomy, Forensic Anthropology

Gerard Ahern (MBBS)

PG Co-ordinator, Lecturer, Prosector ACB Monash U. Lecturer Southern Health Clinical School Senior Fellow in Anatomy U of M Hon. Ass. Professor Oceania University Contributor, RACS Surgical Skills for GPs Contributor, Diploma of Surgical Anatomy Course U of M Today’), General Practitioner.

Priscilla Barker (PhD)

Principal Prosector ACB/ U of M Contributor, Physiotherapy and Science Anatomy Courses Physiotherapist and Member, APA Research: Lumbar Spine, Anatomy and Biomechanics Young Investigator of the Year Award (2005)

Ivica Grkovic (MD)

Head, Department of Anatomy/University of Split, Croatia Coordinator & Professor, UG Anatomy Courses Contributor, PG Anatomy Courses Professional associations Research:

Alexander Pitman (FRANZCR)

Professor of Medical Imaging, University of Melbourne Director of Medical Imaging, St Vincent's Hospital Senior Fellow, Department of Anatomy U of M Generalist & Specialist Radiologist Author Radiology Core Review Councillor A & NZ Ass. of Physicians in Nuclear Medicine.

Acknowledgements

Department of Anatomy and Cell Biology, the University of Melbourne Courseware Development Unit, the University of Melbourne Department of Anatomy and Cell Biology, Monash University St. Vincent’s Hospital, Melbourne The Fiji School of Medicine, Fiji National University, College of Medicine, Nursing & Health Science The Visible Human Project (National Library of Medicine) 16 figures have been used (with permission) Ch.: 1, 13, 14 & 25)

Contents

i

Preface

SECTION I: THE HUMAN BODY Page

Chapter 1: Human Anatomical Terms 3

Chapter 2: Human Form and Structure 6

Chapter 3: Human Sexual Characteristics 15

SECTION II: BODY SYSTEMS AND ORGAN STRUCTURE

Chapter 4: Skeletal System and Bones 19

Chapter 5: Articular System and Joints 29

Chapter 6: Muscular System and Muscles 41

Chapter 7: Integumental System and Skin 55

Chapter 8: Visceral Systems and Viscera 68

Chapter 9: Nervous System and Nerves 86

Chapter 10: Arterial System and Arteries 105

Chapter 11: Venous System and Veins 117

Chapter 12: Lymphatic System and Lymph Vessels 126

SECTION III: BODY REGIONS AND ORGAN POSITION

Chapter 13: Regions of the Body 135

Chapter 14: Arrangement of Body Regions 140

Chapter 15: Body Compartments and Fascial Planes 144

Chapter 16: Body Wall and Cavities 147

Chapter 17: Neurovascular Pathways 151

SECTION IV: HUMAN DEVELOPMENT AND VARIATION

Chapter 18: Growth and Development 157

Chapter 19: Normal Variation 162

Chapter 20: Anatomical Variation in Structure 166

Chapter 21: Anatomical Variation in Position 172

Chapter 22: Pathological Changes 177

SECTION V: PRACTICAL PERSPECTIVES

Chapter 23: Surface and Functional Anatomy 200

Chapter 24: Radiographic Anatomy and Imaging 200

Chapter 25: Sectional Anatomy, CT and MRI 200

Chapter 26: Ultrasound Imaging 200

Chapter 27: Endoscopic Anatomy 200

Chapter 28: Clinical Procedures 200

Chapter 29: Postmortem Examination of Organs 200

Chapter 30: Cadaver Dissection 200

Appendices

Glossary

Index

Annulus

Preface

ii

1

Section I

THE HUMAN BODY

Introduction: ‘Anatomy accommodates ancestry'

Chapter 1: Human Anatomical Terms

Chapter 2: Human Form and Structure

Chapter 3: Human Sexual Characteristics

Introduction: ‘Anatomy accommodates ancestry’

2

Evolutionary history of the human body

All animals evolved from a common ancestor. Humans (Homo sapiens) share many features with other animals

on our family tree but may be categorised via a hierarchy of progressively more specific characteristics.

The human 'identity card' is:

Kingdom: Animal Superphylum: Coelomate Phylum: Chordate Subphylum: Vertebrate Class: Mammal Order: Primate Family: Hominid Genus: Homo Species: Homo sapiens

Developmental history of the human body

During development from a single cell (itself the product of fertilization of an ovum by a sperm) to an adult human (male or female), features from each of the above categories appear, at least transiently. For example, all developing vertebrates acquire precursors of gills and a tail, even though they may subsequently disappear or become modified beyond recognition.

It is also no accident that this reflects the evolution from unicellular organism to Homo sapiens, as at the earliest stages of their development embryos of different animals tend to resemble each other (a human embryo even up to six weeks is almost indistinguishable from that of other mammals). However, from then on they progressively diverge, both in form (external appearance) and in structure (internal construction). The respective genetic blueprint (modified by mutations) predetermines this. According to Haeckel’s Biogenetic Law: ‘Ontogeny recapitulates Phylogeny'.

The developmental history of an individual reflects the evolutionary history of its species.

During development, an individual passes through the

ancestral stages of life forms that progressively acquired modifications (due to gene mutations retained through a succession of environments).

The potentials (and limitations) of cells, tissues and organs are determined by the germ layers from which they are derived.

The embryo develops from three germ layers:

ectoderm, mesoderm and endoderm. 1. Ectoderm (G. ‘outside skin’) forms epidermis (and

skin appendages) plus nerve cells. 2. Mesoderm (‘middle skin’) forms all connective

tissues (including bone, muscle, fascia, dermis and the sheaths of peripheral nerves). Mesoderm also forms vessels.

3. Endoderm (‘inside skin’) forms the epithelial lining

of the digestive tract (gut) and of the respiratory tract (which buds out of the foregut)

Only mesoderm derived structures are vascular.

Chapter 1: Human Anatomical Terms

3

ANATOMICAL POSITION AND PLANES

TERMS OF RELATIONSHIP

TERMS OF COMPARISON

SPECIAL TERMS OF COMPARISON

TERMS OF LATERALITY

TERMS OF MOVEMENT

SPECIAL TERMS OF MOVEMENT

ANATOMICAL POSITION AND PLANES

To avoid confusion in describing the location of one structure relative to another a standard reference position has been adopted.

This arbitrary position is termed the anatomical position, where the body is standing upright with arms by

the sides and palms facing forwards. Other terms of position include sitting, kneeling and lying.

Fig.1.1 The anatomical position

Three sets of planes associated with the anatomical position are sagittal, coronal and transverse.

Sagittal and coronal planes are vertical but at 90 degrees to each other. Vertical slices parallel to the sagittal (L. ‘arrow’) suture of the skull are in sagittal planes. The midline of the body is in the mid-sagittal (or median) plane, as it is directly along the sagittal suture.

Vertical slices parallel to the coronal (L. ‘crown’) suture of the skull are in coronal planes. Horizontal slices are in transverse planes.

Fig.1.2 Planes at 90 degrees to each other

TERMS OF RELATIONSHIP

When describing the relationship between one structure and another, the body is considered to be in the anatomical position.

Terms of relationship are in opposite pairs along the three dimensions of depth, length and width. Superior is above while inferior is below. Medial (L. ‘middle’) is closer to the midline while lateral (L. ‘side’) is further from it. Anterior is in front while posterior is behind.

Fig.1.3 The mid-sagittal plane

THE HUMAN BODY

4

Fig.1.4 A coronal plane

Fig.1.5 A transverse plane

TERMS OF COMPARISON

Alternative pairs of terms may be used when comparing the position of structures, or even the same structure in different species. Terms of comparison apply to any position of the body without it necessarily being in the anatomical position

Proximal (L. ‘nearest’) is closer to the origin of a part while distal (L. ‘distant’) is further from the origin. Superficial is closer to the skin while deep is further from the skin. For a hollow structure, external (or outer) is further from its cavity while internal (or inner) is closer. Ventral (L. ‘belly’) is closer to the belly surface while dorsal (L. ‘back’) is closer to the back surface. The dorsal

surface of the penis is in front of its ventral surface when flaccid but behind it when erect.

Fig.1.6 Terms of comparison

SPECIAL TERMS OF COMPARISON

Special terms of comparison may be used for certain specific regions.

Fig.1.7 Surfaces of hand and foot

1. Human Anatomical Terms

5

The palm of the hand is termed the palmar surface. Its opposite surface is the dorsum of the hand. The sole of the foot is the plantar surface. Its opposite surface is the

dorsum of the foot. In the anatomical position, the dorsum of the hand is posterior to the palm while the dorsum of the foot is superior to the sole.

Fig.1.8 Terms of polarity

Cranial (G. ‘skull’) is closer to the head while caudal (L. ‘tail’) is closer to the tail. Within the head, rostral (L. ‘beak’) is closer to the front while occipital (L. ‘begin’ as in ‘born first’) is closer to the back of the head.

Fig.1.9 Polarity within the head

TERMS OF LATERALITY

Paired structures on both right and left sides of the body are regarded as bilateral. Unpaired structures may be midline or, if they are only on one side, unilateral. Ipsilateral refers to structures on the same side of the body while contralateral refers to those on the opposite

side.

TERMS OF MOVEMENT

Movements at joints occur in pairs, each in an opposite direction. Flexion (L. ‘bend’) is to bend, decreasing the angle between two levers while extension (L. ‘stretch’) is to straighten, increasing the angle. Abduction is to move away from the midline while adduction is to move towards it. Medial or internal rotation is to turn inward around the long axis while lateral or external rotation is to turn

outward. This pair of movements tends to occur in the transverse plane.

Fig.1.10 Examples of movement pairs

SPECIAL TERMS OF MOVEMENT

Special terms may be used for certain movements. For example, the cervical spine, with a series of joints

between its seven vertebrae, can (collectively) decrease the angle between the head and the shoulder on each side. The term for this pair of movements is lateral flexion of the

neck to the left and to the right. Another example is the complementary rotatory

movements that occur at proximal and distal radio-ulnar joints between the two bones of the forearm. It is much simpler to use the terms pronation (L. ‘face down’) and supination (L. ‘back down’) of the hand.

Fig.1.11 Examples of special movements

Other examples include movements of the foot (plantar flexion and dorsi flexion as well as inversion and eversion) and of the scapula (protraction and retraction as well as elevation and depression).

Chapter 2: Human Form and Structure

6

ANIMAL FEATURES

COELOMATE FEATURES

CHORDATE FEATURES

SEGMENTATION

POLARITY

VERTEBRATE FEATURES

MAMMALIAN FEATURES

PRIMATE FEATURES

HUMAN FEATURES

ERECT POSTURE AND EXPANDED BRAIN

BIPEDAL LOCOMOTION

SPEECH VIA A LENGTHENED PHARYNX

Fig.2.1 Mouse embryo at 5 weeks

Although humans have a particular form (external appearance) and structure (internal construction) all belong to the animal kingdom, the coelomate superphylum and the chordate phylum. We share characteristics of each of

these, particularly seen during embryonic development (representing a record of preceding evolution).

ANIMAL FEATURES

An animal (L. 'breath') is a living organism capable of

independent movement. Animals require an external energy source from

oxygen and organic foods (in contrast to plants producing sugar via photosynthesis).

Animal cells are surrounded by a cell membrane (rather than a rigid cell wall).

COELOMATE FEATURES

A coelomate has a segmented body wall (allowing greater movement) but a fluid-filled internal body cavity, termed a coelom (G: ‘hollow’) situated towards its ventral

aspect.

Fig.2.2 A coelomate

The gut tube is suspended in the coelom. Although

there is an opening at both ends of the gut tube (one becoming a mouth, the other an anus) the coelom is a closed body cavity which does not communicate with the outside environment.

Germ Layers

The organism develops in three germ layers: ectoderm, mesoderm and endoderm.

Ectoderm is exposed to the external environment and also forms nerve cells.

Mesoderm develops into structures providing support and splits to form the coelom around the gut and the lining of the body wall. It also conveys and forms vessels.

Endoderm forms membranes for absorbing nutrients and becomes continuous with ectoderm at openings to the external environment.

CHORDATE FEATURES

Chordates (G. 'cord') have the following

characteristics, at least during some stage of development: - a dorsal hollow nerve cord (neural tube) - a notochord and a tail - pharyngeal pouches

Fig.2.3 A chordate

2. Human Form and Structure

7

Neural tube of ectoderm

A longitudinal midline thickening of ectoderm (termed the neural plate) along the dorsum of the embryo forms a groove. This neural groove has folds that meet and

become buried as the neural tube. The brain develops from the expanded cranial end of the neural tube while the spinal cord develops from its narrowed caudal part. The cavity of the neural tube remains relatively wide throughout most of the brain (as its ventricles) but becomes very narrow in the spinal cord (as its central canal).

Fig.2.4 Neural tube development

Notochord of mesoderm

The notochord (G. ‘back’ + ‘cord’) is a mesoderm-

derived flexible rod, providing support. The nerve cord lying dorsal to it is ectoderm-derived, while the gut tube ventral to it is endoderm-derived. The notochord and the tail disappear almost completely (the notochord remnant as the nucleus pulposus of each intervertebral disc, the tail remnant as the coccyx).

Pharyngeal pouches of endoderm

At the cranial end of the gut tube, a series of endoderm bulges termed pharyngeal (G. 'throat') pouches abut overlying ectoderm depressions termed branchial (G. 'gill') clefts. In certain caudates (water breathers) the

pouches communicate with the corresponding clefts via gill slits.

Fig.2.5 Endoderm pouches and ectoderm clefts

Other caudates (air breathers) develop lungs (from an outpouching of the foregut) instead. In humans, a middle ear cavity develops from the first pharyngeal pouch with the tympanic (L. 'drum') membrane intervening between it and the external auditory meatus derived from the first branchial cleft.

SEGMENTATION

Segmentation and polarity are important anatomical

features of all chordates, including humans. Blocks of muscle termed myomeres (G. 'muscle parts') are arranged

segmentally along the body of a chordate. Although only present briefly during human development, their derivatives persist into adulthood. Segmentation is clearly manifested along the trunk but modified in the head.

Somites

Segmentation along the trunk ('metamerism') is seen

in the embryo as mesoderm arranged in a paired series of similar paraxial segments, termed somites (G. ‘body’). Each somite subsequently develops into a sclerotome (G. ‘hard’ + ‘cut’) and a derma-myotome (G. ‘skin’ + ‘muscle

cut’).

Fig.2.6 Segmentation in embryo at 3 weeks

Fig.2.7 Section through trunk of an embryo at 4 weeks

Sclerotomes form the vertebral skeleton, while derma-myotomes form segments of skin and skeletal muscle that become incorporated into the trunk and the limb buds. These mesoderm segments are located lateral to the notochord.

In the adult, segmentation is still seen in the short muscles of the back and intercostal muscles, vertebrae and ribs, spinal nerves and vessels of the trunk. It is also reflected in the segmental nerve supply of skin (as dermatomes), even in the limbs.

THE HUMAN BODY

8

Fig.2.8 Segments of the trunk in an adult

Segmentation relies upon the differential expression of sets of genes in the long axis (about the fourth week of development). These are known as homeotic genes. They determine regional characteristics. These genes have a sudden onset of expression then fade out once their segmental job is done. This produces an acute onset of the next body segment and gradual loss of the previous segment. Genetic variation can produce loss of segments, extra segments, transposed segments or changes in segment number (e.g. with cranial or caudal shift of the vertebral column, common anatomical variations).

Branchial arches

Head segmentation ('branchiomerism') is modified

from that in the trunk. Bones, cartilages and muscles of the jaw, face, pharynx and larynx are derived from the branchial arches. These commence as six paired masses

of mesoderm situated between branchial clefts (ectoderm depressions overlying endoderm pouches). Branchial arches may also be termed pharyngeal arches as pouches associated with them line the developing pharynx.

Branchial arch derivatives retain their nerve supply despite migration.

Fig.2.9 Segmentation in an embryo at 5 weeks

The nerve supply of branchial arch derivatives is from a specific cranial nerve (designated for each arch) rather than from a spinal nerve.

Fig.2.10 Paired branchial arches of mesoderm

Although structurally identical to skeletal muscles and capable of voluntary movement (e.g. in speech), branchial muscles develop from splanchnic (rather than somatic)

mesoderm. These muscles, which tend to be covered by mucous membrane or located near a mucocutaneous junction, are also functionally related to smooth muscle. They are effectors for a special group of superficial reflexes (e.g. swallow and cough reflexes) arising from mucous membranes (of upper digestive and respiratory tracts) and often act in conjunction with visceral reflexes involving glandular secretion (e.g. salivation).

POLARITY

All chordates exhibit polarity with a cranial (head) end

and a caudal (tail) end. The cranial part of the neural tube expands to form the brain, while the remainder develops into the much narrower spinal cord, terminating caudally.

Pharyngeal pouches and cloaca

At the head and tail ends of the gut tube endoderm abuts ectoderm at the buccopharyngeal membrane and cloacal (L. 'sewer') membrane, respectively. These

membranes break down to create orifices (with endoderm becoming continuous with ectoderm).

Fig.2.11 Longitudinal section through embryo


9

The pharyngeal pouches arise from the cranial end of the gut tube. In humans, the middle ear (tympanic) cavity develops as an extension of the first pharyngeal pouch (remaining in continuity with the pharynx via the pharyngotympanic tube).

Fig.2.12 Division of cloaca in the human

In humans, the cloacal membrane becomes divided into urogenital and anal membranes, creating separate orifices.

Axial borders

Fig. 2.13 Axial borders on limb buds of embryo (5 weeks)

In the embryo, a limb bud develops like a paddle (or flipper) with a pre-axial border (along the radius/thumb

aspect of the upper limb and the tibia/big toe aspect of the lower limb) and a post-axial border (along the ulna/little

finger aspect of the upper limb and the fibula/little toe aspect of the lower limb).

During later development the limb buds lengthen considerably and the lower limb buds rotate medially. In the anatomical position the pre-axial border of the upper limb is located laterally (and post-axial border medially), with flexor compartments anteriorly and extensor compartments posteriorly. The pre-axial border of the lower limb is located medially (and the post-axial border laterally), with flexor compartments posteriorly and extensor compartments anteriorly.

Fig.2.14 Axial borders on limbs in the anatomical position

Sequence of dermatomes and myotomes

The combination of segmentation with polarity is reflected in how dermatomes and myotomes are

arranged. A dermatome (G: 'skin' + cut') is the area of skin

supplied by a particular spinal cord segment. The segmental arrangement of dermatomes in an adult

is less disguised when the limbs are shifted from the anatomical position (to the 'welcoming position') by

abduction of both upper and lower limbs as well as lateral rotation of the lower limbs. The pre-axial border of each limb is then located cranially and the corresponding post-axial border, caudally (like the original limb buds).

Fig. 2.15 Axial borders and relation to dermatomes

A myotome (G: 'muscle' + 'cut') is the mass of muscle supplied by a particular spinal cord segment.

THE HUMAN BODY

10

The segmental arrangement of muscles in the trunk (particularly for intercostal muscles and short muscles of the back) is easily seen, with myotomes oriented from cranial to caudal. However, myotomes are oriented from proximal to distal along a limb and are disguised by even more overlap than occurs with dermatomes.

Fig.2.16 Segment in the trunk

The nerve supply to a muscle is retained even if the muscle migrates during development.

This also applies to both peripheral nerve supply and segmental nerve supply of limb muscles.

Left-right axis

With establishment of polarity along the vertical axis, there is also the development of the left-right axis, thought to be due to the beating of certain cilia (L. ‘eyelashes’)

These hair-like mobile cellular projections direct a net leftward flow of influential local fluid. The differential chemical concentration induces changes in left-right symmetry. A defect in ciliary motility may disrupt this process and even cause situs inversus, a rare anatomical variation where the thoraco-abdominal viscera are in mirror image to normal.

VERTEBRATE FEATURES

Humans are vertebrates. A vertebrate (L. 'jointed') is

characterised by the presence of a backbone (the vertebral column or spine).

Spine, skull and skeleton

Vertebrates are also characterised by a skeleton (L.

'dried up') for protection, support and locomotion, including a skull (housing the brain, derived from the expanded cranial end of the neural tube). The head also has special sense organs (associated with eyes, ears and a nose) and teeth. Vertebrates have a heart pumping blood under pressure into blood vessels. Lungs develop from a diverticulum arising from the foregut in air breathers.

Spinal cord and spinal nerves

The spinal cord is enclosed within the vertebral

(spinal) canal with an associated series of paired segmental spinal nerves exiting on each side. The

segmental arrangement of the trunk is also seen with the vertebrae and ribs. It is less visible with the muscles, although still reflected in their nerve supply.

Fig.2.17 Foetus at 5 months with spinal cord exposed

The bony vertebral column (derived from mesoderm of the sclerotome) replaces the notochord as the primary structural support. The notochord is incorporated as gelatinous material (the nucleus pulposus) within each intervertebral disc. In humans, the coccygeal vertebrae regress and the tail disappears.

Four limbs with five digits

Vertebrates include fish and quadrupeds (L. ‘four’ + “footed’) as well as humans. Except for fish, vertebrates

possess two pairs of jointed limbs attached to the vertebral column via girdles (pectoral girdle to upper limb and pelvic girdle to lower limb, respectively).

Each limb develops with a principal bone proximally, a pair of long bones distal to it, then short bones and five digits.

Although modified by development in other vertebrates (e.g. birds, horses etc.) for specific roles, this pattern remains in mature humans.

Fig.2 18 Developing limb in quadrupeds and humans

MAMMALIAN FEATURES

Humans are mammals. A young mammal (L. 'breast')

is suckled by its mother.


11

Skin appendages, cheeks and lips

Mammals are characterised by skin with appendages

(hair, sweat glands and sebaceous glands) and in particular mammary glands (modified sweat glands that

produce milk). Mammals also have muscular cheeks (for sucking) and

lips.

Placenta

True placental mammals also develop within a uterus (L. ‘womb’) of the mother connected by an umbilical (L. ‘navel’) cord to the foetus.

The placenta (L. ‘cake’) is incorporated in the lining of

the uterus. The umbilical vessels convey blood between the foetus and the placenta (until birth).

Fig 2.19 Human foetus within uterus

Pulmonary and systemic circulations

Mammals have a higher body temperature, being warm-blooded, with a circulation pumped by a four-chambered heart.

This enables two vascular systems arranged in parallel: pulmonary (to and from the lungs) and systemic (to and

from the rest of the body). Within the trunk, a muscular diaphragm is located between the thorax and the abdomen.

Forebrain, mandible and ear ossicles

Mammalian features include a forebrain (and cerebral cortex), a lower jawbone (hinged at the temporomandibular joint) and teeth replaced a maximum of

once in a lifetime (if at all). Other bones associated with the jaw are much reduced in size, creating the chain of three linked middle ear ossicles (to conduct sound-derived

vibrations from tympanic membrane to the inner ear)

PRIMATE FEATURES

Humans are primates. A primate (L. 'first') is able to

grasp objects.

Binocular vision and opposable thumb

Primates are characterised by a large brain (within surrounding skull), well-developed eyes (located at the front of the head enabling binocular vision) and a short nose (reflecting less reliance on smell). There are 4 upper and 4 lower incisor teeth (for biting).

Primates are brachiators (L. ‘arm’) and are able to

grasp objects with their hands. They possess a clavicle, enabling free movement of the upper limb. The limbs are also freed from the body with no webs of skin between the hip and the shoulder. The five digits (which include a first digit that may be partially opposed to the other digits) also possess nails (rather than claws). These features, coupled with the greater brain and visual capacity, provide primates with their hand/eye coordination.

Fig.2.20 Foetal head and hand

Less heat loss and long life stages

Primates tend to have an extended period of growth and development, primarily associated with their body size. Large body size enables better conservation of energy and thermoregulation. Large primates, including humans, are

characterised by greater longevity, duration of pre-natal life, lactation period (and interval between births) and age at maturity than small primates. They also have lower metabolic needs, smaller litter size (with humans generally producing only one offspring at a time) and only one pair of nipples. Large male primates tend to have a pendulous penis (longest in mature human males).

HUMAN FEATURES

Humans belong to the hominid (L. 'man form') family, the homo (L. 'man') genus and sapiens (L. 'wise man')

species.

The most distinctive human characteristic is the habitual adoption of upright stance and locomotion based solely on the two lower (hind) limbs.

THE HUMAN BODY

12

ERECT POSTURE AND EXPANDED BRAIN

Fig.2.21 Adult skeleton in erect posture

Upright stance is associated with evolution of the largest cerebral hemispheres (and corresponding intellectual capacity) within the animal kingdom.

The arched human foot

The principal feature associated with erect posture and bipedal gait is our peculiar foot (which may be regarded as the most distinctly human part of the body).

In non-human primates, feet are also used for grasping, and resemble hands (particularly as the big toe is opposable). The human foot is an arched platform (supporting body weight) and has a non-opposable big toe (sacrificing grasp for gait). It seems that humans owe their highly developed intelligence to evolution of the foot and the domino effect of features associated with bipedalism (all the way up to and including the head).

Bipedalism complements the massive increase in cerebral hemisphere size and literally enables us to think on our feet.

Fig 2.22 The peculiar human foot

Skull supported by S-shaped spine

The bones of the upper limb are shorter than those of the lower limb, allowing manipulation rather than locomotion. The hand has an elongated and fully opposable thumb, and particularly sensitive fingertips. The femur is angulated medially enabling the feet to be placed together in upright stance. The short but broad pelvis, created by the hipbones and sacrum, forms a complete ring. In females, the enclosed cavity provides a birth canal large enough for a foetal skull to pass through.

The adult vertebral column is aligned vertically but has a series of four curves, which are alternately convex forward (cervical and lumbar) and convex backward (thoracic and sacral). This supports a large cranium (in turn containing a particularly large brain) above it. The short, flat face with a relatively small jaw (and associated reduction in teeth) enables better balance of the vertically held head.

Thanks to our skeleton, we can literally hold our heads up high.

Fig.2.23 Features of the human skeleton

Line of gravity and stable joints

In an adult standing upright, the line of gravity passes

between the mastoid processes of the skull, balancing the head.


13

It continues through the S-shaped vertebral column behind the centres of the cervical and lumbar spine and in front of the centres of the thoracic and sacral spine. It then passes behind the centre of the hip joints and in front of the centre of each of the knee and ankle joints.

While standing (with hips and knees extended and ankles dorsi-flexed) the weight bearing joints are in the position of maximal stability. Articular surfaces are apposed and associated ligaments taut (to conserve muscular effort). Minimal skeletal muscle tone is therefore required to maintain upright posture, other than to correct for body sway.

Fig.2.24 Line of gravity in erect posture

Thanks to stable joints the precarious evolution of upright stance did not fall flat on its face.

Site of most stress on spine

Fig.2.25 Weight-bearing stress at angulation of spine

Due to angulation between the lumbar spine and sacrum, weight bearing creates shearing stress through the lumbosacral disc (which is obliquely oriented) and through the fifth lumbar vertebra.

It is no accident that the most commonly disrupted intervertebral disc is the lumbosacral disc and the most common vertebra to sustain a stress fracture is the fifth lumbar.

BIPEDAL LOCOMOTION

In contrast to standing where muscular effort is conserved, bipedal locomotion enlists the actions of

many muscles. Walking on level ground involves cycles (between heel-

strike of the same foot) of swing (limb not in contact with the ground) phase and stance (weight bearing) phase. Muscles not only act to accelerate the swinging lower limb (from the beginning of swing phase to mid-swing), but also to decelerate it (from mid-swing to the end of swing phase).

Fig.2.26 Phases of the walking cycle

The line of gravity moves forwards in the direction of motion. At one phase of the cycle (mid-swing and mid-stance) it passes through both limbs. At all other phases it passes between the limbs.

Roles of the gluteal muscles

The large gluteus maximus muscle is located

posteriorly (creating the unique form of the human buttock) producing powerful hip extension in running and jumping.

Fig.2.27 Stabilisation of the pelvis during locomotion

Gluteus medius and minimus muscles prevent

excessive tilting of the pelvis (supporting the trunk above it) towards the unsupported side during locomotion.

The gluteal muscles literally got us up off our backsides to move.

THE HUMAN BODY

14

SPEECH VIA A LENGTHENED PHARYNX

Another distinctly human characteristic is speech by sounds formed into words. This confers the advantage of using the voice to communicate ideas (via strings of words) but comes at a price. The larynx, the organ of phonation (G. 'voice') consists of cartilages housing the vocal cords

together with special muscles controlling vocal cord vibration. In order to prolong movement of an air column through the mouth, where sounds (particularly vowels) can be shaped for speech, a sufficient length of airway is required between the larynx and the mouth.

Fig.2 28 High larynx in a chimpanzee

Other mammals (and human infants) have a high larynx at the level of the soft palate and can only emit air from the

mouth in short bursts. However, they can breathe and swallow simultaneously as their air and food pathways do not cross. Air is breathed into the larynx via the nasopharynx (situated behind the nose) and food from the mouth swallowed (into the oesophagus) via channels

lateral to the larynx. During human postnatal development (late infancy) the

pharynx lengthens as the larynx descends into the neck.

This creates a new part of the pharynx, termed the oropharynx, situated between the soft palate and the

larynx.

Fig.2.29 Low larynx in an adult human

Risk of choking and protective reflexes

A potentially deadly risk is created by descent of the larynx as the air and food pathways now intersect in this region. Swallow and cough reflexes (elicited by stimulating mucous membranes of the oropharynx and larynx, respectively) as well as gag and vomit reflexes, protect the airway.

An important modification to the swallow reflex is that during swallowing breathing is stopped.

Protective reflexes involving muscles (and associated nerves) of the branchial arches have saved us from literally choking on our own words.

Chapter 3: Human Sexual Characteristics

15

MALE

FEMALE

MALE

Humans are typically either male or female. Although ambiguous sexual development may occur, a genetic male is characterised by the presence of a Y chromosome. There is normally a single X chromosome

and a single Y chromosome. Males are conceived (and born) in about equal proportions with females.

Fig.3.1 Adult male characteristics

Male primary sexual characteristics

Primary sexual characteristics are formed during pre-natal development. In the male they are the testes,

together with the male genital tract and external genitalia.

Male secondary sexual characteristics

Secondary sexual characteristics arise during puberty.

These have widespread effects on the body and are due to hormonal secretion initiated by the pituitary gland. They

include enlargement of the genital organs and the appearance of pubic and axillary hair. Distinctive

masculine features are the extensive growth of facial and body hair together with that of the skeleton and skeletal muscles. Typically, males have a relatively narrow pelvis with broad shoulders. The larynx is also large (associated with deepening of the voice).

Genital organs in the male

The internal genital organs of a male include the

testes, epididymes, deferent ducts, seminal vesicles, prostate and bulbourethral glands, while the penis and scrotum are regarded as external genital organs.

Fig. 3.2 Male genital organs

FEMALE

A genetic female is characterised by the absence of a

Y chromosome. There are normally two X chromosomes.

Female primary sexual characteristics

Female primary sexual characteristics are the ovaries,

together with the female genital tract and external genitalia. These are formed during prenatal development.

Female secondary sexual characteristics

As with the male, female secondary sexual characteristics have widespread effects on the body due to hormonal secretion initiated by the pituitary gland.

They also include enlargement of the genital organs and the appearance of pubic and axillary hair, in addition to the onset of menstruation (L. 'monthly'). Distinctive female features are the growth of the mammary glands (creating

the breast form) and preferential deposition of subcutaneous fat, creating a more rounded body form.

THE HUMAN BODY

16

Typically, females have a relatively wide pelvis compared to that of males. The female pelvis has more capacious internal dimensions (although there is considerable variation) enabling less restricted passage for a foetus along the birth canal.

Fig.3.3 Adult female characteristics

Genital organs in the female

The internal genital organs of a female include the

ovaries, uterine tubes, uterus and vagina while the clitoris and vulva are regarded as external genital organs.

Fig 3.4 Female genital organs

17

Section II

BODY SYSTEMS AND ORGAN STRUCTURE

Introduction: 'Structure mirrors function'

Chapter 4: Skeletal System and Bones

Chapter 5: Articular System and Joints

Chapter 6: Muscular System and Muscles

Chapter 7: Integumental System and Skin

Chapter 8: Visceral Systems and Viscera

Chapter 9: Nervous System and Nerves

Chapter 10: Arterial System and Arteries

Chapter 11: Venous System and Veins

Chapter 12: Lymphatic System and Lymph Vessels

Introduction: ‘Structure mirrors function’

18

Organ structure

The unit or building block of anatomy is termed an anatomical structure (L. ‘build’) or organ (G. ‘tool’). Organs are made up of tissues, which in turn, are made up of cells.

There are four tissue types (see diagram): 1. epithelial 2. connective 3. muscular 4. nervous

Organs may be grouped according to a common

function into systems (L: ‘organised whole’).

Systemic anatomy is concerned with the intrinsic (organisational) properties of organs - their structure and supply.

The body may be classified into twelve systems in three

groups of four.

Somatic systems: - skeletal system - articular system - muscular system - integumental system

‘Somatic’ (L. ‘body’) systems are collectively

responsible for the overall form and shape of the body. They provide support, movement and protection. Somatic systems are the musculoskeletal framework together with the covering skin. The skeletal, articular and muscular systems may be grouped into a single musculoskeletal (or locomotor) system.

Visceral systems: - respiratory system - digestive system - urinary and male genital systems - endocrine and female genital systems

Viscera (L. ‘sticky’) are collectively responsible for

internal regulation. They occupy cavities within the body framework and are involved with secretion, excretion and absorption. Visceral systems are made up of solid glands and/or hollow tubes (of smooth muscle, lined by mucosa). The urinary and genital (reproductive) systems may be grouped into a single urogenital system.

Supply systems: - nervous system - arterial system - venous system - lymphatic and haemopoietic systems

All organs, whether somatic or visceral, require neurovascular supply (although supply of somatic organs

is by a separate set of nerves and vessels to that of viscera). Organs receive their nerve supply via the nervous system. Organs also receive a supply of arterial blood as well as drainage of venous blood and lymph. ‘Vascular’ (L:

‘vessel’) refers to arteries, veins and lymph vessels. Vascular systems are classified as arterial, venous and lymphatic. The heart, together with arterial and venous systems, may be grouped into a single cardiovascular (or circulatory) system.

1.

2.

3.

4.

Chapter 4: Skeletal System and Bones

19

SKELETAL SYSTEM

COMPACT AND SPONGY BONE

PERIOSTEUM AND BONE MARROW

BONES AND BONY FEATURES

CARTILAGE

OSSIFICATION AND PRIMARY CENTRES

SECONDARY CENTRES & GROWTH PLATES

EPIPHYSES AND EPIPHYSIAL LINES

LONG BONE GROWTH AND GROWING END

NEUROVASCULAR SUPPLY OF A BONE

SKELETAL SYSTEM

The skeletal system is made up of bones and cartilages. In an adult there are approximately 206 bones.

However, the number may vary due to the presence of accessory bones (anatomical variants created by bony parts that have separated to become discrete bones). At different stages of development cartilage (and membrane) precursors are converted to bone.

Fig.4.1 Disarticulated skeleton of a newborn

Short bones (e.g. carpal bones in the wrist and most

tarsal bones in the foot) tend to be cartilaginous at birth. One or both ends of long bones, together with parts of

vertebrae, are also cartilaginous at birth.

Bones are typically paired except for those in the

midline. Some bones that are paired during development (e.g. the halves of the mandible and of the frontal bone) unite in the midline to become unpaired.

Subdivisions of skeleton

The skeleton may be subdivided according to the following modules:

Head 29 bones Neck 7 bones Thorax 25 bones Back 19 bones Upper limbs 64 bones Lower limbs 62 bones

Total 206 bones

Fig.4.2 Axial and appendicular skeleton of an adult

The bony pelvis is made up of the paired hip bones (of the lower limb) and the sacrum and coccyx (of the back). The hyoid bone is included in the head (although it may be classified in the neck). The cervical vertebrae are included in the neck while the rest of the spine is included in the back. Bones may also be arranged into the axial skeleton (skull, spine and thoracic cage) and the appendicular

skeleton (limb bones including the pectoral and pelvic girdles).

COMPACT AND SPONGY BONE

Functions of bone Bone provides protection and support for the body as

well as levers for limb movements. Bone houses a major site of blood cell production and is a vast calcium reserve. Although a skeleton (G: ‘dried up’) is the bony

remnant of a dead body, in the living it has the capacity for remodelling and (up to maturity) for growth.

Bone cells and matrix Bone is a dense connective tissue composed of cells

and extracellular matrix.


20

The cells responsible for bone deposition are osteoblasts (G: ‘bone + germ’) and those responsible for resorption are osteoclasts (G. ‘bone + break’). These cells

require a rich blood supply to remain viable. The extracellular matrix is made up of mineralised ground substance (resisting compression) reinforced by collagen fibres (that resist it being pulled apart). Bone therefore possesses compressive strength coupled with tensile

strength and is hard yet not brittle. About two thirds of the dry weight of bone is calcium salts and one-third collagen.

Trabeculae Compact bone forms the hard, protective outer shell

while the inner spongy (cancellous) bone possesses trabeculae (L: ‘beams) that resemble scaffolding with

spaces between them. This trabecular arrangement provides strength with lightness. Bony trabeculae in the upper end of the femur form intersecting arches. Lines of compressive stress are oriented more vertically along the femoral neck while lines of tensile stress run across it.

4.3 Upper end of a dry femur

Bony trabeculae are oriented along lines of stress (both compressive and tensile).

Fig.4.4 Trabeculae along stress lines

Medullary cavity Trabeculae are absent in the medullary cavity of a

long bone (a cylinder is lighter and almost as strong as a solid rod). A long bone is characterised by a shaft between

its proximal and distal ends. The shaft has a central medullary (L: ‘marrow’) cavity. There are no (net) forces in this part of the bone (where compressive and tensile forces cancel each other).

The shaft (particularly midway along it) has a thick shell of compact bone where its perimeter is subject to considerable forces. This rigid tube resists bending forces in all directions without the need for a solid core. In dry bone all fibrous tissue and cartilage have been stripped off.

Fig.4.5 Compressive and tensile forces on the shaft

Dry bone is also devoid of bone marrow (which during life fills the medullary cavity and the spaces within spongy bone).

PERIOSTEUM AND BONE MARROW

Outer and inner layers of periosteum Periosteum (G: ‘around bone’) is the covering around

compact bone, except over joint surfaces (which are covered by cartilage).

Fig.4.6 Upper end of an embalmed femur.

4. Skeletal System and Bones

21

Periosteum has an outer (fibrous) layer. Ligaments

and muscles attach to bone through the blending of their collagen fibres with those around, and of, the bone. Periosteum also has the capacity to form new bone (ossification) from its inner (osteogenic) layer. Both

layers of periosteum have a vascular supply and a nerve supply

Red and yellow bone marrow

The medullary cavity of a long bone has a fibrous tissue lining, termed the endosteum (G. ‘within bone’). Bone

marrow fills the spaces of spongy bone and the medullary cavity of a long bone. Red bone marrow is a prime site of blood cell production. It consists of haemopoietic (G: ‘blood + make’) tissue embedded in fat (adipose tissue). Yellow bone marrow is almost entirely fat. In early life bone

marrow throughout the body is red. Red marrow remains in the axial skeleton for life, but in the limbs becomes yellow marrow during adolescence.

Marrow reversion after blood loss

Yellow marrow retains the capacity to revert to red marrow, particularly after severe blood loss.

BONE TYPES AND BONY FEATURES

Classification of bones

Bones are primarily classified as long, short, flat or irregular. Pneumatic (G: ‘air’) bones surround membrane-

lined spaces of the skull. These house the paranasal air sinuses (L. ‘spaces’) and mastoid air cells. Sesamoid (G: ‘sesame seed-like’) bones are found in tendons where they articulate with bony facets. The patella is by far the largest sesamoid bone. Accessory bones may also occur. They

are anatomical variants, created from bony parts that fail to amalgamate with the ‘parent’ bone.

Fig.4.7 Major types of bones

Mistaking bones for fracture fragments

Although smooth and regular, sesamoid and accessory bones may be mistaken by the unwary for fracture fragments in radiographs.

Bony surfaces

The exterior of bones typically has flattened surfaces separated by sharper borders. Articular (L. ‘joint’) surfaces are the areas of bone that articulate at synovial

joints.

Articular surfaces are the only external surfaces of a bone not surrounded by periosteum.

Articular surfaces are covered by hyaline articular cartilage. Articular surfaces are smooth and may be in the form of a small flat area (articular facet), a large rounded area (head), a knuckle-shaped area (condyle) or a pulley (trochlea).

Fig. 4.8 Bony features

Bony markings

An end of a long bone may include a head with a neck

(between the head and the shaft). A short or flat bone may also have a head with a neck (between the head and the body).

Markings may be classified into four groups: elevations, facets, depressions and holes. An elevation may be a line, a crest, a spine (L: ‘thorn’), a process, a condyle (G. ‘knuckle’), a tubercle, a tuberosity or a trochanter. A facet (Fr. ‘little face’) is a smooth, flat area. A depression may be a fossa (L. ‘ditch’), a fovea (L. ‘pit’), a groove or sulcus (L. ‘furrow’) or a notch. A hole may be a foramen (L. ‘bore’), a fissure, a meatus (L. ‘passage’), a canal or a hiatus (L. ‘aperture’).

Bony elevations are produced at sites of traction

Attachments of fleshy muscle fibres tend not to produce markings.

CARTILAGE

There are three types of cartilage (L. ‘gristle’): - hyaline cartilage - fibrocartilage - elastic cartilage


22

Fig.4.9 Types of cartilage

Hyaline cartilage

Hyaline (G. ‘glass’) cartilage covers bony articular

surfaces. In addition, it is found in the chest wall (as costal cartilages) and is also associated with the respiratory tract (helping keep patent the nose, larynx, trachea and bronchi, particularly during inspiration).

The model for the early foetal skeleton is primarily hyaline cartilage. Hyaline cartilage is composed of a solid matrix that can bear weight, resist compression and be almost frictionless. The glassy appearance is due to the collagen fibres in the matrix being arranged in parallel bundles. A thin fibrous membrane, the perichondrium (G. ‘around + cartilage’) surrounds non-articular cartilage.

There are no blood or lymph vessels and nerve fibres in hyaline cartilage.

Hyaline cartilage is avascular and aneural

Vessels would otherwise be compressed and nerve

endings irritated by the pressures hyaline cartilage may be subjected to.

Cartilage cells (chondrocytes) receive their nutrition

(from synovial fluid in joint cavities or from vessels in the perichondrium) via diffusion through the cartilage matrix.

Fibrocartilage

Fibrocartilage is a mixture of fibrous tissue and hyaline cartilage. It forms special structures in joints (disc, meniscus, labrum) that can withstand prolonged

pressure, contribute to articular surfaces and act as shock absorbers. Fibrocartilage is not glassy in appearance due to the vastly increased numbers of collagen fibres arranged in irregular bundles.

Except for around the periphery where pressure is minimal, fibrocartilage is avascular and aneural. Chondrocytes in fibrocartilage receive their nutrition via diffusion through the matrix.

Elastic cartilage

Elastic cartilage contains bundles of elastic fibres

providing flexibility. It forms discrete structures in the external ear, auditory tube and parts of the larynx.

OSSIFICATION AND PRIMARY CENTRES

Ossification in cartilage and membrane

Bone development is termed ossification. The vast

majority of bones develop from a hyaline cartilage precursor, by intracartilaginous (endochondral)

ossification where a cartilage model is progressively replaced by bone. Some bones develop from a fibrous tissue precursor, by intramembranous ossification. These

bones include the flat bones of the skull as well as parts of the clavicle and mandible. Despite the different precursor (and process), bone formed via intramembranous ossification is identical with that formed via intracartilaginous ossification.

Primary centres of ossification

Ossification commences at a primary centre. This

generally occurs in the middle of each cartilage (or membranous) model. At about the sixth intrauterine week, intramembranous ossification commences. At about the eighth week hyaline cartilage starts being transformed into bone at primary centres (those for the larger bones tending to appear first). By birth the vast majority of primary centres have appeared (except for the short bones in the hand and most of those in the foot).

Fig.4.10 Primary centres in a 12 week foetus

A bone’s blood supply develops during the transformation of cartilage to bone. Blood vessels (from the nutrient artery and vein) invade the primary centre

together with cells that subsequently form bone (osteoblasts).

Unlike cartilage, bone requires a blood supply, as the calcified matrix does not allow diffusion.


23

Fig.4.11 Endochondral ossification of diaphysis

SECONDARY CENTRES & GROWTH PLATES

Secondary centres of ossification

Many bones have additional sites of growth allowing for the subsequent change in demands on them. Secondary centres of ossification are growth centres typically located

in the ends of the cartilage model for long bones. They occur only at one end of a small long bone (e.g. digits and ribs) but at both ends of the large long bones of the limbs.

Almost all secondary centres appear after birth (females generally at an earlier age than males).

Although secondary centres tend to appear at different times for different sites, the vast majority have appeared well before puberty.

Blood vessels, together with osteoblasts, invade each secondary centre.

Fig.4.12 Endochondral ossification of epiphyses

The (epiphysial) arteries are derived from separate

sources to that of the primary centre (which is from the nutrient artery). Hyaline articular cartilage is retained at joint surfaces after the bone has completed ossification.

Diaphysis, metaphysis and epiphysis

A mature long bone is characterised by a shaft (between its proximal and distal ends). The corresponding part in a developing long bone is the termed the diaphysis (G. ‘between growth’).

Fig.4.13 Secondary centres in tibia of a 2 year old

The termination of the diaphysis is the metaphysis (‘changing growth’). An epiphysis (‘upon growth’) is the

end of the developing bone, adjacent to a metaphysis. The primary centre of ossification commences near the middle of the diaphysis then extends along it to reach each metaphysis. A secondary centre of ossification commences near the middle of each epiphysis then invades the cartilage model between the joint surface and the growth plate.

Fig.4.14 Parts of mature and developing bones

Epiphysial plate

The epiphysial (growth) plate is a plate of hyaline

cartilage in the epiphysis, along its junction with the metaphysis.

The epiphysial plate produces new bone at its metaphysial surface (where it is supplied by metaphysial arteries).

Growth in length occurs at the metaphysial surface of an epiphysial plate.


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Fig.4.15 Zones in epiphysial plate

Fig.4.16 Relationship of epiphysial plate to metaphysis

The epiphysial plate disappears (along with the capacity for growth in length at this site) when the epiphysis fuses with the metaphysis. Epiphyseal fusion occurs after puberty (females generally at an earlier age than males).

Although epiphyses at different sites tend to fuse at different times, the vast majority have fused by the end of adolescence.

EPIPHYSES AND EPIPHYSIAL LINES

Pressure and traction epiphyses

Epiphyses are located in the ends of developing long bones (adjacent to the metaphysis) and produce growth corresponding with demands on the bone. The two major types of epiphyses are ‘pressure’ and ‘traction’

epiphyses. Pressure epiphyses are associated with joints. They

enable growth of the apposed articular surfaces while being subjected to compression.

Traction epiphyses associated with bony prominences

enable growth where strong attachments pull on them. Atavistic epiphyses are few and insignificant. They

represent bones that have disappeared during evolution (becoming incorporated with another bone).

Mistaking epiphysial plates for fracture lines

An epiphysial plate undergoing fusion resembles a fracture line. Awareness of the likely sites for epiphyses helps avoid mistaking an epiphysial plate for a fracture line on a radiograph.

An epiphysial plate may also be differentiated from a fracture by obtaining an X-ray of the corresponding bone on the other side of the body.

Fig.4.17 Epiphysial plates and lines

Epiphysial line

Fusion of an epiphysis to a metaphysis is associated with disappearance of the epiphysial plate and cessation of further growth in length at this site.

A thin layer of compact bone, termed the ‘epiphysial line’, is the only remnant of the plate in a mature bone.

Although not visible on the external surface, an epiphysial line can be seen at times in vertical sections and in radiographs.

Determination of skeletal age

Since epiphyses fuse in an ordered sequence, they can assist in determining the age of an individual from radiographic images or forensic skeletal examination. In the latter, a narrow cleft on the external surface of a bone indicates an epiphysis undergoing fusion.

Accessory bone formation

A bony part that was previously an epiphysis may exist as a discrete bone, termed an accessory bone. This

sometimes occurs for traction epiphyses associated with bones of the foot and hand.

Mistaking accessory bones for fragments

An accessory bone can be mistaken for a fracture fragment, although accessory bones have regular rather than jagged edges. If the accessory bone is bilateral it will show a similar appearance on x-ray of the corresponding


25

bone on the other side of the body, excluding a fracture (unless the injury is also bilateral).

LONG BONE GROWTH AND GROWING END

Growing end of a bone

A long bone grows in length at the metaphysial surface of each growth plate. It also grows in width from the inner (osteogenic) layer of the periosteum. An epiphysis is located only at one end of a long bone that is of small size (e.g. in a finger). This end (termed the ‘growing end’) is

responsible for all of its growth in length. Although epiphyses (and growth in length) of a large long bone occur at both ends, one end (also termed the ‘growing end’) is responsible for more growth than the other.

Fig.4.18 Growth of a long bone

The more time an epiphysis (and associated epiphysial plate) exists the greater the opportunity for growth in length at that site. No further longitudinal growth occurs after epiphysial fusion (with disappearance of the epiphysial plate).

The earlier an epiphysis appears the later it fuses.

Epiphyses for larger long bones tend to appear before (and fuse after) those for smaller long bones.

Within a large long bone the epiphysis for one end tends to appear before (and fuse after) that of the other end. More growth therefore occurs at one end (the ‘growing end’).

The first epiphyses to appear (at about birth) are for the lower end of the femur and the upper end of the tibia (the two longest bones in the body).

Epiphysial judgement

The appearance of primary centres of ossification in the distal femur and proximal tibia is of medico-legal importance in determination of maturity and interpretation of radiographic imaging.

Most lower limb growth in length occurs near the knee, as these associated epiphyses also the last to fuse.

Most upper limb growth in length occurs near the ends of the long bones at the shoulder and wrist. The first epiphyses to fuse are at the elbow.

Fig.4.19 The humerus in late adolescence

Nutrient arteries

The major artery supplying a long bone is termed the nutrient artery. This artery occupies a passage (termed

the nutrient foramen) penetrating the shaft of the bone through to the medullary cavity. The canal of nutrient foramen (when viewed from its outside opening) is directed away from the growing end. Differential longitudinal growth results in the nutrient artery taking an oblique path (from outside to inside) through the full thickness of the bone while the bone is still growing (unequally). Equal growth would have resulted in the artery penetrating perpendicular to the shaft.

Fig.4.20 Orientation of nutrient foramen to growing end

Epiphysial damage

In children and adolescents, injury to an epiphysis or a fracture extending through the epiphysial plate carries special significance.


26

Damage to an epiphysial plate will impair subsequent growth.

Bone infections (spread from the blood stream) tend to occur at the metaphysis (a site of vulnerable blood supply) and may also lead to damage of the adjacent epiphysial plate. Interruption of blood supply to the adjacent epiphysis or metaphysis will similarly tend to impair normal growth.

Fig.4.21 Impaired growth from epiphysial damage

NEUROVASCULAR SUPPLY OF A BONE

Supply to bone and cartilage

Bone tissue has a sensory nerve supply and a rich blood supply. The periosteum also possesses vessels and abundant sensory nerve (particularly pain) fibres. In a developing bone the entire cartilage model (including articular cartilage and the growth plate) is avascular and aneural except after conversion to bone at centres of ossification. In a mature bone, hyaline cartilage is retained only on articular surfaces. Cartilage that is not articular (e.g. of nasal septum) receives its only nutrition from perichondrial vessels.

Perichondrial stripping

Extensive stripping of the perichondrium (e.g. from bruising) endangers viability of the cartilage.

Vascular foramina

Fig.4.22 Sites of vascular foramina

Blood vessels enter and leave a bone via numerous vascular foramina. Large vascular foramina (primarily for

veins) are particularly numerous near the articular margin (e.g. on neck of femur) and on parts of bones filled with red bone marrow (e.g. body of a vertebra).

Lymph vessels accompany the arteries and veins.

Nerves also accompany them, providing sensory fibres to bone and motor fibres to vessels.

Vascular foramina are absent from articular surfaces of bone because its covering articular cartilage has no vessels or nerves. Bony articular surfaces therefore receive their vascular and nerve supply via the underlying bone.

Nutrient and periosteal arteries

The shaft of a long bone receives a single large nutrient (medullary) artery via a nutrient foramen

extending obliquely to the medullary cavity (directed away from the growing end) to supply the bone marrow and inner compact bone.

Fig.4.23 Vessels supplying the shaft of a long bone

It divides into superior and inferior medullary branches

(directed towards both ends). The shaft of a long bone also receives multiple small periosteal arteries that supply the

periosteum and outer compact bone.

Periosteal stripping

Although these arteries link with branches of the nutrient artery, extensive stripping of the periosteum (e.g. during surgery) may deprive directly underlying compact bone of blood supply.

Metaphysial and epiphysial arteries

Fig.4.24 End arteries in a developing long bone

The ends of developing long bones typically receive sets of metaphysial and epiphysial arteries. These arteries are ‘end arteries’ because the cartilaginous plate

is avascular and forms a barrier preventing communication between them (until epiphysial fusion).


27

Interruption of blood supply to bone

Interruption of blood supply from metaphysial or epiphysial arteries endangers the adjacent metaphysis or epiphysis, respectively. This may impair normal growth and/or result in death of bone (avascular necrosis)

Vascular circle

The arteries supplying the ends of a mature long bone arise from a vascular circle, (‘circulus vasculosus’),

derived from articular branches of arteries to the associated joint.

Fig.4.25 Arteries at the end of a developing bone

The vascular circle is typically located around the attachment of the capsule to bone. It provides branches both to the capsule and to bone. Early in development the joint capsule attaches around the periphery of the epiphysial plate. The vascular circle around the end of a developing bone is initially located at this site and provides both epiphysial and metaphysial branches.

Anastomoses in the end of a long bone

A mature long bone no longer possesses epiphyses, metaphyses or a diaphysis (the parts of a developing long bone).

Fig.4.26 Anastomoses after epiphysial fusion

The branches of articular arteries that correspond to epiphysial and metaphysial arteries are able to link (‘anastomose’) with each other because the intervening

(avascular) hyaline cartilage of the growth plate has disappeared with epiphysial fusion.

Fig.4.27 Upper end of adult femur after vascular injection

Fractures

A broken bone is termed a fracture (L. ‘break’). A

fracture may be associated with stripping and/ or tearing of the periosteum (particularly if there is displacement of the bone ends). There is typically swelling from bleeding due to the rich blood supply (particularly of bone tissue and marrow). It is accompanied by severe pain due to the rich sensory nerve supply (particularly of periosteum).

Fractures occur commonly in children.

Adults tend to have stronger bones than ligaments, while children have the reverse.

Fig.4.28 Simple and compound fractures

If fractured bone is exposed to the air (by laceration of overlying skin or mucous membrane, e.g. from sharp bone fragments) it is termed a ‘compound’ (open) fracture and has a significant risk of bone infection.

Fracture healing

Bone receives a rich blood supply creating much bleeding at the time of injury.

Vascularity enables numerous vessels to invade the fracture site during repair. This occurs within the mass of connective tissue (termed callus) as a result of periosteal and endosteal proliferation. New bone is formed within the callus then subsequently remodelled. Uncomplicated fractures therefore tend to heal well, provided the bone ends are correctly aligned and immobilised for an appropriate length of time.


28

Fig.4.29 Alignment and immobilisation in fracture healing

Healing, including of fractures is more rapid in children.

Weight bearing bones heal slower than non-weight bearing bones.

Chapter 5: Articular System and Joints

29

ARTICULAR SYSTEM

FIBROUS AND CARTILAGINOUS JOINTS

SYNOVIAL JOINTS

ARTICULAR SURFACES AND CARTILAGE

FIBROUS CAPSULE

SYNOVIAL MEMBRANE AND CAVITY

LIGAMENTS

SPECIAL JOINT STRUCTURES

JOINT STABILITY AND MOBILITY

NEUROVASCULAR SUPPLY OF JOINTS

ARTICULAR SYSTEM

The articular system is made up of joints including associated ligaments. Bones and/or cartilages meet at

joints.

Types of joints

Joints are classified as fibrous, cartilaginous and synovial. The vast majority are synovial joints which are

adapted for movement. Some joints, particularly primary cartilaginous joints of

developing long bones (epiphysial plates) are temporary,

fusing at various ages.

Fig 5.1 Modules of an articulated adult skeleton

Other joints, particularly sutures (fibrous joints between

bones of the skull) tend to become obliterated with aging. Joints are typically paired. However, secondary

cartilaginous joints are unpaired and are located

exclusively in the midline.

Fig 5.2 Major joints of the articular system

FIBROUS AND CARTILAGINOUS JOINTS

Fibrous and cartilaginous joints are solid with no joint cavity.

Fig.5.3 Fibrous and cartilaginous joints


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Fibrous joints

In fibrous joints (suture, syndesmosis and gomphosis), bones are bridged by fibrous tissue. The

periosteum of each bone forming the articulation is continuous with the fibrous tissue of the joint.

Fig.5.4 Structure of a fibrous joint

Sutures (L. ‘seams’) occur between bones of the skull creating characteristic wavy lines occupied by sutural ligaments. With age, skull bones tend to unite, forming a synostosis (G. ‘together + bone’) as the fibrous joint

becomes obliterated. Sutures are not mobile but they allow for growth. A syndesmosis (G. ‘together + band’) holds the distal ends of the tibia and fibula together by a strong interosseous ligament. It permits only a small amount of movement, enough to help absorb compressive forces and avoid fracture. A gomphosis (G. ‘bolt’) is a tooth socket lined by the periodontal membrane (ligament) anchoring the tooth. It does not allow significant movement.

Primary cartilaginous joints

In primary cartilaginous joints, two areas of bone are

bridged by hyaline cartilage. They do not allow movement but allow for growth.

Fig.5.5 Structure of a primary cartilaginous joint

An epiphysial (growth) plate is regarded as a primary cartilaginous joint, even though it is temporary. At epiphysial fusion this joint disappears, becoming a synostosis. Other primary cartilaginous joints occur in the thoracic cage associated with the costal cartilages (particularly the costochondral and interchondral joints).

Secondary cartilaginous joints

Secondary cartilaginous joints consist of a thin layer

of hyaline cartilage lining each bony articular surface, with thick fibrocartilage sandwiched between them. They allow restricted movement yet can withstand considerable pressure. Secondary cartilaginous joints are also termed symphyses (G. ‘together + grow’). They are located

exclusively in the midline of the body. The pubic symphysis, intervertebral discs and the manubriosternal joint are all secondary cartilaginous joints.

Fig.5.6 Secondary cartilaginous joints in midline

SYNOVIAL JOINTS

Synovial joints allow for extensive movement and are characterised by a joint cavity (in addition to the hyaline cartilage that covers the bony articular surfaces).

The shape of the articular surfaces determines the particular movements permitted.

Shape, depth and size of articular surfaces (as well as ligaments and muscles) contribute to the range of movement possible.

Types of synovial joints

The only movements that occur at plane joints are

simple gliding movements (e.g. joints between articular facets of adjacent vertebrae and some joints between carpal bones). Plane joints are characterised by articular surfaces that are almost flat and parallel.

Fig.5.7 Types of synovial joints and associated movements

5. Articular System and Joints

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Synovial joints may be classified according to the number of axes of movement.

Uni-axial joints have a pair of movements around one axis. These joints may be further classified as hinge for flexion (L. ‘to bend’) and extension (e.g. elbow joint and interphalangeal joints), and pivot for rotation (e.g.

radioulnar joints). Bi-axial joints have a pair of movements

(flexion/extension and abduction/adduction) around each of two axes (typically perpendicular to each other). These joints may be further classified according to the shape of their articular surfaces. Bi-axial joints are condylar (e.g.

metacarpo-phalangeal joints of the fingers and toes), ellipsoid (wrist joint) and saddle (metacarpo-phalangeal

joint of the thumb). Multi-axial joints have two pairs of movements

(flexion/extension and abduction/adduction) around two horizontal axes perpendicular to each other and an additional pair of movements (medial rotation/lateral rotation) around a longitudinal axis. These are ball and socket joints (e.g. shoulder and hip joints).

Combinations of movements may occur in bi-axial joints (e.g. rotation in conjunction with flexion or extension) and in multi-axial joints (e.g. circumduction).

Simple and compound joints

Synovial joints may be classified according to the number of articular surfaces. Simple joints have one pair of

articular surfaces (the majority of synovial joints) while compound joints have more than one pair (e.g. elbow and

knee joints). The elbow joint involves the humerus, ulna and radius (in humero-ulnar and humero-radial articulations). The knee joint involves the femur, tibia and patella (in femoro-tibial and femoro-patellar articulations).

Complex joints

The vast majority of synovial joints have a single synovial cavity. In complex joints the joint cavity is

subdivided into more than one compartment. This may be a complete partition by fibrocartilage disc (e.g.

temporomandibular and sternoclavicular joints) or incomplete by menisci (e.g. knee joint). Complex joints

enable separate movements to occur simultaneously on either side of the partition (e.g. gliding with rotation at the temporomandibular joint) whilst maintaining optimal stability.

ARTICULAR SURFACES AND CARTILAGE

The surfaces of structures meeting at a joint are termed articular (L. ‘joint’) surfaces. Typically, there is only one

pair (simple joints) but some joints have more than one (compound joints).

Matching pairs of articular surfaces are smooth and reciprocally shaped. Each is covered by articular cartilage (to bear weight, resist compression and be almost frictionless).

Bony articular surfaces do not come in direct contact with each other unless the overlying articular cartilage has worn away.

All joint surfaces have some degree of curvature. This

may be convex, concave or both. Ovoid (L. ‘egg-like’)

surfaces are convex (‘male’) or concave (‘female’) in all directions. Sellar (L. ‘saddle’) surfaces are concave in one

direction and convex in the other.

Fig.5.8 Articular surfaces and cartilage of hip joint

Bony non-articular surfaces

Although articular surfaces are typically bony, the entire surface on the end of a bone is not necessarily articular.

Bony non-articular areas occur in some joints (e.g. hip and knee). They may be in the form of a pit, fossa or notch providing attachment for intra-articular ligaments

(e.g. ligament of head of femur and cruciate ligaments) or menisci.

A bony non-articular area may be covered by a fat pad

(e.g. in the socket of the hip joint).

Non-bony articular surfaces

Ligaments and fibrocartilage may contribute to articular surfaces.

Ligaments, particularly around pivot joints (e.g. proximal radioulnar) and associated with some weight bearing joints (e.g. hip and ankle), may have a surface that is articular. This surface tends to be lined with cartilage (as it would otherwise be rough).

A fibrocartilaginous labrum (L. ‘lip’) deepens the socket of a ball and socket joint. Fibrocartilaginous discs (e.g. temporo-mandibular and sternoclavicular) and menisci

(e.g. knee) also tend to be articular.

Joint degeneration

With aging and overuse there is a tendency for joint degeneration. With this degeneration (degenerative

arthritis) the articular cartilage becomes progressively thinner. This may be detected on a radiograph as a narrowing of the radiological joint space in contrast to the

anatomical joint space (the synovial cavity). Thinning of the articular cartilage causes the bony

articular surfaces to come into closer proximity, eventually making contact with each other. This causes severe pain due to exposure of underlying bone possessing a sensory nerve supply (in contrast to aneural hyaline articular cartilage).


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Fig.5.9 Thinning and loss of articular cartilage

Osteophyte formation

Degenerative arthritis is also associated with formation of osteophytes (G. ‘bone + growths’) at the joint margins.

Fig.5.10 Osteophytes and encroachment on a foramen

These are produced by proliferation of exposed bone with a rich blood supply (in contrast to the avascular hyaline articular cartilage). The progressive bony proliferation tends to decrease joint mobility (and may ultimately result in bony fusion). Osteophytes may also

encroach on adjacent structures, especially if bordering confined spaces (e.g. spinal canal or an intervertebral foramen).

Articular cartilage damage

Injury to an articular surface (particularly a fracture extending into it) is likely to be associated with damage of the hyaline articular cartilage. If a fragment of cartilage breaks off, a loose body (in the joint cavity) is created,

together with the associated defect that may expose underlying bone.

Damage to articular cartilage triggers an early onset of degenerative arthritis at that joint.

Fig.5.11 Effects of articular cartilage damage

FIBROUS CAPSULE

Description

The fibrous capsule (L. ‘box’) encloses a synovial

joint, defining its boundary. Structures located within the joint are termed intracapsular (or intra-articular). The

fibrous capsule may also be termed the capsular ligament. Like a ligament, it is dense connective tissue made up of collagen fibres.

The fibrous capsule may be reinforced by ligaments or receive muscle attachments at particular sites. It may also have deficiencies (to allow exit for an intracapsular tendon or for a bursa to communicate with the joint cavity).

Fig.5.12 Elbow joint capsule and reinforcements

Attachments of fibrous capsule

The capsule generally attaches to the articular margins (where its collagen fibres merge with those of the periosteum). At particular sites its attachment may deviate along the bone. In certain joints part of the capsule may attach to a ligament rather than to bone.


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The elbow joint capsule permits rotation of the radius by merging with the annular ligament of the proximal

radioulnar joint (instead of attaching to the radius). Although loose enough to allow sufficient mobility, the

fibrous capsule becomes taut on stretch and contributes to stability.

Migration of capsule from epiphysial plate

With long bones, the capsule is initially attached to the periphery of the epiphysial plate then migrates (usually towards, but occasionally away from, the articular margin).

Fig.5.13 Migration of capsule during development

SYNOVIAL MEMBRANE AND CAVITY

Synovial cavity

The space within the interior of a joint is termed the synovial cavity. The vast majority of joints have a single

and discrete joint cavity. For some (complex joints), the joint cavity is either partly (e.g. knee joint) or completely (e.g. temporomandibular joint) subdivided compartments. For others, more than one joint may share the same joint cavity (e.g. elbow joint with proximal radioulnar joint).

Fig.5.14 The interior of a synovial joint

Synovial membrane

Synovial membrane is a serous membrane. It consists

of a layer of flattened cells (mesothelium) on a thin bed of loose connective tissue that is highly vascular and can be thrown into folds or fringes.

Synovial membrane lines the internal surface of the capsule and all non-articular structures on the interior of a synovial joint.

Synovial membrane is delicate and does not extend over the articular cartilage (where it would be damaged).

At sites where the capsule does not attach to the articular margin, synovial membrane is reflected onto bone (covering periosteum between the capsular margin and the articular margin).

Synovial fluid

The volume of a synovial cavity is normally very small

(less than 1ml, even in a large joint). The synovial membrane secretes fluid into the joint cavity thus providing nutrition for the articular cartilage. Synovial (L. ‘with + egg’ i.e. consistency of egg white) fluid acts as an adaptable

lubricant for the articular cartilage. Its viscosity decreases with increased loading minimising friction (because the contained muco-polysaccharide hyaluronic acid can

change its configuration accordingly). A film of synovial fluid lies between apposed articular cartilage surfaces, particularly during movement.

Fig.5.15 Section through the shoulder joint

Synovial effusion

Synovial membrane has a rich blood supply derived from articular arteries (via branches from the vascular circle, around the capsular attachment).

Irritation of the delicate synovial membrane (e.g. by mild, repetitive trauma) results in an increased blood supply to it (due to vessels dilating) and a subsequent increase in secretion of synovial fluid. An accumulation of synovial fluid is termed a synovial effusion. It produces (often visible)

swelling of the joint (particularly where it is least supported). Tissue resistance limits the degree of effusion.


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Haemarthrosis

Tearing the vascular synovial membrane produces bleeding into a synovial cavity. The accumulation of blood in a synovial cavity is termed a haemarthrosis (G. ‘blood + joint’). It is typically due to severe trauma, particularly when structures lined with synovial membrane are torn. However, it may even occur with minimal trauma in a haemophiliac.

Blood progressively accumulates in the joint cavity (until limited by capsular expansion). A haemarthrosis produces a swelling of the joint that tends to be warm to touch.

Septic Arthritis

Introduction of microbes into a synovial cavity (septic arthritis) is a potentially serious event likely to produce an

accumulation of pus. Septic arthritis, as with any inflammatory arthritis, may lead to permanent joint destruction due to erosion of articular surfaces.

Loose body

A fragment of cartilage may survive as a ‘loose body’

in a joint cavity (or even grow) because it receives adequate nutrition from the synovial fluid. A loose body may produce ‘locking’ of the joint if trapped between the

articular surfaces. This interference with movement tends to be episodic.

Fig.5.16 Loose body surviving in synovial fluid

LIGAMENTS

Ligaments (L. ‘bind’) are fibrous connections between

bones. The vast majority of ligaments are primarily composed of collagen fibres (for tensile strength), which blend with the fibrous covering (periosteum) of the bones taking part in a joint.

Elastic ligaments

Some special ligaments, termed ‘elastic ligaments’,

contain large numbers of (yellow) elastic fibres. Being able to stretch (and recoil) they are less susceptible to injury. They have a poor nerve supply (as pain fibres would otherwise be triggered by stretch). Ligamenta flava (L. ‘yellow’) of the vertebral column are elastic ligaments.

Intrinsic and extrinsic ligaments

The majority of ligaments are intrinsic ligaments.

Intrinsic ligaments are thickenings of the fibrous capsule of a synovial joint. Their primary role is to reinforce the capsule. Extrinsic ligaments are separate from the

capsule. They may be extracapsular or intra-capsular. Most of them are extracapsular. The cruciate ligaments are an

example of intracapsular ligaments. They are located near the axis of movement at the knee joint, enabling them to contribute to stability without impeding mobility. The same applies for extrinsic ligaments situated between two joints acting as a functional unit (e.g. the interosseous ligament between the two joints under the talus and the interosseous membrane between the two radioulnar joints). Such ligaments also tend to be located along the axis of movement.

Ligaments, within a joint or between two joints acting as a functional unit, are positioned along the axis of movement.

Collateral ligaments

Collateral (L. ‘with + side’) ligaments are typically

located on the medial and lateral sides of hinge joints (e.g. elbow, ankle, knee and interphalangeal joints of the fingers and toes) perpendicular to the axis of movement. They tend to blend with the joint capsule, forming intrinsic ligaments.

Collateral ligaments are important contributors to stability by preventing unwanted side-to-side movement.

Some part of a collateral ligament tends to remain taut throughout the full range of flexion/extension.

Fig.5.17 Ligaments of the elbow complex

Uniaxial joints enable no other (pairs of) normal movements. Modified hinge joints (e.g. knee) permit some rotation when collateral ligaments become slack during flexion. This slackness is facilitated by the ligaments being situated eccentrically (i.e. not perpendicular to the axis of movement during the entire range of flexion).

Accessory ligaments

Accessory ligaments are extrinsic ligaments of a joint

that are located at a distance from it. Although structurally separate, they function with the associated joint.


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Accessory ligaments are found in the spine (e.g. ligaments of the vertebral arches and bodies) at a distance from the intervertebral joints. They are also found on the clavicle (e.g. costoclavicular and coracoclavicular ligaments) at a distance from the joints at each end of the bone (sternoclavicular and acromioclavicular joints, respectively). The interosseous membrane of the forearm and the leg may be regarded as an accessory ligament of the radioulnar and tibiofibular joints, respectively.

Grades of ligament injury

Ligament injuries (‘sprains’) are common, particularly in adults (where bones are relatively much stronger than in children).

Children are more likely to fracture a bone before tearing a ligament.

Fig.5.18 Grades of ligament injury

In ligament injuries there is damage to the collagen fibres. Degrees range from microscopic sprain (grade I) where a few fibres rupture to partial tear (grade II) and complete tear (grade III), where all fibres rupture.

Fig.5.19 Avulsion fracture

Tears and avulsion at ligament attachments

Ligaments tend to tear at their weakest point.

The weakest points of a ligament are at or near their attachments, rather than between them.

Sometimes a fragment or flake of bone is avulsed (L.

‘tear away’) with (or even instead of) ligament rupture.

Ligament vulnerability

A ligament that is arranged in discrete parts rather than a continuous band allows more joint mobility but is weaker and therefore more vulnerable.

Fig.5.20 Vulnerable bands of ankle joint lateral ligament

Ligament stress test

Extensive ligament damage produces great impairment of function and increased potential for instability.

Fig.5.21 Stress test for cruciate ligaments of knee joint

Ligament integrity may be tested clinically by stressing

the ligament (putting it on stretch) and comparing the observable movement between the injured and uninjured sides. This can be confirmed on X-ray by performing ‘stress views’ for suspected complete rupture of a ligament. With a ligament sprain, pain tends to be exacerbated by stressing the ligament.

Masking of ligament tear by muscle spasm

Abnormal or excessive joint movement is an important diagnostic feature in an acute ligament injury, particularly a grade III injury. This may be masked initially by the other stabilising structures at a joint, particularly muscles (due to protective reflex muscle spasm).

Masking of pain by nerve fibre rupture

Stressing a ligament to elicit pain is also a diagnostic feature in an acute ligament injury (particularly for grade I or grade II sprains). This may be masked in grade III injuries as sensory nerve fibres (including pain fibres) within the ligament are also likely to be severed.


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Laxity and loss of proprioception

Laxity can predispose a ligament to future injury by

allowing excessive range of normal movement and/ or abnormal movements.

Loss of proprioception also predisposes to future

injury by impairment of (voluntary and reflex) muscle control (that normally contribute to joint stability).

Fig.5.22 Predisposition to further injury

SPECIAL JOINT STRUCTURES

Additional components of certain joints are termed special structures. These structures are made up of

fibrocartilage, tendon, serous membrane or fat.

Fig.5.23 Special structures of knee joint

Labrum

A labrum (L. ‘lip’) deepens the socket of a ball and

socket joint (e.g. hip and shoulder). A labrum is made of fibrocartilage. Being articular it is not covered by synovial membrane and is avascular (receiving its nutrition from the synovial fluid).

Disc and menisci

A complete disc subdivides a synovial cavity (e.g. temporomandibular and sternoclavicular) while menisci (L. ‘little half-moons’) partially subdivide a synovial cavity (e.g. knee).

Discs or menisci create compartments, allowing different movements to occur simultaneously on each side of the partition.

Flexion/extension occurs in the upper compartment of the knee joint (between the femur and menisci) while rotation occurs in the lower compartment (between the menisci and tibia). Discs and menisci are made of fibrocartilage. Being articular, they are not covered by synovial membrane (although they assist with spreading synovial fluid). They are able to resist compression and may be weight-bearing.

Discs and menisci tend to be thicker at their periphery (increasing stability) where they attach to the fibrous capsule and receive a vascular supply. The central parts of these structures are avascular (receiving nutrition from the synovial fluid).

Labrum or meniscal tears

A torn labrum tends not to heal, as it is avascular.

Fig.5.24 Vascularity and healing of meniscal tears

Tears to discs or menisci tend not to heal, except around the periphery (where they receive a blood supply along their capsular attachment). A meniscus trapped between bony condyles while weight bearing may split longitudinally. A dislodged fragment of meniscus may survive as a ‘loose body’ in a joint cavity (since it receives

adequate nutrition from the synovial fluid).

Intracapsular tendon

Tendons attaching to a bony area between the articular margin and the periphery of the capsule occur in two major joints (shoulder and knee). The associated muscles (long head of biceps and popliteus) by the location of their tendon, contribute to shoulder stability or enable rotation that unlocks the knee joint, respectively.

An intracapsular tendon leaves a joint through a

defect in the fibrous capsule. The tendon is covered by synovial membrane throughout its intracapsular course.

Bursae

A bursa (L. ‘purse’) is a double fold of serous

membrane (containing a small amount of fluid) interposed between structures that rub together (e.g. skin, bone, ligament, tendon) reducing friction.

Bursae tend to be more numerous at joints with greater mobility.


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Certain bursae (e.g. subscapular and suprapatellar) communicate with a synovial cavity (of shoulder and knee, respectively). The communication occurs via a deficiency in the fibrous capsule. Synovial fluid may pass between the synovial cavity and its communicating bursae.

Fig.5.25 Types of bursae

Bursitis

Irritation (e.g. by unaccustomed or repetitive movement) trauma or infection of a bursa may result in inflammation (bursitis) with associated accumulation of synovial fluid,

blood or pus, respectively.

Fig.5.26 Olecranon bursitis ('student's elbow')

Joint cavity communication

Infection introduced into a bursa that communicates with a synovial cavity may easily spread directly by the synovial fluid into the joint and lead to septic arthritis.

Fig.5.27 Penetrating injury to a bursa and septic arthritis

Fat pads

Fat pads are intracapsular but extra-synovial. They fill

unoccupied space in a joint (e.g. below the patella at the knee joint and in the bony fossae of the humerus at the elbow joint).

Fat pads help absorb compressive forces between bones. They also contribute to the spread of synovial fluid by acting as a swab and create extra folds of synovial membrane to greatly increase its surface area.

Pinched fat pad

A fat pad may occasionally become trapped (pinched) between bony surfaces (e.g. between the large femoral and tibial condyles). This may produce pain associated with a synovial effusion.

JOINT MOBILITY AND STABILITY

The dual joint properties of mobility (capacity for movement) and stability (capacity to resist excessive or

unwanted movement) vary in degree for different joints. Movements are described as occurring ‘at’ a joint

(rather than ‘of’ a joint). Movements may also refer to the (distal) part or bone moved (e.g. movements at the shoulder joint may also be termed movements of the arm or humerus).

Passive and active movements

Movements are either passive or active. A movement at a joint is passive when it is not directly due to contraction of its associated muscles (e.g. purely via gravity).

Assessment of joint mobility

An external agent may also be utilised to assist a passive movement throughout its full range of motion (‘passive assistance’).

This enables a clinical assessment of joint mobility (the potential range for each movement at a joint) that may be otherwise masked by muscle weakness or paralysis.

Pairs of movements

The shape of the articular surfaces primarily determines the type of movements allowed.

These movements may be gliding (at plane joints) or pairs of movements (one pair at uni-axial joints, two pairs at bi-axial and three pairs at multi-axial joints).

The most common pairs of movements are flexion/ extension, abduction/ adduction, and medial rotation/ lateral rotation. Other specific pairs of movement include plantar flexion/ dorsiflexion (at the ankle joint), inversion/ eversion (of the foot) and pronation/ supination (of the forearm).

Roll, slide and spin motions

There are three possible motions that may occur at joint surfaces for any particular movement. Roll (like a wheel), slide (like a ski) and spin (like a top) occur in varying

combinations and degrees. Flexion/ extension and abduction/ adduction tend to

have a combination of roll and slide. This minimises the necessary length of articular surfaces while maximising the range of a movement. Normally, rotation is pure spin.

Factors responsible for joint stability

Joints represent a trade-off between mobility and stability (e.g. the shoulder joint is the most mobile but least stable joint in the body).


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The factors responsible for stability (and limiting mobility) are classified as bony, ligamentous and muscular. These factors are involved to varying degrees

for different joints.

Fig.5.28 Factors stabilising the shoulder joint

The contribution to joint stability from bones is dependent on the congruence of their articular surfaces.

Fig.5.29 Types of joint stabilising factors

Ligaments (both intrinsic and extrinsic) act in addition to the fibrous capsule to prevent unwanted movements and movements beyond normal range. They also resist distraction of articular surfaces.

Muscles (and tendons) surrounding the joint give it support. Around the most mobile joints (shoulder and hip)

tendons blend with the capsule to form a cuff and act as dynamic ‘ligaments’.

This arrangement helps compress the ball against its socket, without limiting mobility (unlike ligaments).

Muscles are the most important stabilising factor for mobile joints, providing the first line of defence against dislocation.

Muscle's role in joint support can be controlled automatically by stretch reflexes (e.g. when part of a

capsule is on stretch the overlying muscles contract more strongly).

Close-packed position

The position of maximal stability is termed the ‘close-packed’ position. Articular surfaces (and articular

cartilages) are most apposed and the majority of ligaments are maximally taut (including the capsule, which may spiral and tighten). This is also the position of least volume in the synovial cavity (and most discomfort, if there is a synovial effusion).

All other positions of a joint are ‘loose-packed’,

allowing normal movement to take place (but with greater potential for unwanted movement). Articular surfaces will not be fully apposed and, in multi-axial joints, not all ligaments will be taut.

Fig.5.30 Close-packed position of a joint

Joint dislocation and subluxation

Dislocation of a joint means its articular surfaces are completely separated. Subluxation is a partial dislocation

where there is still some contact between the articular surfaces.

The vulnerability for dislocation and subluxation is greatest when overlying muscles are relaxed (or weakened).

Dislocation and subluxation may stretch (or tear) the joint capsule and ligaments; damage its associated structures (e.g. synovial membrane, bone, articular cartilage, menisci) or structures adjacent to the joint (particularly nerves and vessels). Reflex spasm of overlying muscles tends to protect the joint from further damage (but also makes reduction of a dislocation more difficult).

Dislocation or subluxation may result in subsequent ligamentous laxity, which predisposes to future injury.


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Fig.5.31 Complete and partial joint separation

NEUROVASCULAR SUPPLY OF JOINTS

Certain joint structures possess a rich nerve supply, while in others it is poor or absent. The same applies for blood supply.

Innervated underlying bone

The bony articular surfaces receive pain fibres, but these are normally protected from exposure and excessive pressure by the overlying hyaline articular cartilage (which is aneural).

Pain from degenerative arthritis

Degeneration of articular cartilage may lead to the progressive exposure of underlying bone, resulting in severe pain (especially with movement or weight bearing).

Fig.5.32 Bone exposure with cartilage loss

Innervated capsule and ligaments

The main fibrous tissue elements of a joint (capsule and ligaments) receive a rich supply of proprioceptive fibres conveying deep somatic sensation such as stretch and joint position. They also receive a rich supply of pain fibres (together with associated periosteum).

Sensory effects of ligamentous injury

Capsular and/ or ligamentous injury tends to be painful and results in significant loss of proprioception, particularly joint position sense. This loss of proprioception predisposes to future injury by impairment in both voluntary and reflex control of muscles contributing to joint stability.

Articular branches of nerves

Nerves supplying muscles that produce movements at a joint also typically supply the joint.

Nerve supply is via articular branches that convey

sensation (particularly proprioception and pain). When capsule on the flexor aspect of a joint is stretched

it stimulates proprioceptive sensory fibres in articular branches of the associated nerve. This leads (via reflex control involving the spinal cord then motor fibres in muscular branches of the same nerve) to increased contraction of the overlying flexor muscle group, helping to protect it from further stretch. Similarly, stretch of the capsule on the opposite aspect of the joint leads (via reflex control involving articular, then muscular branches of a different nerve) to increased contraction of its overlying (extensor) muscle group.

Vascular synovium and bone

The synovial membrane and the bone beneath articular surfaces receive a rich blood supply (from branches of articular arteries). Numerous vascular foramina are located on all non-articular parts of the bone, but hyaline cartilage covering articular surfaces is avascular (receiving its nutrition from the synovial fluid).

Effects of injury on vascular joint tissues

Mild injury of the synovial membrane produces an effusion of synovial fluid.

A severe injury tearing the synovial membrane produces bleeding into the joint cavity. Intracapsular bone fractures are also associated with bleeding into the synovial cavity.

When degeneration of articular cartilage exposes vascular bony surfaces, bone proliferation tends to occur (creating osteophytes).

Poorly vascular capsule and ligaments

Although the fibrous capsule and ligaments receive a blood supply, it is poor, particularly compared to that of the highly vascular surrounding muscles.

Effects of capsular or ligamentous injury

Injuries to capsule and ligaments tend not to bleed much compared to injured muscles or bones. They repair slowly and often inadequately, with resultant ligamentous weakness, lengthening and loss of proprioception.

Aneural and avascular hyaline cartilage

Hyaline articular cartilage does not possess blood vessels, lymph vessels or nerve fibres

Effects of articular cartilage injury

Injury to hyaline cartilage does not directly produce pain but may expose (or be accompanied by injury to) bone that does produce pain. Although a fragment of hyaline cartilage may survive in the synovial cavity as a loose body, the defect on the articular surface where it was previously located tends not to undergo normal healing.


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Articular vessels

The major artery of a limb tends to give branches as it passes near a joint. The branches link with each other (anastomose) particularly within the surrounding muscles, to ensure adequate blood supply. They form alternative pathways when the artery is kinked (e.g. by flexion at the joint).

Fig.5.33 Anastomosis around the hip joint

The anastomosis around a joint also provides articular branches to the joint forming a vascular circle located at

the capsular attachment. Arteries to the capsule (also supplying the synovial membrane) and arteries to the associated bone arise from the vascular circle. Joint structures receiving an arterial supply have a corresponding venous drainage. Many of the vascular foramina in bone, particularly those near the articular margin, are for veins.

Fig.5.34 Vascular supply to a joint

Chapter 6: Muscular System and Muscles

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MUSCULAR SYSTEM

MUSCLE STRUCTURE AND ATTACHMENTS

TENDON, APONEUROSIS AND RAPHE

DEEP FASCIA AND RETINACULA

FASCIAL SEPTA, SHEETS AND SHEATHS

FIBROUS & SYNOVIAL TENDON SHEATHS

MOVEMENT AND SKELETAL MUSCLE FORM

MUSCLE CONTRACTION AND ACTION

NEUROVASCULAR SUPPLY & MYOTOMES

MUSCULAR SYSTEM

The muscular system is made up of skeletal muscles together with associated structures. These are

condensations of fibrous tissue (including tendons and fibrous tendon sheaths) as well as synovial tendon sheaths.

The other types of muscle are smooth muscle and cardiac muscle. The former is found throughout the visceral and vascular systems in the wall of a tubular viscus or blood vessel. The latter is found only in the walls of the heart.

Muscles are arranged in groups that tend to share a common fascial compartment and produce a common action.

Muscles are typically paired, except for those in the midline.

Fig.6.1 Muscular system within body modules

Fig.6.2 Major groups of skeletal muscles

MUSCLE STRUCTURE AND ATTACHMENTS

Types of muscle

Muscle (L. ‘mouse’) is the active producer of movement. There are three types of muscle: skeletal, smooth and cardiac.

Skeletal muscle typically moves bones and is capable of voluntary movement. It is composed of large striated muscle fibres, which are dependent on a (somatic) motor

nerve supply to generate contraction. Each fibre is a discrete unit.

The strength of skeletal muscle contraction is proportional to the number of fibres recruited.

Fig. 6.3 Types of muscles


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Smooth muscle forms the walls of blood vessels and hollow viscera. It is composed of small non-striated

muscle fibres that function as a collective unit in contributing to regulation of the body’s internal environment. Smooth muscle is under autonomic nervous

control. Cardiac muscle forms the walls of the heart. The atria

and the ventricles are composed of striated muscle fibres that function as a collective unit. Cardiac muscle contracts

automatically and rhythmically to pump blood around the body.

Connective tissue in skeletal muscles

A skeletal muscle is composed of bundles of large striated muscle fibres (the contractile elements) surrounded by collagen and elastic fibres (the connective tissue elements). A thin tubular sheath of connective tissue termed endomysium (G. ‘within muscle’) surrounds each

individual skeletal muscle fibre. Bundles of skeletal muscle fibres are surrounded by

connective tissue termed perimysium (‘around muscle’).

The whole skeletal muscle is wrapped in a layer of connective tissue termed the epimysium (‘upon muscle’).

Fig.6.4 Bundling of skeletal muscle fibres

Bony markings from muscle attachments

Skeletal muscle attachments to bone may be fleshy (directly from the muscle belly) or tendinous (via a tendon

or aponeurosis). In each case their collagen fibres blend with those of the bone (via the periosteum).

Tendinous attachments to bone, in contrast to those of fleshy muscle fibres, produce bony markings.

The attachment of a tendon occupies much less area on a bone. The force generated by the muscle pulls on the periosteum with correspondingly greater pressure leading to prominent bone markings. These may be in the form of a roughening, a line, a crest or a tubercle.

A large tendon attaching to a developing bone is likely to be associated with a traction epiphysis (to allow for growth of the bone at the site of attachment).

Origins and insertions

A skeletal muscle has at least one attachment at each end. Generally muscle attachments are to bone but may be to skin, fascia, ligament or even other muscles. The origin is typically fixed and more proximal while the insertion is

typically mobile and more distal. There may be one or more heads of origin. Biceps (G:

‘two + heads’) muscle and triceps muscle have two and

three heads of origin, respectively (although each of these muscles inserts via a single tendon).

A muscle may consist of more than one part. Adductor magnus has adductor and hamstring parts originating

separately (from the pubis and ischium of the hip bone, respectively) and inserting separately (on the femur). These parts act as discrete functional units that even receive a different nerve supply, reflecting their development from different compartments

Fig.6.5 Parts of a skeletal muscle

Fig.6.6 Muscle attachments

Muscle belly

Skeletal muscles have a variety of shapes determined by the arrangement of their components. A muscle belly

lies between the attachments of a skeletal muscle. In many cases there is also a tendon connecting a muscle belly to

the site of attachment.

6. Muscular System and Muscles

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The site where a muscle belly becomes continuous with a tendon is termed the musculotendinous junction.

A fusiform (L. ‘spindle + shape’) muscle has a single

muscle belly with a tendon at each end. A few muscles have more than one belly. Digastric (G.

‘double + stomach’) muscle has two bellies, with a tendon between them. Rectus abdominis has a series of muscle bellies joined by tendinous intersections.

Triangular, rhomboid or quadrate shaped muscles have a flat muscle belly.

A circular muscle (e.g. orbicularis oris around the lips)

has a continuous muscle belly with its fibres arranged concentrically. Muscular contraction narrows the opening, while relaxation widens it.

Fig.6.7 Forms of muscle bellies

Fig.6.8 Shapes of flat muscle bellies

Regressive and atavistic variations

Skeletal muscles are subject to considerable anatomical variation.

A few muscles (e.g. palmaris longus in the forearm and plantaris in the leg) have evolved a small belly in

conjunction with a very long tendon. They are becoming vestigial, being no longer functionally important and may

even be absent. This type of variation is termed a regressive variation.

Fig.6.9 Regressive variations

A muscle attachment (e.g. of adductor magnus) that has retreated from one bone to another (e.g. tibia to femur) during evolution may remain as a ligament between them (e.g. medial ligament of knee joint). Occasionally, some muscle fibres may also remain. This type of variation is termed an atavistic (L. ‘forefather’) variation.

Fig.6.10 Atavistic variations

Grades of muscle injury

In muscle injuries (‘strains’) there is damage to the muscle and/or collagen fibres.

Degrees range from microscopic strain (grade I) where a few fibres rupture to partial tear (grade II) and complete tear (grade III), where all fibres rupture. Each grade

generally produces different results on testing. Grade I tears may be painful with strength testing (but do not reduce strength). Grade II strains are more extensive, producing pain with strength testing (and reduced strength) as well as pain when the muscle is stretched. Grade III possess a visible and palpable defect, together with a total loss of function.


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Fig.6.11 Degrees of muscle injury

Sites of muscle tears

In contrast to a ligament, a muscle tends to rupture at other sites in addition to its attachments.

This typically occurs within the muscle belly, at the musculotendinous junction or through its tendon.

Fig.6.12 Sites of muscle injury

Fig.6.13 Avulsion fracture

A tear is particularly likely to occur at a previous scar (which is a site of weakness).

Sometimes a fragment of bone is avulsed (L. ‘tear

away’) as well as (or even instead of) muscle or tendon rupture at an attachment.

Muscles prone to strain

Muscles crossing more than one joint are particularly prone to injury from over-stretching.

The muscles are more easily stretched beyond their limit by the collective movements at the (two) joints. However, these muscles enable extra force to be generated (as stretching unto a critical point creates stronger contraction).

Fig.6.14 Extra stretch by crossing 2 joints

Contracting gastrocnemius with an extended knee and

dorsiflexed ankle permits a strong contraction, but increases vulnerability of the muscle to injury from overstretching. This also occurs in contracting the hamstrings with a flexed hip and extended knee.

TENDON, APONEUROSIS AND RAPHE

Fig.6.15 Fibrous condensations in muscles


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Condensations of fibrous tissue associated with a skeletal muscle may be in the form of tendons, an aponeurosis or a raphe.

Tendon

A tendon (L. ‘stretch out’) is a prolongation of connective tissue linking a muscle belly to its attachment site(s).

Tendons enable force to be concentrated over a smaller area and/ or transmitted over a longer distance.

Tendons are found at many sites throughout the body, particularly where muscles compete for space (e.g. in the peripheral parts of the limbs). A tendon is much less bulky than a muscle belly. Some muscles (e.g. long flexors and extensors to fingers and toes) have multiple tendons.

Aponeurosis

An aponeurosis (L. ‘from + tendon’) is a broad, flat sheet of connective tissue linking a muscle belly to the site of attachment. Aponeuroses enable force to be spread over a greater area and may enclose other muscles to increase their efficiency of contraction (e.g. rectus abdominis in the rectus sheath).

Aponeuroses are found at many sites throughout the body. The most extensive aponeuroses are found in the trunk (e.g. associated with anterior abdominal wall muscles). An aponeurosis is much thinner than either a muscle belly or a tendon. Some muscles (e.g. biceps brachii) insert via both a tendon and an aponeurosis.

Raphe

A raphe (L. ‘seam’) is a line of fibrous tissue where one

muscle joins another. Raphes are often located in the midline of the body, uniting a muscle with its fellow from the other side.

A raphe provides a relatively long attachment, utilising minimal connective tissue and occupying little space.

A raphe enables its associated muscles to provide greater support, while maintaining flexibility.

The floor of the mouth (formed by the mylohyoid

muscles and their raphe) supports the tongue while the pelvic floor (formed by the levator ani muscles and their

raphe) supports the pelvic viscera (while enabling them to expand).

Sites where muscle fibres are substituted

Fig.6.16 Substitution of muscle fibres at pressure site

Fleshy muscle fibres tend to be replaced by tendons at sites of pressure or friction.

Fleshy muscle fibres tend to be replaced by aponeuroses at sites of increased tensile loading or where enclosed muscles benefit from a mechanical advantage.

DEEP FASCIA AND RETINACULA

Deep fascia

Fascia (L. ‘bandage’) keeps certain structures together

and keeps others apart. Deep fascia is a thin, tough sheet primarily made of

collagen fibres (dense connective tissue). It is found deep to the looser subcutaneous tissue (superficial fascia)

under the skin of the limbs, neck, back and perineum. Deep fascia is strong and non-expansile. It gives

protection and support to underlying structures while providing an extensive surface area for muscle attachment. Deep fascia is thickened in the palm (as the palmar aponeurosis) and in the sole (as the plantar aponeurosis) where it is bound to the overlying skin by numerous fibrous connections. Deep fascia is also thickened over muscles requiring power (e.g. calf muscles) restricting their (radial) expansion and increasing their (longitudinal) efficiency.

Deep fascia is usually in one layer. However, in the neck it forms concentric layers, including one deep to the skin (investing fascia) and one around the cervical spine (prevertebral fascia). This enables structures (e.g.

trachea, oesophagus and major vessels) to glide between the two layers during neck movements.

Sites where deep fascia is absent

Deep fascia is absent from the face (where muscles of facial expression are in the subcutaneous tissue, inserting directly to skin).

The thoracic and abdominal walls are able to expand without the restriction of a continuous layer of non-yielding deep fascia. Their muscles insert into extensive aponeuroses, which provide support while permitting distension of underlying viscera (stretching the muscles). Deep fascia is not found as a continuous sheet around parts of the body that expand significantly. The deep fascia of the leg is absent over the subcutaneous surface of the tibia. It attaches to the bony edges by blending with periosteum.

Deep fascia is not found over the subcutaneous surface of a bone

Retinacula

A retinaculum (L: ‘tether’) is a thickening of deep fascia

that holds down tendons. Retinacula are located near major peripheral joints where long tendons cross en route to more distal joints (to exert their primary action).

Retinacula are found near the wrist and ankle joints, particularly associated with the many long flexor and extensor tendons destined for the digits. Retinacula prevent tendons from ‘bow-stringing’ and can create a pulley mechanism that enhances movement of the joints distally (e.g. In the fingers and toes).

Being maintained close to the axis of motion at the wrist and ankle joints, the digital tendons do not have much influence on movement at these joints. In contrast, the tendo calcaneus (‘Achilles tendon’) crosses the ankle joint but is not covered by a retinaculum as the tendon inserts immediately distal to the joint rather than to the toes.

The absence of a retinaculum allows tendo calcaneus to ‘bowstring’ and to exert great leverage at the ankle joint (by being located at a distance from the axis of motion). This leverage would otherwise be reduced by a retinaculum.


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Retinacula are generally regarded as flexor or extensor, according to the tendons transmitted. The space beneath a retinaculum may be subdivided into compartments (for one or more tendons) by fibrous partitions.

Fig.6.17 Deep fascia and its thickenings

FASCIAL SEPTA, SHEETS AND SHEATHS

Fascia may be arranged in perpendicular septa, parallel sheets or in cylindrical sheaths.

Fascial septa

A fascial septum (L. ‘partition’) is an extension of dense

connective tissue that separates structure(s) from each other. Fascial septa typically form a perpendicular connection between deep fascia and periosteum (at these junctions the collagen fibres blend). Fascial septa also bind down skin to underlying deep fascia (e.g. in the palms, soles and scalp) or aponeurosis.

Fig. 6.18 Fascial compartments in calf

Fig.6.19 Septum separating compartments

They also may subdivide the space under a retinaculum into separate compartments for tendons.

Intermuscular septa are fascial extensions forming

partitions between muscles, providing them with additional area for attachment as well as routes for the passage of vessels and nerves (along the septa).

Muscles with a common action are generally located in the same fascial compartment.

Fig.6.20 Compartments for muscles with a common action

Fascial sheets

Deep fascia is generally in the form of a single sheet although occasionally it is in two parallel or concentric sheets (allowing mobility between them). It typically forms the roof of a compartment for muscles.

Additional sheets of fascia may be located between muscle layers within a fascial compartment (e.g. in the posterior compartment of the calf).

Fig.6.21 Types of fascial sheets


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An interosseous membrane is a fibrous sheet

between parallel bones, connecting the periosteum of one with that of the other and providing increased area for muscle attachment. It creates a major partition between muscle groups and contributes to the formation of compartments. Flexor muscles are located on (and gain attachment to) one side of an interosseous membrane with extensor muscles on the other.

Fascial sheaths

Fascial sheaths surround glands. They also surround nerves and vessels.

Where nerves and vessels have a common course they tend to be enclosed within a common fascial sheath (as a neurovascular bundle).

Fascial sheaths are not as tough as septa or sheets of deep fascia. They provide support, yet allow some movement or expansion.

Fig.6.22 Sheath of a neurovascular bundle

Where more expansion occurs (e.g. around large veins) the fascial sheath is thinner or even absent.

FIBROUS & SYNOVIAL TENDON SHEATHS

Fibrous tendon sheaths

The long flexor tendons in the digits are covered by dense connective tissue, termed fibrous tendon sheaths. Fibrous tendon sheaths form the roof of fibro-osseous tunnels for long tendons passing over (the flexor aspect) of

a succession of bones (in the digits).

Fig.6.23 Fibro-osseous tunnels for a tendon

Tendons are bound down by fibrous tendon sheaths to prevent them from ‘bow-stringing’. A pulley mechanism created may enhance movement (e.g. on the fingers and toes). The sheath is thicker (with its fibres aligned) along lines of stress.

Synovial tendon sheaths

Synovial sheaths surround tendons that are enclosed

by fibrous sheaths or pass under a retinaculum (e.g. the long flexor and extensor tendons to the digits). A synovial sheath may also surround a tendon that lies in a bony groove (e.g. tendon of long head of biceps in the bicipital groove of the humerus). The role of a synovial sheath is to minimise friction between moving parts in a confined space.

Fig.6.24 Fibrous and synovial tendon sheaths

A synovial sheath is an elongated pouch made up of two layers of serous membrane. Each serous membrane consists of a layer of flattened cells (mesothelium) on a

thin vascular bed of loose connective tissue. One layer of the serous membrane pouch lines the internal surface of a fibrous tendon sheath, while the other layer covers the tendon.

Fig.6.25 Development of a synovial tendon sheath


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A small amount of fluid is secreted into the potential space between the two layers of the pouch minimising friction (when the tendon glides back and forth within its overlying sheath during movements). A synovial sheath develops from a single layered pouch that becomes invaginated (L. ‘in + sheath’) by the tendon. The serous

membrane forming the connecting stalk between the lining of the fibrous sheath and the reflection onto the tendon breaks down (except at a few sites along the synovial sheath).

Tenosynovitis

Irritation (e.g. by unaccustomed or repetitive movement) or infection of synovial tendon sheaths may result in inflammation (tenosynovitis) with accumulation of fluid or

pus. Inflammation of the tendon sheath may also be associated with inflammation of the tendon (tendinitis) or of its fibrous tendon sheath (tenovaginitis).

Infection of a synovial sheath

Infection introduced into a synovial sheath (e.g. by a penetrating injury to a digit) may spread long distances by tracking along the fluid-filled conduit provided in the sheath.

The long flexor tendon to the little finger is surrounded by a synovial sheath that is continuous with the large common sheath (at the wrist) for all long flexors of the fingers. Infection introduced into the synovial sheath for the little finger is likely to spread proximally to the palm and wrist.

Fig.6.26 Spread of infection along synovial sheaths

Mesotendons

Small remnants of the connecting stalk that existed between the two layers of a synovial sheath convey blood vessels. These remnants are termed mesotendons (L.

‘middle + tendons’).

Fig.6.27 Blood supply to a tendon

Effect of mesotendon injury

Damage to the mesotendons, by interruption of blood supply, may lead to tissue death and subsequent rupture of the tendon.

Fig.6.28 Mesotendon formation

MOVEMENT AND SKELETAL MUSCLE FORM

Skeletal muscles have a dual role. They move bones, but also hold bones together. Complementing the dual role of joints in mobility (capacity for movement) and stability (capacity to resist excessive or unwanted movement) they vary in degree for different muscles. A particular movement may be described as occurring at its fulcrum (e.g. flexion at the elbow joint). Alternatively, it may be described as a movement of the lever distal to the joint (e.g. flexion of the forearm).

Third order levers

Three types of levers occur in the body. The vast majority of muscles (e.g. flexors at the elbow joint) act on third order levers to move a load through the greatest range of movement (along a wider arc and at more speed). The power (P) via the muscle attachment is applied between the fulcrum (F), or axis of the joint, and the load (L) at the end of the bony lever.

Fig.6.29 Third order levers (for range and speed)


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Second order levers

Second order levers provide greatest power. The load (e.g. body weight transmitted through the ankle) is between the fulcrum (e.g. the joints of the toes) and the power (e.g. via the Achilles tendon).

Fig.6.30 Second order levers (for power)

First order levers

First order levers provide greatest stability. The fulcrum (e.g. the joint above the atlas) is between the load (e.g. the head) and the power (e.g. via neck muscles).

Fig.6.31 First order levers (for balance)

Rotatory and translatory motion

Rotatory (angular) motion consists of a bony lever moving around a fixed axis. Translatory (linear) motion

consists of a bony lever being pulled directly away from or pushed directly towards a joint. Skeletal muscle contraction generally results in a combination of rotatory and translatory motion, which vary for different stages of a movement. There tends to be a trade-off between rotatory and translatory motion (depending on the line of pull of the muscle). The rotatory component typically produces the visible movement of a lever.

Distance of line of pull

Leverage of a muscle acting at a joint is determined by the distance of the ‘line of pull’ from the axis of that joint.

The line of pull is (represented by) a line connecting the muscle’s origin and insertion. The axis of the joint is located at the centre of the arc of movement.

Fig.6.32 Leverage determined by distance from line of pull

Trade-off between power and range

The active range of movement at a joint is dependent on muscle shortening. There is a trade-off between power and range of movement.

A powerful movement requires a strong contraction acting at a distance along a lever (creating great leverage). A large range of movement requires shortening of a muscle that inserts close to the fulcrum (creating a wide arc of movement of the distal lever). The power and range of a particular movement may be deduced by considering the sites of skeletal muscle attachments (for determining leverage versus lever arc) and muscle form (for determining strength of contraction versus degree of shortening).

Muscle form

The form of a skeletal muscle is determined by the arrangement of its fibres.

Fig.6.33 Types of fibre orientation


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Some muscles have long parallel fibres (e.g. strap and fusiform muscles) while others have obliquely oriented fibres. Pennate (L. ‘feather’) muscles contain obliquely

oriented fibres. These may attach on one side of the tendon (uni-pennate), both sides of it (bi-pennate) or be

packed around a series of intramuscular tendons (multipennate).

Length and orientation of fibres

Under normal conditions a muscle fibre cannot shorten passively. During contraction an individual muscle fibre is capable of shortening up to about half of its resting length.

Fig.6.34 Pennate muscles

The active range of movement at a joint is proportional to the length of muscle fibres. This is reflected in the length of the muscle belly (its contractile part), rather than the whole muscle (which may include a non-contractile tendon).

The active range of movement at a joint is proportional to the length of muscle belly.

The orientation of fibres is also important as muscles with parallel fibres shorten more than those with oblique fibres. If a muscle is purely fleshy with parallel fibres, the shortening of the whole muscle will equal the degree of contraction of its individual fibres. If a muscle contains a tendon (non-contractile element) and/ or obliquely oriented muscle fibres, the shortening of the whole muscle will equal only the longitudinal component of the contraction of its individual fibres.

Cross-sectional area of muscles

Fig.6.35 Strength and cross-sectional area

There is a trade-off between degree of shortening and strength of contraction. Muscles with long parallel fibres tend not to have as great a cross-sectional area as those with intramuscular tendons. Pennate muscles can fit more muscle fibres in the belly (to produce greatest cross-sectional area). This is most pronounced in multipennate muscles. They tend to be packed with muscle fibres to enable the strongest contractions.

Strength is proportional to the cross-sectional area of the muscle.

Assessment of muscle function

Muscle function may be tested using active range or resisted contraction. A movement at a joint is active when

it is directly due to contraction of its associated muscles. Active movements may also be assisted (active assistance) or resisted (active resistance) by an external

agent. In clinical assessment of muscle function, the active range of movement (associated with muscle contraction) is compared to the passive range (allowed by joint mobility), to determine which structures may limit movement (or produce pain).

Muscle strength is gauged by the degree of active resistance required to prevent movement.

MUSCLE CONTRACTION AND ACTIONS

Types of muscle contractions

Usually, when muscle fibres contract the muscle as a whole shortens (a concentric contraction). This may be an isotonic (G. ‘equal stretch’) contraction where the muscle

as a whole shortens (while contracting) to move a load. In an eccentric or paradoxical contraction the muscle as a

whole lengthens (due to further stretching of its elastic elements, despite the contractile mechanism operating) while resisting the load. This occurs when slowly lowering a load against gravity.

In an isometric (‘equal length’) contraction, the muscle

as a whole maintains the same length (while the muscle fibres contract) without movement (due to simultaneous stretching of the elastic elements). This occurs when attempting to lift a load that cannot be overcome.

Fig.6.36 Isometric and eccentric contractions


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Length/tension relationship

At rest, muscles are generally in a state of mild stretch. Stretching a muscle (to a degree) before contraction

produces a stronger contraction (generating more tension).

Fig.6.37 Muscles crossing 2 joints can generate extra force

This is due to maximal overlap of the contractile units (myofilaments). However, stretching beyond this point

prior to contraction leads to a weaker contraction (tension is reduced, as the myofilaments are too far apart).

Muscles crossing more than one joint can generate extra force but are also prone to overstretch.

Flexor and extensor musculature

Although skeletal muscles produce individual movements they usually do so as part of a group. Although they may also be involved in other pairs of movements, skeletal muscles are considered as being either flexor musculature or extensor musculature. The major pair of

movements (even at biaxial or multi-axial joints) is flexion and extension.

Fig.6.38 Flexor and extensor compartments

Flexor and extensor muscles tend to be located on their respective (flexor or extensor) aspect of a joint, separated by a fascial intermuscular septum, on each side. Not only do flexors and extensors occupy separate compartments but they also have a separate nerve supply.

Prime movers and antagonists

A muscle acts as a prime mover or agonist (G. ‘contest’) when its contraction produces a particular movement.

In order to contract effectively a prime mover needs to contract from a position with an optimal degree of stretch.

Movement results when the antagonist muscles relax

and lengthen. The degree of stretch of antagonist muscles is often the

major factor limiting range of motion for a particular movement.

Active insufficiency

Even a normal muscle will produce a weak contraction if there is insufficient overlap of its myofilaments. This is termed active insufficiency.

Profound weakening of contraction occurs when it is attempted from an excessively shortened position of the prime mover.

Passive insufficiency

The prime mover action is restricted when an antagonist is unable to relax or to stretch sufficiently from lack of flexibility. This is termed passive insufficiency.

Fig.6.39 Types of muscle action

Fixators as dynamic ligaments

Certain muscles are designed to provide stability at a joint. A fixator muscle is located close to the axis of

movement at a joint. It is characteristically a short muscle that stabilises by acting as a dynamic ligament. Muscles

(and tendons) surrounding the joint give it support. Those that blend with the capsule around the most mobile joints (shoulder and hip) form a cuff. This arrangement prevents distraction of the ball from its socket (without impeding mobility).


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Fig.6.40 Fixator muscles positioned close to a joint

Fixator muscles tend to be the most important stabilising factor and the first line of defence (against dislocating forces). Their tone can be controlled automatically by stretch reflexes (when part of a capsule is on stretch, the overlying muscles contract more strongly).

Synergists as balancers

Excessive shortening of a prime mover may occur if the muscle crosses more than one joint because it tends to exert an unwanted action on the proximal joint. This undermines the desired effect at the distal joint. A prime mover crossing more than one joint enlists the support of synergists (G. ‘with + work’) that oppose the movement at

the proximal joint(s). Synergists augment contraction by keeping the prime mover on stretch.

Fig.6.41 Prime mover enlisting a synergist

Opposing flexion at the wrist joint (which is crossed by the long flexor tendons to the fingers) enables the wrist extensors to act as synergists for finger flexion. Biceps and triceps are prime movers for flexion and extension at the elbow joint, respectively. In addition, the long head of biceps and of triceps cross the shoulder joint.

The long head of triceps acts as a synergist for elbow flexion by opposing the flexor action of the long head of biceps (at the shoulder joint). Their roles are reversed for elbow extension.

Prime movers tend to be located superficially and fixators deep.

However, the same muscle may act as prime mover, antagonist, fixator or synergist for different movements.

NEUROVASCULAR SUPPLY & MYOTOMES

Neurovascular hilum and motor point

A skeletal muscle receives its nerve supply via muscular branches from a peripheral nerve. A nerve

typically enters a muscle accompanied by vessels. This site, the neurovascular hilum (L. ‘slit’) is where the

muscle moves least in relation to the major artery of the limb. This is often near the middle of the muscle belly and on its deep surface. Clinically the neurovascular hilum may be identified as the ‘motor point’ (where electrical

stimulation of the nerve most easily elicits contraction of the muscle).

Fig.6.42 Neurovascular hilum in a muscle

Motor unit

A muscular branch of a peripheral nerve contains both somatic motor and sensory nerve fibres. Each skeletal muscle fibre receives an independent supply from a branch of a motor nerve fibre.

Fig.6.43 A motor unit

A motor unit (within a particular muscle) is the total

number of muscle fibres innervated by a single motor nerve fibre (from a peripheral nerve).

A motor unit is the functional neuromuscular unit. Gross movements are possible using a few large motor units (each with many muscle fibres). More precise movements require many small motor units (each with fewer muscle fibres).


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Sensory nerve fibres to muscles

Almost half of the nerve fibres to a skeletal muscle are sensory. Proprioceptive (L. ‘one’s own receiver’; i.e. from

internal rather than external receptors) fibres arise primarily from stretch receptors and are particularly important in the (unconscious) control of posture.

Stretching a muscle (or its tendon) beyond a threshold stimulates a reflex contraction of the associated muscle fibres (via a circuit involving sensory nerve fibres, the spinal cord and motor nerve fibres).

Muscles also receive a supply of pain fibres.

Skeletal muscle tone and its assessment

Skeletal muscle tone (G. ‘tension’) is measured as

resistance to stretch. Muscle tone is under reflex control. It is dependent on a nerve supply (both motor and sensory) and is modulated by the recruitment of more or fewer motor units.

Skeletal muscle tone may be either increased or decreased by certain lesions of the nervous system. Assessment of skeletal muscle tone involves resistance to stretch of a major muscle group ideally through its full range of movement (with increasing velocity). This is an important step in a neurological examination.

Muscle hypertrophy and atrophy

Muscle is a very highly specialised tissue. Even though mature muscle cells have lost the capacity to replicate they respond to changes in demand. Muscle fibres undergo progressive enlargement, termed hypertrophy (G. ‘over-nourishment’) with increased demand. Muscle fibres progressively waste away with inactivity (‘disuse atrophy’)

and particularly after loss of their motor nerve supply (‘denervation atrophy’).

Being structural changes, muscle hypertrophy and atrophy are not evident immediately but only after a variable period of time. Assessment of skeletal muscle wasting involves comparing both sides of the body and, where possible, measurement of circumference. This is also an important step in a neurological examination.

Reciprocal innervation

Skeletal muscles with a common action often share a common nerve supply (as well as occupying a common compartment).

Muscular branches (containing both sensory and motor nerve fibres) from particular peripheral nerves supply flexor muscles. Extensor muscles receive their branches from different peripheral nerves to those supplying the flexor muscles.

Contraction of a prime mover (whether flexor or extensor) is associated with relaxation and stretch of the antagonist group. Stimulating motor nerves to a prime mover is therefore associated with a subsequent stimulation of proprioceptive fibres from the antagonist. This reciprocal innervation enables coordination between prime movers and antagonists during normal movements.

Dual nerve supply

A muscle located on the border between two compartments may receive a dual nerve supply (and have dual prime mover actions).

Adductor magnus is located in the medial compartment of the thigh but it also forms the floor of the posterior compartment.

It is made up of two parts: an adductor part, supplied by the nerve of the medial compartment (obturator nerve) and a hamstring part, supplied by the nerve of the posterior compartment (sciatic nerve).

Fig.6.44 Dual nerve supply of muscle on a border

Nerve supply to muscles

Muscles that migrate during development retain their original nerve supply.

The nerve supply to a muscle reflects its developmental origin (nerves remain ‘faithful’ to their muscles).

The two parts of adductor magnus developed as separate muscles becoming incorporated into one. Each part retains its original nerve supply. The diaphragm developed as separate parts (with separate sensory nerve supplies) initially far from each other. The central part of the diaphragm commenced development in the neck. It then migrated inferiorly retaining its original nerve supply, the phrenic nerves (derived from cervical spinal cord segments) which explains their long course.

Myotomes

A myotome (G. ‘muscle + cut’) is the mass of muscle

supplied by a particular spinal cord segment.

The segmental pattern of nerve supply in the trunk is in a simple cranial to caudal sequence.

Each intercostal nerve (the continuation of a single thoracic spinal nerve) supplies the muscles in its corresponding intercostal space. The peripheral pattern of motor distribution therefore matches the segmental pattern.

Two consecutive spinal segments

In the limbs (where there are 50 muscles in each), the arrangement of myotomes (involving primarily 5 segments) is much less apparent than in the trunk. A major peripheral nerve (having emerged from a plexus) therefore contains motor fibres derived from a number of spinal cord segments. As a result, the peripheral pattern obscures the segmental pattern of distribution.

An individual limb muscle typically receives its supply from two consecutive spinal cord segments.

These are generally distributed via a single peripheral nerve (although the nerve may contain fibres from additional segments, to supply other muscles).


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Muscles in the same group share a common action and tend to receive their nerve supply from the same spinal cord segments.

Flexor musculature for a particular limb joint usually receives two segments while extensor musculature receives the next two in series. Thus a total of 4 segments are associated with a joint (although a segment may be involved in more than one joint).

Cranial myotomes are proximal

Proximal flexor muscle groups are supplied from more cranial (pairs of) segments than those for distal flexor muscles.

The same applies for extensor muscle groups (although they receive the two segments more caudal than those for

the corresponding flexors at a particular joint).

The most caudal segment distributed via the limb plexus supplies the most distal muscle group for the upper limb and for the lower limb (intrinsic muscles of palm and of sole, respectively).

Fig.6.45 Upper limb myotomes and associated movements

Muscular branches of arteries

Skeletal muscles normally make up well over one third of body weight and are highly metabolic when exercising. They possess a rich blood supply with the capacity to dramatically increase it on demand. Skeletal muscles receive their (potentially) considerable blood supply via muscular branches from adjacent major arteries and are drained via tributaries of associated veins. Muscles are typically supplied by more than one artery via vascular ‘pedicles’ (stalks containing vessels and providing the

avenues of supply). However, a few muscles have only one major vascular pedicle. These are typically muscles with fleshy bellies, which may possess a long tendon (e.g. gastrocnemius), or a tendon at each end (e.g. biceps brachii). Segmental muscles may have two pedicles of similar size (e.g. rectus abdominis) or multiple pedicles of similar size (e.g. external oblique) arising from separate arteries. Other muscles have a dominant pedicle at one or other end and several small accessory vessels along the belly (e.g. rectus femoris) or a dominant pedicle to the belly and several small accessory vessels peripherally.

Where there is a major source artery (and principal vein) it enters as part of the neurovascular bundle at the hilum, on the deep surface of the muscle.

Vascular territories and networks

Many blood vessels and anastomoses (links between blood vessels) are found throughout muscles (whereas tendons have a poor supply).

The majority of anastomoses in the body are via skeletal muscles.

Anastomoses provide potential alternative pathways

(collateral circulations) if a major artery is occluded. Within muscles there is typically a continuous network of vessels rather than end arteries. However, at the boundaries of the vascular territories arteries are usually linked by anastomoses via reduced calibre (‘choke’) arteries. Veins also link territorial boundaries within muscles. However, unlike arteries they are not of reduced calibre. These veins (‘oscillating’ veins) do not have valves and permit flow in either direction.

The concept of vascular territories within muscles (together with the knowledge of the sites of vascular pedicles for particular muscles) is applied when planning grafts in reconstructive surgery.

Muscle injuries and healing With a muscle injury there may be considerable

bleeding due to the rich blood supply of muscle fibres (although tendon has a poor blood supply).

Subsequent healing tends to be problematic as completely torn muscle fibres do not regenerate, but heal with fibrous scar tissue. In addition, inactive muscle wastes away and extensive bruising may even calcify. The degree of fibrous (scar) tissue is minimised by appropriate first aid management (to reduce bleeding) followed by rehabilitation (including graded stretching and exercise). Scar tissue is inherently weaker than normal muscle and shortens the muscle, predisposing it to future injury.

Chapter 7: Integumental System and Skin

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INTEGUMENTAL SYSTEM

SKIN STRUCTURE AND TENSION LINES

SKIN APPENDAGES AND SPECIALISATIONS

SUBCUTANEOUS TISSUE AND FAT

CUTANEOUS NERVES AND OVERLAP

NEUROSOMES AND REFERRED PAIN

ANGIOSOMES AND SKIN BLOOD SUPPLY

LYMPHOTOMES AND WATERSHED AREAS

INTEGUMENTAL SYSTEM

The integumental (L. 'covering') system consists of skin and skin appendages (including hair and nails),

subcutaneous tissue and the breasts.

Fig.7.1 Modules of the integumental system

SKIN STRUCTURE AND TENSION LINES

Skin covers the body and is its largest organ.

Roles of skin

The major roles of skin are protection, sensation and thermoregulation. Skin provides a mechanical barrier as

well as protection from microbe invasion and fluid loss. Skin may also be regarded as a sense organ, due to its contact with the external environment coupled with a rich nerve supply.

Body temperature is controlled primarily by regulating

blood flow to skin and by sweating. An additional role is in vitamin D synthesis (required for normal bone formation)

via absorption of ultraviolet rays, from exposure of the skin to sunlight.

Fig.7.2 Skin functions

Epidermis

Skin is made up of two components. A connective tissue layer, the dermis (G. ‘skin’) is covered by an epithelium (G. ‘upon + nipple’, i.e. a surface lining) termed

the epidermis. Epidermis is primarily a stratified squamous (L.

‘scale’) epithelium. It contains many layers of cells that become progressively flatter towards the exterior. The outermost horny layer (stratum corneum) is dead and

forms a protective layer composed mainly of the protein keratin, preventing fluid loss and acting as an additional barrier. The innermost basal layer (stratum basalis) is a

single layer of cells resting on the dermis. Some of these cells, the melanocytes, produce melanin (G. ‘black’)

pigment.

Fig.7.3 Epidermis and its major layers

Dermis

Fig.7.4 Dermis and its contents

BODY SYSTEMS AND ORGAN STRUCTURES

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The dermis contains collagen fibres and elastic fibres. Loss of elasticity with aging results in wrinkles. Damage to collagen fibres (e.g. in skin of anterior abdominal wall from pregnancy) may result in stretch marks (termed ‘striae’).

Folds termed dermal papillae (L. ‘nipples’) project

upward under the epidermis, increasing the surface area for attachment and for diffusion between the vascular dermis and the avascular epidermis.

The dermis may be subdivided into two merging layers. The papillary layer adheres to the epidermis, while the reticular layer, with thicker elastic fibres and bundles of

collagen, adheres to the subcutaneous tissue.

Fig.7.5 Relative thicknesses of dermis

The dermis on extensor surfaces tends to be thicker and tougher increasing protection from injury.

The dermis on flexor surfaces is more adapted for

discriminative sensation.

Skin pigmentation

A readily visible characteristic of skin is pigmentation.

The melanocytes in the basal layer of the epidermis produce melanin pigment and transfer it to overlying cells protecting against ultraviolet damage to their nuclei. Skin pigmentation decreases the risk of sunburn and skin cancer.

Intrinsic pigmentation is determined by genetic factors. An albino (L. ‘white’) has a genetic lack of melanin pigment

in the skin, hair and eyes. Environmental pigmentation, produced by exposure to ultraviolet light, is reversible. Environmental pigmentation may also be due to hormones. In pregnancy there tends to be increased skin

pigmentation, particularly of nipples and areolae.

Relaxed skin tension lines

General skin characteristics also include mobility, elasticity and tension. These are primarily determined by

the attachments, composition and alignment of dermal connective tissue fibres. They vary in different areas and with aging.

Direction of skin incisions and scarring

Incisions made parallel to lines of tension heal with a fine scar, while those at right angles to lines of tension tend to produce a wide scar.

The relaxed skin tension lines (of Kraissl) are different to the skin cleavage lines (of Langer). The latter are the

lines along which dead skin tends to split in cadavers with a sharp spike.

Fig.7.6 Relaxed skin tension lines on the back

Connective tissue in living skin is oriented along the relaxed skin tension lines.

Body surface area

Skin (including its specialisations) covers the entire external surface of the body. The surface area of an average adult male is approximately two square meters.

Fluid loss in burns and ‘rule of nines’

In burns, fluid loss is proportional to the surface area affected.

This is calculated to determine the amount of fluid replacement required.

According to the ‘rule of 9’s’: trunk = 4x9% lower limbs = 4x9%, upper limbs = 2x9%, head & neck = 1x9% Total = 99% (+ genitals the remaining 1%).

Fig.7.7 'Rule of nines’ for adult body surface area

7. Integumental System and Skin

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SKIN APPENDAGES AND SPECIALISATIONS

Skin consists of more than connective tissue, vessels and nerves. Additional skin structures are termed skin appendages.

Pilosebaceous units

Pilosebaceous (L: ‘hair’ + ‘grease’) units are made up of hairs and hair follicles together with their associated sebaceous glands and muscles. Hair follicles (L. ‘small bags’) are located in the dermis. Hairs project from hair follicles through the epidermis to the exterior.

Fig.7.8 A pilosebaceous unit

Hairs are modified in various locations. These include hairs of the scalp (capilli), eyebrows, eyelashes, nostril hairs (vibrissae), axillary and pubic hairs. Hairs are absent

from thick skin (palms and soles), lips (and labia minora) and glans penis (and clitoris).

Sebaceous glands are also located in the dermis. A duct from each opens into an associated hair follicle. Sebum is the oily secretion that helps make skin waterproof. Arrector pili (L. ‘hair’) muscles are small

bundles of smooth muscle in the dermis attaching to hair follicles and when stimulated, make hairs ‘stand on end’. Contraction of smooth muscle associated with the nipple can make it become erect.

Fig.7.9 Pilosebaceous units around eye and nipple

Sudoriferous and odoriferous glands

Sudoriferous (L. ‘sweat’) glands are located in the

dermis. Their ducts pass through the epidermis to open on its external surface. The bases of some sweat glands extend through the dermis into the underlying subcutaneous tissue.

Fig.7.10 Site of sweat gland and associated duct

Sweat glands are particularly abundant in the palms and soles. They are absent from nail beds, lips, nipples and eardrums. Odoriferous glands (modified sweat glands) are

located in the skin of the armpits, genitals and around the anus. Ceruminous (L. ‘wax’) glands are present in the external auditory meatus. Mammary (L. ‘breast’) glands

are also modified sweat glands.

Regeneration of skin after burns

Skin may regenerate from its appendages provided at least some fragments remain.

In severe burns involving the dermis regeneration may occur from the bases of deeply located sweat glands.

Nails and nail beds

Fig.7.11 Section through distal phalanx of a finger

Nails are primarily composed of keratin and are derived

from an outer layer (the stratum lucidum) of the epidermis. A nail is made up of a nail root (deep to the proximal fold of skin), lunule and nail plate (between the lateral fold of

skin on each side) projecting towards the distal margin of the digit. A nail bed lies under the nail plate and is derived

from the epidermis (deep to the stratum lucidum). The thickened part of the nail bed deep to the nail root and lunule is termed the matrix.

Effect of nail bed damage

Damage to the matrix may result in permanently deformed nails.

The underlying subungual (L. ‘under’ + ‘nail’) dermis is

thick and vascular with numerous fibrous attachments directly to the periosteum of the distal phalanx. There is no intervening subcutaneous layer.

Subungual haematoma

Even a small degree of swelling from a bruise under a nail (subungual haematoma) causes considerable pain. Drainage (e.g. using a hot paper clip) suddenly releases the pressure bringing rapid relief.

Fig.7.12 Drainage of a subungual haematoma

Nail beds and associated dermis have migrated dorsally from tips of the digits. This is reflected in the neurovascular supply (from palmar/plantar digital nerves and vessels), which is retained.


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Fig.7.13 Ventral supply for nail bed

Thick skin

Skin covering all of the body, except for the palms and soles, is termed thin (hairy) skin. Thick (hairless) skin is

located on the palms and soles, where the epidermis is greatly thickened (particularly its outermost layer).

Thick skin is strongly bound down to underlying dense connective tissue (improving grip).

Fig.7.14 Sites of thick skin

Sweat glands are numerous but hair follicles (with sebaceous glands and arrector pili muscles) and pigmentation are absent. Paradoxically thick skin has a thinner dermis than thin skin.

Friction ridges

Thick skin has prominent surface ridges termed friction (or papillary) ridges. These epidermal ridges are

associated with the dermal papillae in the palms and soles. They help increase grip and also enhance sensation.

The pattern of friction ridges is permanent and unique.

Fingerprinting

Fingerprints, being unique, unchanging and accessible, can be used for identification of individuals.

Flexure lines and skin creases

Flexure lines occur where the skin is bound down over

joints to form prominent creases. These are found particularly over the joints of the fingers (and toes) and in the palm of the hand. In the face and neck, skin creases become permanent wrinkles when elasticity is lost due to aging. They tend to become aligned perpendicular to the direction of contraction of underlying muscles.

Cutaneous openings and special areas

Cutaneous openings are typically associated with interfaces between skin and mucous membrane.

These interfaces are termed mucocutaneous junctions and are located near openings to the respiratory

and digestive tracts (nares and oral fissure) and to the terminations of the urogenital and digestive tracts (urethral, vaginal and anal orifices).

Fig.7.15 Cutaneous openings on female perineum

Other openings involving skin are for the external auditory meatus (of the ear) and the palpebral fissure (of

the eyelids). Associated epidermal modifications form the epithelia of the eardrum (tympanic membrane), conjunctiva and cornea.

Fig.7.16 Cutaneous openings for mouth and eye

Specialised areas of skin occur on the breast (nipples and areolae) and on the genitals (penis and scrotum, clitoris and labia).

Skin surgery

Extra care is required with incisions and in surgical repair of wounds to areas with skin specialisations. This is to ensure accurate alignment, minimise tension and prevent disfigurement (most important on the face,

particularly with lips and eyelids). Wounds tend to be under greater tension where skin is tightly bound down (e.g. to cartilage of the ears and the nose). Incisions across flexure lines and hairlines (particularly eyebrows) should be avoided. Where possible, incisions (particularly in the face and neck) should be placed along skin creases. Specialised areas of skin and mucocutaneous junctions also tend to have a particularly rich blood supply and sensory nerve supply.

SUBCUTANEOUS TISSUE AND FAT

Subcutaneous fat

Subcutaneous tissue (also known as superficial fascia) is a loose connective tissue layer, of variable thickness. Subcutaneous tissue contains a variable amount of adipose (L. ‘fat’) tissue.


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Fig.7.17 Thick layer of subcutaneous tissue in thigh

Fat is deposited in preferential sites depending on genetics, age and gender (typically abdomen in males, buttocks and thighs in females). Fat is not present in the subcutaneous tissue of the eyelids, ear, scrotum, penis and clitoris.

Fig.7.18 Layers of skin and underlying tissue

Sites of subcutaneous septa

Fig.7.19 Compartments within subcutaneous tissue

Fibrous strands in subcutaneous tissue bind the overlying skin to the underlying dense connective tissue. These occur particularly in the palms, soles and scalp where they are thickened as septa.

Fibrous septa form boundaries of numerous small compartments within the subcutaneous tissue. In the breast prominent fibrous septa radiate from beneath the nipple, demarcating lobes of the mammary gland within the subcutaneous tissue. The upper septa are termed suspensory ligaments.

Sites of subcutaneous muscles

Muscles are found in the subcutaneous tissue of the face, neck, palm and scrotum. Muscles of facial expression are located in the subcutaneous tissue of the face (where there is no deep fascia). They insert directly into the skin. Platysma is a sheet of skeletal muscle that extends into the subcutaneous tissue of the neck. Palmaris brevis

corrugates the skin over the medial aspect of the palm. Dartos is a sheet of smooth muscle that wrinkles the

scrotum.

Superficial nerves and vessels

Cutaneous nerves and vessels are transmitted to the skin via subcutaneous tissue. Superficial veins (accompanied by lymphatics) and cutaneous nerves run for considerable distances in subcutaneous tissue. They tend to be located close to the underlying deep fascia.

Fig.7.20 Location of cutaneous nerves and vessels

Fixed skin is supplied by short (‘indirect’) arteries

running along the fibrous septa after passing through and supplying underlying muscle. In contrast, mobile skin tends to be supplied by longer (‘direct’) arteries that pass

between rather than through underlying muscles.

CUTANEOUS NERVES AND OVERLAP

Superficial somatic afferents

Even though the epidermis does not possess nerve fibres, the underlying dermis receives a particularly rich nerve supply via cutaneous (L. ‘skin’) branches of peripheral nerves. Skin is a superficial somatic structure.

Cutaneous sensory nerve fibres are superficial somatic afferents (in contrast to deep somatic afferents which supply bones, joints and muscles).

Fig.7.21 Major types of dermal receptors


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Cutaneous sensory nerve fibres arise from numerous receptors almost exclusively located in the dermis. These may be classified as mechanoreceptors (touch and pressure), nocioceptors (pain) and thermoreceptors (hot

and cold).

Vasomotor, sudomotor and pilomotor fibres

Even though there are no visceral organs (visible to the naked eye) in the skin there are microscopic collections of visceral tissue (smooth muscle and glands).

Fig.7.22 Types of motor fibres to skin

Visceral motor nerve fibres (sympathetics) are particularly important for thermoregulation, being distributed to the smooth muscle of dermal blood vessels (‘vasomotor’ fibres), sweat glands (‘sudomotor’ fibres) and even to the arrector pili muscles (‘pilomotor’ fibres).

Cutaneous nerve territories and overlap

The skin of the body may be mapped into territories (‘peripheral cutaneous neurosomes’) supplied by the cutaneous branches of peripheral nerves.

Fig.7.23 Peripheral cutaneous neurosomes

Territories supplied by peripheral nerves derived from consecutive spinal segments overlap extensively (and their branches intermingle).

Fig.7.24 Sensory overlap for different modalities

A lesion of a single peripheral nerve may produce only a small area of complete cutaneous sensory loss, or none at all.

Overlap for pain and temperature is more extensive than that for touch.

Internervous lines

An inter-nervous line is an imaginary line of non-

overlap between adjacent territories supplied by particular peripheral nerves. Inter-nervous lines on the skin are located where cutaneous nerve branches do not enter adjacent territories. The major inter-nervous line of the body is along the midline (the major line of fusion during development).

Nerve branches do not cross the midline of the body.

Fig.7.25 The major Internervous line of non-overlap

The cutaneous branches of a particular peripheral nerve do not cross the midsagittal plane to intermingle

with those from the opposite side of the body.

Fig.7.26 Internervous line through a body segment

Area of anaesthesia in a nerve block

Fig.7.27 More than one nerve may need to be blocked

The area of skin anaesthetised by injection of local anaesthetic around a peripheral nerve corresponds to the cutaneous sensory distribution of the nerve (distal to the site of infiltration) minus the area of overlap from adjacent nerves.


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Therefore, more than one peripheral nerve may need to be blocked to ensure an adequate area of anaesthesia. For example, intercostal nerve blocks should also include the nerve above as well as the nerve below the targeted nerve involved.

NEUROSOMES AND REFERRED PAIN

A neurosome (L. ‘nerve + body’) is the total sensory

territory (a 3-dimensional block of tissue) supplied by a particular peripheral nerve (peripheral neurosome) or spinal cord segment (segmental neurosome).

Dermatomes and overlap

A dermatome (G: ‘skin + cut’) is the area of skin

supplied by a particular spinal cord segment. It is better termed a segmental cutaneous neurosome.

Fig.7.28 Segmental cutaneous neurosomes

Adjacent dermatomes that are consecutive overlap extensively.

Damage to a single spinal cord segment (or posterior nerve root) may produce only a small (or even no) zone of complete cutaneous sensory loss.

As for peripheral nerves, overlap for pain and temperature is more extensive than that for touch across consecutive dermatomes that are adjacent to each other.

Fig.7.29 Consecutive dermatomes overlap extensively

Pre-axial and post-axial borders

In the embryo a limb bud develops with a pre-axial border (along the radius/thumb of the upper limb and the tibia/big toe of the lower limb) and a post-axial border

(along the ulna/little finger of the upper limb and fibula/little toe of the lower limb).

The pre-axial border of the upper limb is located laterally (and post-axial border medially) in the anatomical position, with flexor compartments anteriorly and extensor compartments posteriorly. The pre-axial border of the lower limb is located medially (and the post-axial border laterally)

in the anatomical position, with flexor compartments posteriorly and extensor compartments anteriorly.

Fig.7.30 Spinal cord segment distribution

Cranial spinal cord segments are distributed progressively along the skin of the pre-axial border of a limb (from proximal to distal) while caudal spinal cord segments progressively supply the post-axial border (from distal to proximal).

Middle segment to distal skin

The middle segment of a limb plexus is distributed to the most distal skin.

Fig.7.31 Distribution of middle segment of brachial plexus

Fig.7.32 Distribution of middle segment of sacral plexus

In the upper limb C7 is the middle segment of the brachial plexus (C5, 6, 7, 8 and T1) and is distributed to the skin of the hand including both palmar and dorsal aspects of the middle finger. In the lower limb S1 is the middle segment of the sacral plexus (L4, 5 and S1, 2, 3) and is distributed to the skin of the foot including both plantar and dorsal aspects of the middle toe.


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Fig.7.33 Incorporation of middle dermatome in limb buds

Anterior and posterior axial lines

Axial lines are imaginary lines located where non-consecutive dermatomes are adjacent to each other.

Adjacent dermatomes that are not consecutive do not overlap.

Fig.7.34 Development of an axial line

Cutaneous nerve branches do not cross axial lines.

Fig.7.35 Axial lines in the anatomical position

Axial lines therefore correspond to inter-nervous lines (of non-overlap). They represent buried areas of skin during development of the limb buds. Each limb has an anterior and a posterior axial line (midway between the pre- and post-axial borders) continuing proximally onto the trunk. However, development of the lower limb is complicated by medial rotation with the anterior axial line spiralling medially around to the posterior aspect when viewed in the anatomical position.

Since the lower limb buds rotate medially during development, their dermatomes also spiral in the same direction. The anterior axial line of the lower limb and the arrangement of dermatomes in an adult untwist when the lower limbs are shifted from the anatomical position (to the 'welcoming position') by abduction and lateral rotation.

The upper limbs are simply abducted to adopt this position.

Fig 7.36 Anterior axial lines in the welcoming position

The pre-axial border of each limb is then located cranially and the corresponding post-axial border, caudally (like the original limb buds).

Assessing skin sensory loss

Clinical testing for diminished cutaneous sensation (due to a specific lesion involving either a spinal cord segment or a peripheral nerve) is best performed across axial lines. It is recommended to commence from an area of normal sensation and proceed across the axial line to the suspected area of sensory loss.

Differing dermatome maps

Dermatome maps are not only artificial constructs but

they show sharp lines of demarcation between each strip (which should be blurred where there is overlap).

They also depend on results from testing living people and may be mapped differently with different modalities (e.g. overlap for pain is greater than that for touch). Two types of maps have been constructed.

Fig.7.37 Map based on sensory loss (Foerster)

One type of map (according to Foerster) is based on the area of sensation remaining after nerve roots from segments above and below a single segment were severed. These have both anterior and posterior axial lines


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(of non-overlap) along the limbs where dermatomes from non-consecutive spinal cord segments lie adjacent to each other. This map is preferred for assessing sensory loss.

Fig.7.38 Map based on pain radiation (Keegan & Garrett)

Another type of map (according to Keegan and Garrett) is based on loss of pain sensation due to compression of a particular nerve root. These have continuous strips radiating along a limb and no posterior axial line. This map is preferred for identifying spinal segments involved in pain referral.

Shingles dermatomal distribution

The skin of the body can be mapped into territories (dermatomes) of cutaneous supply derived from each spinal cord segment (via its associated spinal nerve).

Herpes zoster (G. ‘creep’ + ‘girdle’) also known as ‘shingles’ is due to reactivation of Varicella zoster

(‘chicken pox’) virus in the sensory ganglion of a spinal or cranial nerve. This condition is characterised by vesicles

and pain along the cutaneous distribution of the affected nerve.

The pain may precede the rash and can be severe. The rash does not extend across the mid-sagittal plane because branches of cutaneous nerves do not cross the midline of the body (being the major line of fusion). For example, the rash for shingles affecting a thoracic spinal nerve appears as a band around left or right half of the trunk, mapping the territory supplied by it. This, in turn, corresponds to the associated dermatome (including the area of overlap with adjacent dermatomes).

Fig.7.39 Rash in shingles of a thoracic spinal nerve

Distribution of referred pain

Referred pain is pain that is experienced at a site

different from its source. Pain is typically mapped on the body surface (which has a topographical representation on the cerebral cortex). There is no map drawn so far, for the interior of the body (including its contained viscera).

Pain from skin (superficial somatic pain) is sharp and particularly well localised (providing accurate information regarding the surface of the body). Pain from deep structures is both of a different quality and location.

Fig.7.40 Sites of referred pain from heart

Deep somatic pain and visceral pain are dull and ill-defined (especially visceral pain which is particularly poorly localised). Deep pain is also often accompanied by referred pain. The site of referred pain tends to be in a pattern that has an anatomical basis. Thus, there can be two sets of pain. The first is the actual deep (somatic or visceral) pain from the source, experienced deeply (but often poorly localised). The second is the associated pain that is (deceptively) referred elsewhere (referred pain).

Referred pain may mask pain directly from the source, making diagnosis difficult. However, awareness of the anatomical basis of referred pain can overcome this.

Pain from a deep source is referred to the same neurosome.

This can be territory supplied by the same spinal cord segment (segmental neurosome) or the same peripheral nerve (peripheral neurosome). An example of the former is pain referred to the umbilicus from an inflamed appendix. An example of the latter is pain referred to the ear from an impacted wisdom tooth.

Fig.7.41 Structures sharing spinal cord segments


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Afferents from a 3 dimensional block of somatic structures (both superficial and deep) converge on the same part of a spinal cord segment as those arising from certain deep organs (particularly viscera). This common area of the spinal cord can be excited by impulses along neighbouring neurons conveying pain from these organs. The brain seems to interpret impulses arriving at a particular spinal cord segment as originating from a source mapped on the body surface even though experienced deep to it. The deep source may be a viscus (visceral referred pain) or a somatic structure (somatic referred

pain).

Migration and referred pain

Although referred pain generally overlies the associated deep structure, it is often experienced at a site distant from the source. This phenomenon occurs when viscera have migrated (with their nerves) during development or when somatic structures have migrated (with their nerves), during development.

Fig.7.42 Migration and referred pain to neurosomes

The three locations of referred pain relative to its (deep) source are:

- the neurosome overlies a deep organ supplied by the same spinal cord segment (e.g. joint capsule) - the neurosome remains but the deep organ has migrated (e.g. appendix, diaphragm) - the deep organ remains but the neurosome has migrated (e.g. limb buds).

Referred pain to midline

Viscera have different patterns of pain referral depending on whether they are unpaired or paired.

Unpaired viscera receive a bilateral nerve supply.

This is retained even if they have migrated away from the midline.

Pain from an unpaired viscus is referred to the midline.

This occurs because impulses are received simultaneously in both left and right sides of the associated spinal cord segments. The stomach is an unpaired viscus that has migrated to the left during its development. Pain is referred from it to the midline (epigastric region of abdomen) because its sensory nerve fibres enter both sides of the spinal cord.

Fig.7.43 Unpaired organs and midline pain referral

The bladder is an unpaired viscus located in the midline. Pain is referred from it to the midline (suprapubic region of the abdomen) because it also receives a bilateral nerve supply.

Referred pain to same side

Paired viscera develop and are subsequently located

on both sides of the body. Only visceral nerves from the same side of the body supply a paired viscus.

Pain from a paired viscus is referred to the same side.

Fig.7.44 Paired organ and ipsilateral pain referral

The kidneys are paired viscera located on each side of the posterior abdominal wall. Pain from the left kidney is referred to its own side of the body (the left loin) while pain from the right kidney is referred to the right loin.

ANGIOSOMES AND SKIN BLOOD SUPPLY

The epidermis does not have blood vessels. Its cells receive their nutrition by diffusion from the underlying dermis.

Vessels, being derived from mesoderm, develop only within mesoderm-derived tissues.

The dermis (together with connective tissue in general) contains vessels because it is also derived from mesoderm. Epidermis is not derived from mesoderm and therefore cannot develop vessels.

Superficial and deep dermal plexuses

The dermis receives a rich blood supply at two levels. A superficial (papillary) plexus of arterioles (and venules)

located adjacent to the epidermis communicates with a deep (reticular) plexus located adjacent to the

subcutaneous tissue.


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Fig.7.45 Dermal plexuses

Communications (arteriovenous anastomoses) also

occur directly between arterioles and venules of the deep plexus, particularly in terminal areas exposed to cold (e.g. digits, ears and nose). Blood may be diverted from one set of vessels to another in thermoregulation.

Skin, being a continuous sheet, is supplied by sets of anastomoses (linking arterioles lumen to lumen). There are no end-arteries in skin

Continuous arteries supply continuous organs.

Effects of lacerating dermal vessels

Skin lacerations extending through the dermis bleed due to the rich blood supply. For this reason they also tend to heal rapidly (after the wound is closed).

Angiosomes

Cutaneous arterial supply may be mapped into territories that represent the surface of a block of tissue (which includes bone, muscle and skin) supplied by a primary source artery. An angiosome (L. ‘vessel’ + ‘body’)

is the 3-dimensional block of territory supplied by a particular (named) artery and associated vein. It consists of a matching arteriosome and a venosome. Arteries and

veins follow the connective tissue framework of the body, a continuous mesh between the outer dermis and inner skeleton (where it is calcified).

Fig.7.46 Angiosomes

Planning grafts based on angiosomes

The concept of an angiosome, coupled with knowledge of specific vascular territories, is vital when planning grafts in plastic and reconstructive surgery. A block of skin, with or without deeper structures, can be successfully transplanted to another site provided it is done so with the primary source artery and is within the boundaries of the associated angiosome.

Vascular planes

Arteries travel with connective tissue via fascial planes particularly associated with muscles.

Connective tissue is derived from the mesoderm surrounding specialised structures (bones, muscles, fat, vessels and fibrous nerve sheaths) that have developed from it.

Vessels run within the connective tissue mesh along mobile planes where muscles slide under deep fascia and where skin glides over deep fascia or bone.

Vessels do not cross mobile planes.

Fig.7.47 Arteries pass along mobile planes

Vessels cross planes at sites (of least mobility) where connective tissue is anchored.

This occurs particularly at the periphery of muscles, over intermuscular septa, under flexure lines (and skin creases) and where deep fascia attaches to bone.

Arteries course from fixed (concave) areas to mobile (convex) areas.

Direct and indirect arteries to skin

There are two types of arteries to the skin. Direct

cutaneous branches tend to course at the periphery of a muscle (piercing the deep fascia where it is anchored). Indirect musculocutaneous branches pierce muscles and

supply them before reaching the skin. Direct arteries are associated with mobile skin because

vessels do not cross mobile planes (otherwise they would be ruptured or would restrict mobility). Indirect arteries are associated with fixed skin (e.g. bound by septa overlying

the muscles). These tend to be shorter and more perpendicular than direct arteries. Branches of direct and indirect arteries communicate with each other within muscles and within skin.

Directional and oscillating veins

The venous drainage of skin is by two sets of veins communicating with each other:

- Directional veins converge (with valves directing

blood flow) towards the centre of a venosome. They are longer and are associated with direct arteries (and with mobile skin).

- Oscillating veins (without valves, hence flow in either

direction) are shorter and associated with indirect arteries (and with fixed skin). They occupy the territory between the directional veins (equilibrating flow and pressure).

Veins converge on fixed areas from mobile areas.


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Communications via choke vessels

Adjacent source arteries and their branches are linked together, forming a continuous network. Some meet as large calibre anastomoses between arteries. However, most adjacent arteriosomes communicate via small calibre anastomoses between arterioles, termed choke vessels.

Adjacent angiosomes meet at each connective tissue layer (skin, fat, muscle, bone and even fibrous nerve sheaths) via choke vessels and communications between oscillating veins. The boundary of an angiosome typically passes across a muscle.

The vast majority of muscles are part of more than one angiosome.

LYMPHOTOMES AND WATERSHED AREAS

Dermal lymph capillary networks

The epidermis does not contain vessels. The dermis contains blood vessels and lymph vessels, because unlike epidermis it is derived from mesoderm.

Lymph capillaries are not present in epithelia (including epidermis) but are abundant directly under an epithelial surface.

The dermis has an abundance of lymph capillaries and although most have a ‘blind’ origin they link freely to form extensive communicating networks.

Fig.7.48 Lymph capillary plexuses in the dermis

These networks are arranged in superficial and deep plexuses adjacent to the epidermis and the subcutaneous tissue, respectively (accompanying the associated superficial and deep dermal plexuses of blood vessels).

Subcutaneous afferent lymphatics

Lymph vessels tend to accompany veins.

Superficial veins run in the subcutaneous tissue accompanied by lymph vessels directed towards the major superficial lymph node groups. These groups (cervical, axillary and inguinal), located at the junction of the head with the neck and the trunk with the limbs, are readily palpable on clinical examination.

Lymph vessels passing to a lymph node are termed afferent lymphatics. Although very numerous in subcutaneous tissue they are normally not visible because they are thin-walled and contain colourless lymph.

Lymphangitis

Inflammation of lymphatics (lymphangitis) in the

subcutaneous tissue (e.g. due to infection) may cause red streaks along the overlying skin.

Lymph node groups draining skin

Ultimately lymph is returned to the venous system.

Lymph from the skin passes through at least one set of lymph nodes before reaching the venous system.

Fig.7.49 Lymph flows from superficial to deep

The skin of almost the entire body drains first to a superficial lymph node group before draining to a deep group.

Two areas of skin are peculiar in their lymph drainage. Skin on the front of the thorax and skin of the glans penis (and clitoris) drain directly to deeply located lymph nodes without first draining to a superficial lymph node. Lymphatics draining skin of the front of the thorax, including part of the breast, accompany tributaries of the internal thoracic vein (which perforate the intercostal spaces) to the deeply located parasternal nodes. Lymphatics draining the glans accompany tributaries of the deep external pudendal vein to the deep group of inguinal nodes (along the femoral vein).

Lymphotomes

The cutaneous lymph drainage may be mapped into territories that drain to the first group of lymph nodes encountered.

The area of skin that drains to a particular lymph node group is termed a lymphotome.

Fig.7.50 Lymphotomes


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Watershed areas of lymph drainage

Fig.7.51 A watershed

Extensive overlap of lymph drainage occurs across adjacent lymphotomes due to the presence of numerous communicating networks of lymph capillaries. These zones of overlap are termed watershed areas. A watershed area

of lymph drainage is of particular significance as lymph may drain in more than one direction from it.

Vertical and horizontal watersheds

Watershed areas are located at sites of extensive communicating networks of lymph capillaries. These correspond to zones of more than one direction of venous drainage. Cutaneous watershed areas may be classified into two groups: vertical (along developmental lines of fusion) and horizontal (between the most important

superficial lymph node groups).

Fig.7.52 Vertical watershed

The major vertical cutaneous watershed area is centred along the midline of the body where lymph drains to the corresponding lymph nodes on both sides of the body. There is also a vertical cutaneous watershed area centred on the nipple lines over the thorax. There are two major horizontal cutaneous watershed areas, one across the level of the clavicles (where lymph drains to both cervical and axillary lymph nodes) and the other across the level of the umbilicus (where lymph drains to both axillary and inguinal lymph nodes).

Fig.7.53 Horizontal watersheds

Lymph spread from watershed areas

Fig.7.54 Tumour spreads in more than one direction

Watershed areas of lymph drainage are clinically important in the spread of cancer or of infection. Tumour cells or microbes may be carried in different directions to more than one group of lymph nodes.

If the alternative routes are not anticipated, early detection/treatment is more likely to be incomplete and the venous system entered (with further dissemination). This is particularly important regarding the spread of cancer, including skin cancers and breast cancer.

Chapter 8: Visceral Systems and Viscera

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VISCERAL SYSTEMS

HOLLOW VISCERA

EXOCRINE GLANDS AND DUCTS

ENDOCRINE GLANDS

PAIRED AND UNPAIRED VISCERA

SEROUS MEMBRANE AND MESENTERIES

MUSCLE COATS AND SPHINCTERS

MUCOUS MEMBRANE AND JUNCTION ZONE

HILUM AND VASCULAR SEGMENTS

NEUROVASCULAR SUPPLY OF A VISCUS

VISCERAL SYSTEMS

Viscera (L. ‘sticky’) have a variety of structures and

functions. Collectively they are responsible for regulating the internal environment of the body. Viscera occupy cavities within the body framework and are involved with secretion, excretion, digestion and absorption.

Viscera are either hollow or solid. They are typically

organised into systems comprising a tract of hollow tubes with associated solid glands.

Respiratory system

Fig.8.1 Respiratory system

The respiratory system consists of the respiratory tract and the lungs. The tract is made up of the nasal cavity,

pharynx (nasal and oral parts), larynx, trachea and

bronchial tree. It is shared with the digestive tract where the pathways for air and for food intersect.

Digestive system

The digestive system consists of hollow tubes, the digestive (alimentary) tract, together with solid viscera

(the associated glands). The tract extends from the mouth to the anus. It is made up of the pharynx (oral and laryngeal parts), oesophagus, stomach, small intestine and large intestine. The associated glands are the (paired)

salivary glands and the unpaired pancreas. The digestive system also includes the biliary system, made up of the

liver, gall bladder and biliary tree.

Fig.8.2 Digestive system

Urinary and male genital systems

Fig.8.3 Urinary and male genital system

8. Visceral Systems and Viscera

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The urinary system consists of a pair of solid organs (the kidneys) together with the hollow urinary tract

(ureters, bladder and urethra). The male genital system also consists of solid internal

genital organs (testes, epididymes, seminal vesicles,

prostate and bulbourethral glands) together with a hollow tract (ductus deferens and ejaculatory ducts). The external genital organs are the penis and scrotum.

Part of the urinary tract (the urethra) is shared with the male genital tract. This combined system may also be regarded as the male urogenital system.

Fig.8.4 Endocrine and female genital systems

Endocrine and female genital systems

The endocrine system consists of discrete endocrine glands together with endocrine tissues in other organs.

The endocrine glands are the pituitary, pineal, thyroid, parathyroids and suprarenals. Clusters of endocrine tissue occur as the islets in the pancreas. The ovaries in the female (and testes in the male) also have an endocrine function.

The female genital system consists of internal genital organs, the paired ovaries and uterine tubes together with the unpaired uterus and vagina, and external genital organs (the clitoris and vulva).

HOLLOW VISCERA

A hollow viscus is typically tubular, characterised by a cylindrical wall surrounding a central channel, termed the lumen (L. ‘light’, as at the end of a tunnel). Some hollow viscera are saccular, being more spherical in shape. A duct (itself a tubular viscus) conveying secretions from an exocrine gland may pass through the wall of a hollow

viscus.

Structure of a hollow viscus

The wall of a hollow viscus consists of three principal layers from external to internal: the serosa (serous membrane), the muscularis (muscle wall) and the mucosa (mucous membrane).

Fig.8.5 A typical hollow viscus (small intestine)

Fig.8.6 Layers of the wall of a hollow viscus

A serosa typically covers all or part of the external surface of a hollow viscus (and may be continuous with a mesentery attaching to the body wall). It consists of a single layer of flat cells (mesothelium) covering vascular

connective tissue. A serous membrane minimises friction from movement

due to extrinsic changes in position (mobility), intrinsic propulsive contractions (motility) or expansion. The

muscularis consists of at least one muscle coat. Visceral smooth muscle can produce waves of contraction (termed peristalsis) to propel its contents. The mucosa consists of

epithelium covering vascular connective tissue. Mucous membranes may have numerous folds to increase their surface area for absorption (e.g. small intestine).

Sites of normal constrictions

The lumen of a tubular viscus may have a dilatation termed an ampulla (L. ‘flask’) or constrictions at particular

sites.

Normal constrictions of the lumen tend to occur at the beginning and end of a tubular viscus.

These are often associated with orifices, mucosal folds or thickenings of the muscle wall to control passage

through the lumen.


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Fig.8.7 Normal constrictions of urinary tract

The beginnings and ends of the ureters and the urethra have normal constrictions of the lumen. Normal constrictions may also occur where adjacent structures compress a tubular viscus at particular sites along its course. Such normal constrictions occur where the ureter crosses the pelvic brim and where the urethra (in the male) passes through the urogenital diaphragm.

Obstruction of a tubular viscus

Impairment of propulsion through a tubular viscus is termed visceral obstruction. This may occur directly by

mechanical factors or indirectly by interfering with its neurovascular supply (affecting wall function and/or vitality). Obstruction of a tubular viscus may be classified anatomically into three types (according to its relationship with the wall).

Extramural (external) obstruction is from outside

compression of a tubular viscus (e.g. by a tight hernial orifice, fibrous adhesions).

Intramural obstruction arises from within the wall of a

tubular viscus (e.g. by a mucosal tumour, spasm of smooth muscle, occlusion of arteries supplying the wall).

Intraluminal (internal) obstruction is from a blockage in

the lumen of a tubular viscus (e.g. by a foreign body).

Fig.8.8 Types of visceral obstruction

Obstruction of a tubular viscus causes impaired passage of luminal contents. This, in turn, tends to produce distension (proximal to the obstruction), pain

(due to stretching of the distended viscus) and altered peristalsis (to overcome the obstruction, initially). As an example, intestinal obstruction typically produces the triad of constipation (reduced passage of faeces and flatus), abdominal distension and pain. These symptoms may be accompanied by altered bowel sounds (from peristalsis),

detected on auscultation.

EXOCRINE GLANDS AND DUCT

A gland (L. ‘acorn’) is made up of clusters of secretory cells. Glands may be organised into an outer cortex (L. ‘shell’) and an inner medulla (L. ‘middle’).

Glands may also be subdivided into lobes, then lobules (which contain the secretory units). Although typically enveloped by a capsule, glands tend to be

surrounded by an additional covering.

Fig.8.9 Exocrine gland (kidney) and its duct (ureter)

Although the kidney excretes rather than secretes urine, it may be regarded as an exocrine gland, with the ureter its duct.

Serosal covering or fascial sheath

Fig.8.10 Serosa around organs in the peritoneal cavity

Some glands are enclosed by fascia that splits to form an investing sheath.


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Fig.8.11 Fascial sheath around a gland

These fascial sheaths occur particularly in the head and neck (e.g. around salivary glands and the thyroid gland) where they provide both support and protection.

Glands associated with a body cavity (e.g. liver, ovaries) are covered almost entirely, or at least in part, by a serosa, which reduces friction.

Shape, grooves and impressions

The shape of an organ (together with its borders and surfaces) may be determined by the structures adjacent to it. The left suprarenal gland is a crescent shape while the right is a triangular pyramid (wedged between the liver, inferior vena cava and right kidney).

Fig.8.12 Organ shaped by its direct relations

Structures directly related to an organ tend to produce grooves or impressions on it.

The aorta grooves the left lung and the azygos vein grooves the right lung. Although the lungs contain air, their external form is similar to a solid viscus. Their internal structure resembles an exocrine gland with air conveyed via duct-like bronchi rather than secretions filling ducts.

Ducts

Glands secreting into a duct are termed exocrine (L. ‘outside + secrete’).

Fig.8.13 Exocrine gland and duct

A duct (L. ‘lead’) is formed from a system of internal collecting channels and emerges from the hilum of an exocrine gland. The duct transmits secretions towards its orifice opening into the lumen of a hollow viscus (e.g. bile duct into duodenum) or onto an external surface (e.g. lacrimal ducts onto conjunctiva).

At the hilum of the kidney the ureter arises from the renal pelvis, formed by the union of its collecting channels (calyces).

Orifice of a duct

A duct opening into the lumen of a hollow viscus tends to narrow as it traverses the wall.

The narrowest part of a duct is its orifice (L. ‘opening’).

A calculus (stone) will most likely lodge at the orifice of a

duct. The narrowest part of the ureter is its orifice in the bladder (the most likely site for a ureteric calculus to lodge).

Fig.8.14 Narrowest part of duct at orifice

Types of duct obstruction

The major ducts from exocrine glands may also be regarded as tubular viscera. Obstruction of a duct (as with a tubular viscus) may be classified anatomically into three types. Extramural (external) obstruction is from outside compression of a duct (e.g. by a tumour in a neighbouring structure). Intramural obstruction arises from the wall of a duct (e.g. by a fibrous stricture following inflammation). Intraluminal (internal) obstruction is from a blockage within a duct (e.g. by a calculus).


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ENDOCRINE GLANDS

Glands secreting directly into the blood stream are termed endocrine (G. ‘inside + secrete’). Hormones (G.

‘rouse’) are chemicals produced in one part of the body that regulate cells in another part of the body. Certain hormones (e.g. growth hormone) have widespread effects on many tissues throughout the body.

Fig.8.15 Endocrine gland (suprarenal)

Ductless glandular organs

Endocrine glands do not have ducts (hence they may also be termed ductless glands). Their secretions

(hormones) are conveyed directly into the blood stream carrying them to their target organs.

Fig.8.16 Blood flow through an endocrine gland

Endocrine glands have a very rich blood supply.

Purely endocrine glands are the pituitary gland and pineal gland (in the head), the thyroid gland and parathyroid glands (in the neck) and the suprarenal

glands (in the abdomen). However, endocrine glandular tissue is not only clustered into discrete organs.

Sites of endocrine tissue

Some solid viscera (e.g. pancreas, kidneys and gonads) have both exocrine and endocrine roles. While primarily regarded as exocrine glands they contain

microscopic collections of endocrine tissue. The pancreatic islets are particularly important collections of

endocrine tissue which secrete both insulin and glucagon to regulate blood glucose. The testes and ovaries contain

cells that secrete sex hormones. These are responsible for secondary sexual characteristics. The thymus is a

lymphoid organ but also has an endocrine role.

PAIRED AND UNPAIRED VISCERA

Fig.8.17 Paired and unpaired viscera

Paired viscera and unilateral supply

The lungs (and bronchi) and most exocrine glands (lacrimal and salivary glands, kidneys, ovaries, testes and seminal vesicles) are paired viscera. Similarly, their respective ducts (including ureter, uterine tube and ductus deferens) are paired. The suprarenal and parathyroid glands are paired endocrine glands.

A paired viscus develops and is subsequently located on one side of the body. Its nerve and vascular supply lines are directed to that same side of the body.

A paired viscus receives a unilateral neurovascular supply and refers pain to the same side.

For example, right kidney pain is felt in the right loin.

Fig.8.18 Ipsilateral supply of paired kidneys


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Unpaired viscera

There are two types of unpaired viscera, midline and non-midline. This also has implications regarding their

neurovascular supply. Some hollow viscera (trachea, bladder, urethra, uterus and vagina) an exocrine gland (prostate) and most endocrine glands (pituitary, pineal and thyroid) are unpaired viscera located in the midline.

Midline viscera and bilateral supply

Midline unpaired viscera develop by fusion from each side of the body.

Fig.8.19 Bilateral blood supply to the uterus

Midline unpaired viscera receive nerve and vascular supply lines from both sides

These subsequently form a broad band of overlap across the wall of the viscus.

Non-midline unpaired viscera

The digestive system developed from the primitive gut, originally in the midline of the body. This includes the gastrointestinal and biliary tracts, with their associated glands (liver and pancreas) and ducts. These viscera subsequently migrate to one side of the body, or are located asymmetrically across the midline. However, the uppermost end of the digestive tract (mouth and pharynx) and lowermost end (anal canal) remain in the midline, as do their orifices.

Fig.8.20 Migration of gut away from midline

Unpaired vascular supply to gut

Non-midline unpaired viscera have an arterial supply from unpaired branches of the aorta (arteries of the foregut, midgut and hindgut) and venous drainage into an unpaired system of veins (the ‘portal’ system).

Fig.8.21 Unpaired arteries supplying the gut

Unpaired branches (coeliac, superior mesenteric and inferior mesenteric arteries) arising from the front of the aorta supply the stomach, small intestine and large intestine. The venous drainage is to the liver via the portal vein.

Bilateral nerve supply to gut

Unpaired viscera receive a bilateral nerve supply. This applies for all unpaired viscera (midline and non-

midline). Although the nerves to a non-midline unpaired viscus are derived from both sides of the body they converge to accompany the unpaired arteries (which originate from the front of the aorta).

Fig.8.22 Nerve supply of gut and pain referral


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Both sensory and motor nerve fibres overlap extensively across the wall of an unpaired viscus.

Pain from an unpaired viscus is felt over the midline of the body as impulses are simultaneously received by the left and by the right side of the spinal cord.

Pain from the small intestine is referred to the umbilical region; pain from the uterus is referred to the suprapubic region.

SEROUS MEMBRANES AND MESENTERIES

Serous membranes

A serous membrane typically covers most of, the

external surface of a viscus within a body cavity. The interior of a body cavity is lined by serous (L. ‘serum’) membrane forming a closed sac.

Fig.8.23 The mesentery and its contents

A serous membrane consists of a single continuous sheet of flat cells that secrete a small amount of fluid (into the enclosed potential space) minimising friction between structures. This mesothelium (G. ‘middle + nipple’, i.e. a

surface lining) is on a thin bed of vascular connective tissue.

Parietal and visceral serous membranes

Fig.8.24 Visceral and parietal layers of a serous membrane

The parietal (G. ‘wall’) ‘layer’ of a serous membrane

lines the interior of the body wall and receives its nerve and vascular supply via the body wall (i.e. by somatic nerves and parietal vessels).

The visceral ‘layer’ covers the viscera (as the serosa)

and receives the same nerve and vascular supply as the viscera (i.e. by visceral nerves and vessels).

During development, viscera (with their neurovascular supply lines) invaginate the serous sac of a body cavity.

Fig.8.25 Supply lines to serous membranes

Mesenteries

The parietal layer of a serous membrane is continuous with the visceral layer via connecting roots to the viscera. These are termed mesenteries (G. ‘middle intestine’ - an

intermediary structure). A mesentery consists of two sheets of serous

membrane with loose connective tissue (containing a variable amount of fat) between them. The sheets of serous membrane of the mesentery become continuous with the parietal layer of serous membrane at the parietal attachment of the mesentery (adjacent to the body wall).

Fig.8.26 Development of a mesentery

Roles of a mesentery

A mesentery has two major roles. A mesentery (particularly a long mesentery) provides an attachment enabling mobility. A mesentery also contains the supply lines. Vessels and nerves are transmitted in the connective

tissue between the two sheets of serous membrane forming the mesentery.


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Fig.8.27 Roles of a mesentery

Viscera suspended in body cavities

Viscera projecting into body cavities (pleural and peritoneal) are usually suspended by a mesentery.

Fig.8.28 An intraperitoneal viscus

The lungs are suspended in the pleural cavities (of the thorax) and (intraperitoneal) segments of the gastrointestinal tract in the peritoneal cavity (of the abdomen and pelvis). The ovaries and the uterus also have mesenteries as they are suspended in the peritoneal cavity (of the pelvis).

Fig.8.29 Intraperitoneal abdomino-pelvic viscera

Typically, a viscus has a single mesentery. However, some viscera have more than one (e.g. stomach and liver).

Posterior and subperitoneal viscera

The paired abdominal viscera (suprarenal glands, kidneys and ureters) are located on the posterior abdominal wall and are covered by the unpaired viscera (which project much further into the peritoneal cavity).

Fig.8.30 Posterior peritoneal viscera

Most unpaired pelvic viscera (bladder, prostate gland, vagina and rectum) are partly located below the level of the peritoneum, in the pelvic cavity, without a mesentery.

Fig.8.31 Subperitoneal viscera

Viscera associated with a body cavity, but without a mesentery, still tend to be partly covered by a serous membrane. This applies both to ‘posterior peritoneal’ and to ‘subperitoneal’ viscera.

Retroperitoneal viscera

Fig.8.32 Retroperitoneal abdomino-pelvic viscera

The gut tube (foregut, midgut and hindgut), forming the gastrointestinal tract and associated glands, commences development with a dorsal mesentery throughout its length.


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During subsequent development the dorsal mesentery fuses with the parietal peritoneum at particular sites (duodenum, ascending colon, descending colon and rectum) while it is retained at others (stomach, small intestine, transverse colon and pelvic colon). The former become retroperitoneal while the latter remain intraperitoneal.

Mobility and fixation trade-off

The gastrointestinal tract (GIT) has fixed segments

(which have fused their mesentery with the parietal peritoneum) alternating with free segments (which have

retained their mesentery).

Fig.8.33 Parts of gut alternately fixed and free

For a very long tube such as the gut there is a trade-off between mobility and fixation. The parts of the

gastrointestinal tract are alternately free (intraperitoneal) and fixed (retroperitoneal) to enable mobility while maintaining stability, respectively. In addition to altering their relative position (by mobility) those segments suspended by a mesentery have more capacity to distend (by expansion).

Torsion of a viscus

A viscus suspended on a mesentery (e.g. intestine) is

in potential danger of twisting (torsion). This may subsequently cut off its blood supply.

Fig.8.34 Torsion and compromise of blood supply

A loop of intestine may also become twisted by adhesions or by protrusion through a hernial orifice.

The testis, being suspended in the scrotum by a long vascular stalk, is particularly prone to torsion of its blood

vessels.

Fig.8.35 Torsion of the testis

MUSCLE COATS AND SPHINCTERS

Circular and longitudinal coats

The wall of a tubular viscus consists of smooth muscle coats (the muscularis) between serous membrane (the serosa), externally and mucous membrane (the mucosa), internally. Concentric rings of smooth muscle, organised into a circular coat, surround the mucosa in a tubular viscus. A complete or partial longitudinal coat (around the

circular coat) is also present in many tubular viscera. In the stomach there is an oblique muscle coat in addition to the

circular and longitudinal. This (incomplete) coat is located internal to the other two.

Fig.8.36 The stomach wall

Motility and expansion

Motility refers to the process by which luminal contents

are transmitted along tubular viscera (and ducts). This is


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due to the wave of visceral smooth muscle contraction, termed peristalsis.

Peristalsis typically involves alternate contractions of longitudinal and circular muscle coats (the former expanding the lumen ahead of a bolus, the latter constricting the lumen behind it).

Fig.8.37 Intrinsic movement of a viscus

Solids, liquids and gas are ingested at the proximal end of the gastrointestinal tract and propelled to its distal end, where the remnants are expelled.

Visceral smooth muscle fibres (unlike skeletal muscle fibres) may be stretched without increasing their force of contraction.

The property of stretch without increased force of contraction is termed plasticity and is particularly

important in organs that may expand to store large volumes (e.g. stomach, bladder and rectum). The bladder and rectum gradually accumulate urine and faeces, respectively. At a critical point of stretch (micturition and defecation) reflexes are elicited, with expulsion of the contents. The stomach expands to accommodate a meal then gradually releases its contents.

Voluntary and involuntary sphincters

A sphincter (G. ‘strangle’) is a localised muscular

thickening of, or around, the wall of a tubular viscus controlling passage of contents through its lumen.

A sphincter may be smooth muscle (an internal sphincter) of the visceral wall itself or skeletal muscle (an external sphincter) around the viscus. Visceral nerves supply the former, also termed ‘involuntary’ sphincters

(under reflex control), while somatic nerves supply the latter, also termed ‘voluntary’ sphincters (under voluntary

control).

Fig.8.38 Skeletal and smooth muscle sphincters

Fig.8.39 Voluntary and involuntary anal sphincters

Sites of sphincters

Sphincters are found at the distal end of viscera that act as a reservoir (e.g. bladder, stomach).

Fig.8.40 Sphincter at distal end of a reservoir

Sphincters may also be located at the distal end of a duct (e.g. bile duct and pancreatic duct).

Fig.8.41 Sphincters at distal end of ducts

Sphincters are often located near an external orifice (particularly on the perineum).


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Fig.8.42 Sphincters at external orifices on perineum

Both voluntary and involuntary sphincters are present near the external orifice of the anal canal, vagina and urethra as a dual safeguard against unwanted passage (from either direction).

Muscular functional sphincters

Passage can also be controlled without an ‘anatomical’ sphincter. A sphincteric mechanism without any localised muscular thickening is a ‘functional’ sphincter.

Sphincteric mechanisms can arise from muscle contraction around (extrinsic) or within (intrinsic) a viscus.

A functional sphincter may be created from extrinsic muscle contraction by the oblique path of a tube through a gap in a muscle (e.g. the termination of the ureter through the bladder wall). Contraction of the (extrinsic) muscle presses against the tube from two directions creating a shutter-valve without need for an anatomical sphincter.

Fig.8.43 Functional sphincter of ureter

A functional sphincter may be created by intrinsic muscle contraction by a localised increase of muscle tone in the wall of a viscus. The functional sphincter at the junction of the lower end of the oesophagus with the cardia of the stomach (cardiac sphincter) may be contrasted with the distinct muscular thickening at the distal end of the stomach (pyloric sphincter).

Mucosal functional sphincters

A fold of mucosa can create a functional sphincter. The ileocaecal valve has two folds of mucosa helping

maintain one-way passage from the small intestine to the large intestine. The spiral valve in the cystic duct is a raised fold of mucosa, helping control passage of bile to and from the gallbladder. This special mechanism can allow flow in both directions (but at different times).

Fig.8.44 Functional sphincter for gall bladder

Functional sphincters at orifices

Fig.8.45 Direction of orifices on perineum

A functional sphincter may also be created at an orifice.

The direction of the orifice is at right angles to the direction of apposition of the walls of the tubular viscus (or duct) immediately proximal to it.

This is particularly important for orifices opening onto the exterior of the body at the termination of the urethra, the vagina and the anal canal. The external urethral, vaginal and anal orifices are aligned in the mid-sagittal plane, while the walls of the urethra, vagina and anal canal are apposed antero-posteriorly. The anal canal is particularly well guarded. It has an involuntary sphincter (of smooth muscle) surrounded by three voluntary sphincters (of skeletal muscle) and an additional external sling of skeletal muscle (from levator ani). In addition, (although created by the anatomical sphincters) the orientation of the orifice (relative to that of the walls of the canal) may be regarded as a functional sphincter.

MUCOUS MEMBRANE AND JUNCTION ZONE

A mucous membrane (L. ‘slippery + thin skin’) forms

the inner component of the wall of a tubular viscus.


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Epithelium and lamina propria

The mucous membrane (mucosa) consists of epithelium covering a vascular connective tissue layer termed the lamina propria (L. ‘plate + special’).

Fig.8.46 The major layers of a mucous membrane

The epithelial lining of viscera is avascular (as is the epithelium of skin).

Mucus is secreted onto the epithelium of respiratory

and digestive tracts. This may be directly from surface cells or from microscopic exocrine glands deep to the epithelium.

The underlying connective tissue of the lamina propria is highly vascular (as is the dermal layer of skin).

In particular, there are numerous lymph capillaries strategically located directly under a surface lining where they contribute to the first line of defence (by draining invaders to a lymph node). Although most have a blind origin they link freely to form extensive communicating networks.

Mucosal folds and papillae

The internal features on a mucous membrane are usually created by the pattern of mucosal folds, projecting

into the lumen. Numerous small folds (e.g. circular folds of small intestine) increase surface area, particularly when coupled with microscopic projections (villi and microvilli)

for absorption. Large folds of mucosa may help create a functional sphincter (e.g. ileocecal valve, rectal valves). Mucosal folds within a duct (e.g. spiral fold of cystic duct) may also create a functional sphincter. Elevations, termed mucosal papillae (L. ‘nipples’) may have openings of ducts

on their summits. These ducts are derived from associated exocrine glands. The bile and pancreatic ducts enter the duodenum on a raised fold on the mucosa, termed the duodenal papilla.

Horizontal and vertical junctions

Junction zones are mucosal interfaces where supply

lines approach each other from different directions to meet or overlap.

Junction zones may be classified into two groups. At a horizontal junction, nerves and/or vessels approach each

other from above and from below. Horizontal junctions may be mucocutaneous (between skin and mucous membrane) or transmucosal (between different mucosal territories). At a vertical junction, nerves and/or vessels approach each

other from each side. A midline tubular viscus (e.g. bladder, uterus and vagina) may be regarded as a vertical junction, possessing a bilateral neurovascular supply overlapping across the width of the mucosa. This is in contrast to skin,

where no sensory nerve overlap occurs across the midline (creating an inter-nervous line of non-overlap).

Fig.8.47 Horizontal and vertical junctions of supply

Developmental interfaces

Vertical junction zones typically correspond to lines of fusion. Horizontal junction zones typically correspond with

tissues of differing embryological derivation (e.g. endoderm or ectoderm derived epithelium).

Fig.8.48 Major developmental interfaces of gut

Endoderm forms the epithelial lining of the digestive tract (derived from the primitive gut tube) as well as the respiratory tract and part of the lower urogenital tract (which develop as an outpouching associated with the foregut and hindgut, respectively).

During development, endoderm meets ectoderm at the oropharyngeal membrane and at the cloacal (L. ‘sewer’) membrane, which break down, forming openings at each end of the gut tube.

Additional interfaces occur between the pharyngeal pouches and the foregut, foregut and midgut, midgut and hindgut and between the hindgut and the cloaca. The upper end of the digestive and respiratory tracts (with epithelia derived from pharyngeal pouches) and the lower end of the digestive and urogenital tracts (with epithelia derived from the cloaca) form special zones where mucous membrane covers skeletal muscle.


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At these zones skeletal muscles (derived from pharyngeal arches and cloacal sphincter, respectively) are involved in important reflexes (particularly swallowing and coughing, micturition and defecation), which are coupled with voluntary control.

Fig.8.49 Derivation of the gut in an adult

Fig.8.50 Developmental interface at distal gut

The reflexes coordinated by motor nerves to particular muscles are elicited by stimulation of corresponding sensory nerves that supply the overlying mucosa.

Fig.8.51 Common segments of nerve supply

Mucocutaneous junctions

The most prominent junction zones occur at mucocutaneous junctions. These are located at the

openings to the respiratory and digestive tracts (nostrils and lips) and the terminations of the urogenital and digestive tracts (near urethral, vaginal and anal orifices).

There are three types of transitions that particularly

occur in relation to mucocutaneous junctions: - the epithelial lining (the transition between epithelium

of mucous membrane and epidermis) - the mucosal and cutaneous neurovascular territories

in the underlying connective tissue (at the transition between lamina propria and dermis)

- the type of underlying muscle (the transition between

smooth and skeletal muscle coats and/or sphincters). These transitions in epithelium, neurovascular territories

and muscle type are usually located at or near the same site.

Fig.8.52 Transition of lining and wall at a junction zone

Epithelial interfaces

Fig.8.53 Epithelial interface in anal canal

At mucocutaneous junctions, the epithelium changes from a moist, delicate lining of the mucous membrane into epidermis, characterised by an outermost dead and dry layer (stratum corneum). A line in the anal canal (the dentate line) is created by remnants of the anal membrane

and demarcates the change in epithelium. It represents the site of the endoderm/ectoderm interface.


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Fig.8.54 Mucosa of rectum and anal canal

Neural and vascular interfaces

Arterial anastomoses, venous communications, watershed areas of lymph drainage and inter-nervous lines (of sensory nerve supply) occur at mucocutaneous junctions.

These vascular, lymph and nervous interfaces are located in the connective tissue underlying the epithelium, at the transition between lamina propria and dermis.

The vascular interfaces are broad bands of overlap. The sensory nerve interface is typically a line of non-

overlap. The dentate line overlies the centre of a zone of arterial anastomosis (between branches of the unpaired superior rectal artery, above and paired inferior rectal arteries, below), venous communication (between tributaries accompanying the arteries) and lymphatic communication (between vessels draining to internal lymph nodes, above and bilaterally to external lymph nodes, below). Visceral afferent nerve fibres (pelvic parasympathetics) supply the mucosa above the dentate line. In contrast, below the dentate line, the skin of the anal canal is supplied bilaterally by superficial somatic (cutaneous) afferents (via the inferior rectal nerve).

Fig.8.55 Neural and vascular interfaces at a junction zone

Muscular interfaces

These mucocutaneous junctions usually overlie a

change of muscle tissue and particularly involve sphincters (e.g. an internal smooth muscle and an external skeletal

muscle sphincter are found around the anal canal; similar arrangements occur for the urethra and vagina).

Fig.8.56 Anal sphincters and their innervation

There is a corresponding change of underlying motor supply.

Visceral nerves supply smooth muscle sphincters, while somatic nerves supply skeletal muscle sphincters.

The internal anal sphincter is supplied by sympathetic nerves while the external anal sphincters are supplied by the inferior rectal nerve).

Transmucosal junctions

Junction zones also occur between different mucosal territories. Transmucosal junctions involve epithelial

and/or neurovascular interfaces. They may also be associated with muscular interfaces. A change in epithelium (from stratified squamous to columnar) occurs near the gastro-oesophageal junction.

Fig.8.57 Mucosal and muscular interfaces in oesophagus

Other transitions associated with the oesophagus in its lower half include arterial supply together with venous and lymphatic drainage (in the lamina propria).


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Fig.8.58 Vascular interface along oesophagus

The type of underlying muscle (from skeletal to smooth) coats is also in transition, together with the associated type of nerve fibres.

Fig.8.59 Vascular interfaces along gut

Transmucosal junctions tend to be located where territories of different developmental origin meet.

Transmucosal junctions correspond with neurovascular interfaces (associated with the respective foregut, midgut or hindgut artery and with the visceral afferent pain fibres that pass along them).

Internervous reflex lines

Sensory nerve interfaces at transmucosal junctions in long visceral tracts have major significance regarding reflexes and referred pain.

Internervous lines for reflexes particularly occur where mucosa overlies skeletal muscle.

Internervous lines for reflexes are located in the upper digestive and respiratory tracts at interfaces where specific areas of mucosa (and associated muscles) are each supplied by a designated cranial nerve.

Fig.8.60 Reflex territories along upper airway

At the upper end of the digestive tract, stimulating touch and taste receptors of oral mucosa elicits chewing and salivation reflexes (via cranial nerves V3 and VII, respectively), while stimulating touch receptors of pharyngeal mucosa elicits the swallowing reflex (via cranial nerve IX).

At the upper end of the respiratory tract, stimulating touch receptors of nasal mucosa elicits the sneeze reflex (via cranial nerve V2) and touch of laryngeal mucosa (e.g. by inadvertent entry of food or fluid) elicits the cough reflex (via cranial nerve X). Internervous lines for reflexes (e.g. defaecation and micturition) also occur at the lower end of the digestive and urinary tracts.

Internervous pain lines

Between the oesophagus and the rectum, visceral pain fibres accompany sympathetic nerves (bilaterally) to spinal cord segments T4-L1. Pain is referred to associated neurosomes in the midline, from the sternal (T4-6), epigastric (T7-9), umbilical (T10-11) and suprapubic regions (T12-L1), represented on the surface by the corresponding dermatomes.

Fig.8.61 Visceral pain lines

The ‘thoracic pain line’ projects through the

oesophagus to the sternal angle. Above the oesophagus (to the mucocutaneous junction at the lips) the mucosa is derived from pharyngeal pouches. General afferent fibres convey pain from it via cranial nerves.


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Fig.8.62 Segmental supply of viscera and overlying skin

The ‘pelvic pain line’ projects through the rectum to

the pubic symphysis. Below the rectum (to the mucocutaneous junction in the anal canal) the mucosa is derived from the cloaca. Visceral afferent fibres accompanying parasympathetic nerves convey pain from it to spinal cord segments S2-4 (and refer to the perineum).

HILUM AND VASCULAR SEGMENTS

Hilum

A hilum (L. ‘slit’) of a viscus is the site where nerves and vessels typically enter and leave it. The hilum is also the site where a duct exits from an exocrine gland.

The site of the hilum is usually associated with an indentation. The cavity in the hollow of the kidney (renal sinus) contains the renal pelvis (which gives origin to the ureter) together with the neurovascular supply lines.

Vascular segments

Fig.8.63 Hilum and vascular segments of kidney

Solid viscera (e.g. liver, kidney and lung) are often subdivided into discrete segments termed vascular segments.

Fig.8.64 Renal vascular segments

Vascular segments may be associated with subdivisions of ducts (e.g. bronchopulmonary segments of a lung); they receive a separate, exclusive arterial supply.

There tends to be no arterial anastomosis across vascular segments although there may be some venous communication.

Surgical removal of a segment

The concept of a vascular segment, together with knowledge of the specific territories, is applied in the surgical removal of part of a solid viscus. It is also important in considering the consequences of ligating a particular artery (e.g. ligating a segmental branch of a renal artery is likely to result in infarction of that kidney segment).

NEUROVASCULAR SUPPLY OF VISCERA

Viscera receive an entirely separate nerve supply to that of somatic structures.

Visceral nerves supply smooth muscle and glands, while somatic nerves supply skeletal muscle.

Derivation of visceral muscle and its supply

Fig.8.65 Gut smooth muscle from splanchnic mesoderm


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All smooth muscle of the gut (with its connective tissue and vessels) is derived from splanchnic (G. ‘visceral’)

mesoderm, which in turn developed from part of the mesoderm lateral to the somites.

Originally, this lateral plate mesoderm splits into two layers (around the developing serous body cavities), the somatic mesoderm and the splanchnic mesoderm.

Fig.8.66 Differing paths of visceral and somatic nerves

Fig.8.67 Distribution of visceral and somatic nerves

The body wall and the (parietal) layer of serous membrane lining it are supplied by somatic nerves, while the gut and the (visceral) layer of serous membrane around it is supplied by visceral nerves.

Visceral afferents and efferents

Hollow viscera generally receive a single sensory supply of visceral afferents. Clinically, the most important are pain fibres normally stimulated by stretch but also by tissue damage (e.g. associated with inflammation) or deprivation of blood supply.

Fig.8.68 Dual motor supply of tubular viscera

Hollow viscera generally receive a dual motor supply of visceral efferents (parasympathetic and sympathetic).

Parasympathetic nerve fibres are distributed primarily to the muscle coats. They augment motility and promote expulsion. Sympathetic nerve fibres inhibit motility. They are primarily distributed to sphincters and promote retention (as well as supplying blood vessels of the viscus).

Exocrine glands also tend to receive a dual motor supply. This is primarily by parasympathetic nerve (secretomotor) fibres that promote secretion.

The majority of sympathetic fibres to exocrine glands supply the associated blood vessels (vasomotor fibres).

Fig.8.69 Dual motor supply of glands

Neuroendocrine connections

Certain endocrine glands have major connections to the nervous system.

The pineal gland and the posterior lobe of the pituitary gland are outgrowths of the brain. The suprarenal medulla developed as a modified sympathetic

ganglion and is supplied directly by sympathetic nerve fibres. The anterior lobe of the pituitary gland receives

chemicals (releasing factors) released into its blood vessels indirectly from the neighbouring area of brain (the hypothalamus). The other endocrine glands function independently from the nervous system (although their associated blood vessels receive sympathetic fibres).

Highly vascular endocrine glands

Fig.8.70 Bilateral arterial supply to thyroid gland


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Viscera being actively metabolic have a rich vascular supply. Endocrine glands have a particularly rich supply as they secrete directly into the blood stream. The thyroid gland is the most vascular major organ in the body.

Like other highly vascular midline viscera (uterus and vagina) it is supplied bilaterally, from above as well as from below. The thyroid gland is not only drained by paired veins (directed upwards and downwards) but also by a large unpaired vein.

Fig.8.71 The multiple vessels of the thyroid gland

Arterial links along tubular viscera

Tubular viscera, being continuous organs, have a continuous series of links between adjacent arteries (anastomoses). A long tube (e.g. large intestine) however, may be vulnerable at particular sites (between the vascular reinforcements).

Dual vascular supply of lungs and liver

The lungs and the liver not only possess a dual blood supply but this supply is from different vascular systems. One (‘public’) supply is primarily for the benefit of the rest of the body, while the other (‘private’) supply is primarily

nutritional for the organ itself. The lungs receive a huge pulmonary circulation (via

pulmonary vessels) for gas exchange and a small systemic supply (via bronchial vessels) to keep the airways viable.

Fig.8.72 ‘Public and private’ vascular supplies to lung

The liver receives the hepatic artery and the portal vein (carrying blood to it from the gut via the portal venous

system) before being drained by the hepatic veins (into the systemic venous system).

Strangulation of a viscus

The blood supply to a viscus may be endangered from external compression. Strangulation (L. ‘choke’) affects

veins earlier than arteries, being thinner walled (and with a lower blood pressure). Swelling from venous congestion may further aggravate the compression and ultimately the arterial supply is compromised.

Strangulation of a tubular viscus (e.g. loop of intestine) irreversibly lodged in a tight hernial orifice (e.g. the

femoral ring), occurs when its blood vessels are compressed. Strangulation may also occur from associated twisting of blood vessels to a viscus suspended by a

long vascular stalk (e.g. torsion of the testis) or by a mesentery (e.g. volvulus of the sigmoid colon).

Fig.8.73 Strangulation of a herniated gut loop

Chapter 9: Nervous System and Nerves

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NERVOUS SYSTEM

NERVE FIBRES AND REFLEX ARCS

BRAIN AND SPINAL CORD STRUCTURE

SPINAL NERVES AND FIBRE TYPES

CRANIAL NERVES AND FIBRE TYPES

NERVE GANGLIA

SYMPATHETIC TRUNKS AND FIBRE PATHS

NERVE PLEXUSES

NERVE DISTRIBUTION AND BRANCHES

VASCULAR SUPPLY OF A NERVE

NERVE INJURIES AND NEUROGENIC PAIN

NERVOUS SYSTEM

The nervous system consists of the Central Nervous System (CNS) and the Peripheral Nervous System

(PNS). The CNS is made up of the brain and spinal cord. The brain consists of the forebrain (primarily the paired cerebral hemispheres), the midbrain and the hindbrain (pons, medulla and cerebellum).

Fig.9.1 Nervous system within body modules

The brainstem is the (unpaired) midbrain, pons and medulla. The PNS is made up of the somatic and visceral

(autonomic) nervous systems that are distributed via 12 pairs of cranial nerves and 31 pairs of spinal nerves.

The visceral nervous system consists of the sympathetic nervous system, the parasympathetic nervous system, visceral afferent fibres and the enteric

nervous system.

Fig.9.2 Central and peripheral nervous systems

NERVE FIBRES AND REFLEX ARCS

Roles of nerve fibres

Nerve fibres have two special abilities. Excitability

enables an area of cell membrane to change electrical polarity when stimulated. Conductivity enables a wave of

excitation to be propagated along a cell membrane.

Components of a nerve fibre

A nerve fibre is made up of a centrally located axon (L. ‘axis’) and surrounding supporting cells (with or without a

myelin sheath, between them). An axon is a major prolongation of cell membrane and enclosed cytoplasm (axoplasm) from a nerve cell. Nerve cells are termed neurons (L. ‘tendons’ i.e. stretched out). The supporting cell (termed the Schwann cell) is in the form of a sheath, the neurilemma (G. ‘nerve’ + ‘husk’).

Fig.9.3 Nerve fibres and speed of conduction

9. Nervous System and Nerves

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Myelin (G. ‘marrow’) is a fatty insulator (derived from

the Schwann cell) around the axon and internal to the neurilemma, enabling rapid conduction. The wave of excitation jumps between gaps (nodes) that occur at

regular intervals in the myelin, bypassing intervening segments of the axon. Conduction velocity is also proportional to the diameter of the nerve fibre. Large myelinated nerve fibres (e.g. motor fibres to skeletal muscle) are the most rapidly conducting, while small non-myelinated nerve fibres (e.g. pain fibres from viscera) are the slowest conducting (but occupy much less space).

Connective tissue of a peripheral nerve

A nerve (L. ‘string’) is made up of nerve fibres (derived

from ectoderm) and connective tissue (derived from mesoderm).

A tubular sheath of connective tissue termed endoneurium (G. ‘within nerve’) surrounds each individual

nerve fibre. Bundles of nerve fibres are surrounded by connective tissue termed perineurium (‘around nerve’).

The whole nerve is wrapped in a connective tissue sheath termed the epineurium (‘upon nerve’). The connective

tissue associated with nerves is primarily made of collagen fibres, although it may include some fat between the epineurium and perineurium. This occurs particularly at sites where nerves are subjected to stretch or to compression (e.g. the median nerve in the carpal tunnel).

Fig.9.4 Connective tissue sheaths of nerve fibres

Synapses

The central nervous system consists of the brain and spinal cord. The peripheral nervous system and ganglia

are derived from cranial and spinal nerves (arising from the brain and spinal cord, respectively).

An axon of one neuron meets another via a synapse (G. ‘touch’)

Fig.9.5 Major components of a neuron

Although a neuron possesses only one axon, it may receive numerous synapses from axons of other neurons. These typically occur with the cell body (which surrounds the nucleus) and with prolongations of cell membrane termed dendrites (G. ‘trees’) near the cell body.

Fig.9.6 Sites of synapses

Cell membranes of synapsing neurons are separated by a gap, the synaptic cleft. A chemical transmitter

released by arrival of electrical impulses at the pre-synaptic membrane diffuses across the synaptic cleft to affect the excitability of the post-synaptic membrane.

Fig.9.7 Components of a synapse

Typically, synapses with dendrites are excitatory, while those with the cell body are inhibitory

Sensory and motor nerve fibres

Nerve fibres within a peripheral nerve are classified into two types. Sensory fibres conduct impulses from organs that are sensitive to stimuli, while motor fibres conduct

impulses to organs that may respond actively. Sensory and motor fibres are also termed afferent (L. ‘carry to’) and efferent (L. ‘carry from’) being directed to and from the

central nervous system, respectively.

Fig.9.8 Afferent and efferent nerve fibres


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A typical sensory neuron has no dendrites. Instead, a sensory nerve has a short single axon with two axonal extensions branching at right angles. The proximal extension synapses in the CNS, while the (much longer) distal extension travels within a peripheral nerve. The cell body of a sensory neuron is located (in a sensory ganglion) near, but not in, the CNS.

A typical motor neuron has a single long axon arising from its cell body. The axon travels within a peripheral nerve. However, the cell body and dendrites of a motor neuron are located in the CNS.

Although some peripheral nerves are purely motor or purely sensory, the vast majority are mixed.

Impulses pass from distal to proximal along sensory

nerve fibres and from proximal to distal along motor nerve fibres.

Receptors

A receptor (L. ‘receive’) is at the origin of a sensory

nerve fibre. This is typically located on the end of the distal axonal extension. There are several types of receptors. Exteroreceptors are located in the skin and in the special

sense organs. Cutaneous exteroreceptors are mechanoreceptors (for touch and pressure), thermoreceptors (hot and cold) and nocioceptors (for superficial somatic pain). Proprioceptors are located in

somatic structures deep to the skin (in skeletal muscles, joints and bones). They include mechanoreceptors (for stretch, joint position and vibration) and nocioceptors (for deep somatic pain). Interoreceptors are located in internal organs. They include baroreceptors (for arterial blood pressure), chemoreceptors (for arterial oxygen tension) and nocioceptors (for visceral pain).

Effector and neuro-effector junction

An effector is just distal to the termination of the axon

of a motor nerve. An effector may be a muscle (voluntary or involuntary) or a gland.

Fig.9.9 Components of a neuromuscular junction

In contrast to a receptor, an effector is not in direct continuity with a neuron.

A motor neuron meets an effector at the neuro-effector junction. There is a gap (the junctional cleft) between the

cell membrane of the neuron (exposed by local loss of endoneurium, neurilemma and myelin sheath) and that of the effector. The neuro-effector junction for skeletal muscle is termed a neuromuscular junction. At a neuromuscular

junction, a chemical transmitter released by arrival of electrical impulses at the nerve terminal diffuses across the junctional cleft to excite a specialised area of skeletal muscle cell membrane (the motor end-plate).

Somatic and visceral fibre types

There are four major groups of functional fibre types that may occur in a peripheral nerve: somatic afferent, visceral afferent, somatic efferent and visceral efferent.

The functional fibre type of a sensory nerve fibre corresponds to the type of organ associated with the receptor.

This may be somatic (e.g. for skin) or visceral (e.g. for a

mucous membrane). Somatic afferents may be subdivided into superficial and deep. Superficial somatic (cutaneous) fibres convey touch, pressure, temperature and superficial somatic (‘sharp prick’) pain. Deep somatic (proprioceptive) fibres convey joint position sense, vibration sense, skeletal muscle stretch and deep somatic (‘dull ache’) pain.

Fig.9.10 Major types of nerve fibres

Visceral afferent fibres are non-myelinated and conduct slowly, conveying smooth muscle stretch and visceral (e.g. ‘vague’) pain.

The functional fibre type of a motor nerve fibre corresponds to the type of effector.

This may be somatic (for skeletal muscle) or visceral

(for smooth muscle, cardiac muscle and glands). Somatic efferent fibres are large and rapidly conducting, while visceral efferent fibres are smaller and slower conducting.

Autonomic nervous system

The nervous system can be subdivided functionally into somatic and visceral components. The visceral component is the Autonomic (G. ‘self+ law’ i.e. automatic) Nervous System (ANS). It continuously controls vessels and

viscera, including glands (both exocrine and endocrine), without relying on conscious awareness. The ANS is subdivided into sympathetic, parasympathetic and enteric divisions.

Sympathetics and parasympathetics generally have complementary actions (e.g. parasympathetic erection

followed by sympathetic ejaculation). Some of their activities are independent as a few

viscera and almost all vessels (except for arteries supplying the brain and erectile tissue) receive sympathetics only.

Other actions can be opposite (e.g. on heart rate and

pupil size) as many organs receive supply from both. However, even for organs with dual innervation, sympathetics tend to supply different components from those supplied by parasympathetics. For example, sympathetics supply smooth muscle sphincters of hollow viscera as well as blood vessels of both hollow and glandular viscera (with stimulation generally increasing vasoconstrictor tone). In contrast, parasympathetics supply the smooth muscle wall of hollow viscera as well as their associated exocrine glands (with stimulation producing secretion).


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Sympathetics supply dilator pupillae of the eye (stimulating contraction of radially oriented smooth muscle fibres) while parasympathetics supply sphincter pupillae (stimulating contraction of circular fibres). Sympathetics supply ventricles as well as atria of the heart (to suddenly increase stroke volume and heart rate, respectively when necessary), while parasympathetics supply atria only (to maintain a low basal heart rate).

Sympathetics and parasympathetics tend not to be fully activated simultaneously. However, they act in concert on viscera and the vasculature, catering for a wide range of bodily activities and external environments.

Fig.9.11 Reciprocal autonomic activation

Sympathetic nervous system

Sympathetic nerve fibres (‘thoracolumbar outflow’)

emerge from thoracolumbar spinal cord segments (T1-L2). They radiate to the rest of the body via a long sympathetic trunk (situated between the base of the skull and the tip of

the coccyx) on each side of the vertebral column.

Fig.9.12 An example of sympathetic activation

Sympathetic outflow is more divergent and diffuse than parasympathetic. For example, it is directed to almost all arterioles throughout the body, particularly those in skin and skeletal muscle (including of the limbs).

Sympathetics primarily control smooth muscle tone of arterioles.

Blood can be diverted to active organs according to need while blood pressure is maintained (ensuring adequate cerebral blood flow).

Although certain components of the sympathetic nervous system are most active in alarm reactions (‘fright’), stimulating the heart and diverting blood from viscera to skeletal muscles (for ‘fight’ or ‘flight’), this role is relatively minor compared to that when the body is at rest or changing posture.

Sympathetics dilate pupils (increasing peripheral vision) and bronchioles (facilitating lung ventilation). They also stimulate contraction of gastrointestinal and urinary tract involuntary sphincters.

Sympathetic nerves supply the suprarenal medulla, which secretes the hormone adrenaline into the blood stream that, in turn, potentiates the actions of sympathetic stimulation on many of its effectors. Sympathetics also form the efferent pathway for the ejaculation reflex (via contraction of ductus deferens).

Parasympathetic nervous system

Parasympathetic nerve fibres (‘craniosacral outflow’)

emerge from the brain (associated with cranial nerves III, VII, IX and X) and as pelvic splanchnic nerves (‘nervi erigentes’) from sacral spinal cord segments (S2-4). Parasympathetic fibres are distributed to the intervening thorax and abdomen via the continuation of the vagus nerve (cranial nerve X) on each side.

Parasympathetic outflow is more convergent and targeted than sympathetic. For example it is directed to specific sets of arterioles (in the brain and erectile tissues).

Parasympathetic activity maintains a slow heart rate and activates reflexes associated with glandular secretion (e.g. salivation and lacrimation). Parasympathetic stimulation constricts pupils and accommodates the lens of the eye (enhancing visual acuity). Parasympathetics also form the efferent pathway for the erection reflex (via dilation of arteries supplying erectile tissue).

Fig.9.13 An example of parasympathetic activation

Enteric nervous system

The enteric (G. ‘intestine’) nervous system is a special

division of the ANS that contains more neurons than either the sympathetic or parasympathetic divisions. The enteric nervous system includes all nerve cells found within walls of the gastrointestinal and biliary tracts, as well as in the pancreas. It contains complete circuits that can operate in the absence of connections with the CNS.

Extrinsic nerves provide the primary innervation of the digestive tract only near its proximal end and at its distal end (associated with skeletal muscle derived from branchial arches and cloacal sphincter, respectively). For the vast majority of the digestive tract (associated with


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smooth muscle, glands and mucosa of the foregut, midgut and hindgut), there are numerous intrinsic nerve cells, warranting the special classification. Neurons of the enteric nervous system tend to synapse with each other, rather than take part in reflex arcs that pass through the CNS. Their numerous ‘local circuits’ provide the prime neural control mechanism for gut motility, secretion and absorption.

Enteric neurons also have connections with certain extrinsic visceral nerves (sympathetic post-ganglionic fibres and parasympathetic pre-ganglionic fibres) which can modulate their actions (e.g. on peristalsis).

Reflexes

The nervous system is the sole control mechanism for skeletal muscle action. It also complements hormonal and local mechanisms in the control of smooth (and cardiac) muscle and of glands. In particular, it has the capacity to act almost instantaneously on specific distant targets.

A reflex (L. ‘bend backwards’, as in feedback) is an

active response to a stimulus that is involuntary and stereotyped. It is a ‘negative feedback’ mechanism; the

response feeds back on the stimulus (and progressively shuts it off beyond a certain threshold).

A reflex is characterised by an active rather than a passive response. More energy is expended in the response than is provided by the stimulus (i.e. it is ‘primed’ like a spring). Although modified by voluntary control, a reflex may occur without conscious awareness (i.e. it is ‘automatic’) and is stereotyped rather than random (i.e. it is ‘pre-programmed’ along a specific path).

Components of a reflex arc

A reflex arc is the pathway between stimulus and

response. It involves both the PNS and the CNS. There are five components of a typical reflex arc: receptor, afferent, CNS, efferent, and effector.

The simplest reflex arc involves only one synapse within the CNS (between the afferent neuron and the efferent neuron). The vast majority have more than one synapse within the CNS (involving at least one interneuron). A stretch reflex for skeletal muscle (e.g. a tendon jerk in a limb) is monosynaptic. It involves the following sequence of events:

- stretch receptors in the muscle (e.g. biceps brachii) are stimulated by a sudden pull on the associated tendon,

- deep somatic afferents convey impulses via a peripheral nerve (e.g. the musculocutaneous nerve) to the spinal cord,

- a spinal cord segment (e.g. C5 for the biceps jerk reflex) links respective afferent and efferent neurons (via synapses)

- somatic efferents convey impulses via the same peripheral nerve to the muscle,

- the effector contracts in response, moving a lever that simultaneously reduces stretch of the muscle (completing a ‘negative feedback loop’ shutting off the stimulus).

Fig.9.14 Components of a reflex arc

Fig.9.15 Reflex arc for biceps tendon jerk

Somatic and visceral reflexes

There are two major types of reflexes: somatic and visceral.

With somatic reflexes the effectors are skeletal muscles, while with visceral reflexes the effectors are smooth muscle, cardiac muscle or glands. Somatic reflexes may be subdivided into superficial and deep according to the afferent nerve fibre type. Superficial somatic (cutaneous) reflexes (e.g. withdrawal reflexes) arise from skin.

A special group of superficial reflexes (e.g. cough and swallow reflexes) arise from mucous membranes, although they involve skeletal muscle effectors. Deep somatic (proprioceptive) reflexes (e.g. stretch reflexes and tendon jerks) arise from skeletal muscles and joints. Visceral reflexes include pupillary, lacrimal, salivary, baroreceptor and chemoreceptor reflexes.

Fig.9.16 Superficial and deep somatic reflexes


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Fig.9.17 Visceral reflexes

BRAIN AND SPINAL CORD

Neural tube

The central nervous system is made up of the brain and spinal cord with a common derivation (from ectoderm). A longitudinal midline thickening of ectoderm (the neural plate) along the dorsum of the embryo forms a groove.

This neural groove has folds that meet and become buried as the neural tube.

Fig.9.18 Development of neural tube

The brain develops from the expanded cranial end of the neural tube, while the spinal cord develops from its narrow caudal part. The cavity of the neural tube remains relatively wide throughout most of the brain (as its ventricles), but becomes very narrow in the spinal cord (as its central canal).

Islands of ectoderm (neural crest) break away from the

neural folds to form all of the sensory neurons in the PNS. Ganglia and Schwann cells are also derived from neural crest, as well as the suprarenal medulla (a modified sympathetic ganglion) and the inner two membranes covering the CNS.

However, motor neurons that have their cell bodies in the CNS (all somatic and all preganglionic autonomic motor neurons) are not derived from neural crest.

Meninges and Cerebrospinal fluid

The brain and spinal cord are soft, protected by the surrounding meninges (G. ‘membranes’) and cerebrospinal fluid.

The meninges consist of three connective tissue layers. The outer layer is the dura mater (L. ‘hard’ + ‘mother’) a

tough fibrous covering derived from mesoderm. The middle layer is the arachnoid mater (G. ‘spider web-like’ + ‘mother’) lining the dura. The inner layer is the pia mater (L. ‘tender’ + ‘mother’) a delicate covering, directly investing the brain and spinal cord (as well as the blood vessels that supply them). The pia and arachnoid are derived from ectoderm.

Cerebrospinal fluid (CSF) is formed in the ventricles of the brain. The ventricular system communicates with the subarachnoid space (located between the arachnoid and

pia), where CSF circulates, acting as a shock absorber for the brain and spinal cord. CSF is in equilibrium with extracellular fluid surrounding neurons and neuroglia, which it maintains (e.g. by removing potentially harmful metabolites).

CSF is returned to the venous system (via arachnoid granulations projecting into the superior sagittal venous sinus) within the cranial cavity.

Neuroglia

The brain and spinal cord consists of nerve cells plus supporting cells.

Blood vessels (being derived from mesoderm) supply the brain and spinal cord from the exterior and must penetrate them to reach deep parts.

Neuroglia (G. ‘nerve’ + ‘glue’) supports neurons in the

brain and spinal cord. The majority of neuroglial cells (which also outnumber neurons in the CNS) are the astrocytes. These star-shaped cells have processes with

‘foot plates’ that surround blood vessels. Some neuroglial cells are more like the Schwann cells that support peripheral nerve fibres. These myelin-forming cells are the oligodendrocytes.

Grey matter cortex and nuclei

The brain and spinal cord are composed of grey matter and white matter. Grey matter is made of cells, white matter of fibres.

Components have a precise anatomical localisation coupled with a representation of associated body areas (or even of basic mental activities). Grey matter consists primarily of cell bodies of neurons. It also contains non-myelinated axons.

Grey matter is primarily located around the periphery of the cerebral and cerebellar hemispheres, where it is termed cortex (L. ‘shell’). This is in contrast to the spinal cord

where all grey matter is centrally located, with white matter around the periphery.

The grey matter of the cerebral cortex is in folds, termed gyri (G. ‘circles’) with intervening crevices, termed sulci (L. ‘furrows’) greatly increasing its surface area.

Collections of cell bodies of neurons centrally located in the brain (particularly around the ventricular system) or at least submerged in its white matter are termed nuclei (L. ‘nuts’). These include the basal nuclei (in the cerebrum), nuclei of the thalamus and hypothalamus (in the diencephalon), cranial nerve nuclei (in the brainstem) and cerebellar

nuclei.


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Fig.9.19 CNS grey matter and white matter

Spinal cord columns

Grey matter within each side of the spinal cord is arranged as two major columns (previously called horns

when seen in transverse sections), posterior and anterior. The posterior column is continuous with the posterior nerve root (for entering sensory fibres). The anterior column contains the cell bodies of somatic motor neurons. Their axons exit via the anterior nerve roots. Anterior column cells are also called lower motor neurons and are the final

common path for skeletal muscle stimulation.

Fig.9.20 Major motor pathways cross the midline

A lateral column (containing the cell bodies of sympathetic neurons) lies in all thoracic, plus the upper two lumbar, segments of the spinal cord.

Fig.9.21 Upper and lower motor neurons

White matter tracts

White matter consists of axons surrounded by a myelin sheath (derived from neuroglia) and appears white because of the fatty content of the myelin.

Within white matter, nerve fibres are grouped into pathways (tracts). In the brain, tracts of white matter

contain fibres that are classified according to their orientation. Projection fibres pass up (ascending fibres) or down (descending fibres). Association fibres pass

forwards or backwards (on the one side of the brain). Commissural fibres pass from one side of the brain to the

other (a band purely of fibres that cross the midline is a commissure).

Spinal cord funiculi

Each half of the spinal cord is divided into three funiculi (L. ‘little cords’) posterior, lateral and anterior.

These funiculi consist of white matter tracts with descending fibres (to motor neurons) and ascending

fibres (from sensory neurons).

Fig.9.22 Descending fibres to motor neurons

The descending fibres are primarily in the lateral funiculi

and include the voluntary pathway (from the motor area of the cerebral cortex) plus many fibres to control skeletal muscle tone. There are also sympathetic pathways (primarily from the hypothalamus).


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Fig 9.23 Ascending fibres from sensory neurons

The ascending fibres convey most cutaneous sensation (via spinothalamic tracts), conscious proprioception plus fine touch (via the posterior columns) and unconscious proprioception (via spinocerebellar tracts). Within the

funiculi, both descending and ascending fibres are arranged in laminae. In the anterior and lateral funiculi,

fibres associated with caudal segments are superficial to those associated with more cranial segments. In the posterior funiculi, fibres associated with caudal segments are medial to those associated with more cranial segments.

The left cerebral hemisphere controls movements for, and receives conscious sensation from, the right side of the body. Similarly, the right cerebral hemisphere is connected to the left side of the body.

Most neural pathways in the CNS cross the midline.

SPINAL NERVES AND FIBRE TYPES

Posterior and anterior nerve roots

There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal). Each is attached to the corresponding spinal cord segment via nerve roots arising as a series of rootlets from the posterior and the anterior aspects of the spinal cord.

Fig.9.24 Spinal nerve roots

Posterior nerve roots are purely sensory while anterior nerve roots are purely motor.

They contain only afferent fibres and efferent fibres respectively.

Fig.9.25 Posterior and anterior nerve roots

A serrated fold of pia mater (the denticulate ligament)

within the subarachnoid space separates the posterior and anterior nerve roots. A swelling, termed a ganglion, is

found on each posterior root. A posterior root ganglion comprises clusters of cell bodies of sensory nerve fibres (which unlike motor fibres have both proximal and distal axonal extensions).

Afferent fibres in posterior roots

Somatic afferent fibres, both superficial (cutaneous) and deep (proprioceptive), form the vast majority of fibres in a posterior root of spinal nerves at all levels.

Fig.9.26 Somatic afferent fibre paths

In addition, posterior roots from T1 to L2 convey some visceral afferent fibres (which accompanied the visceral efferents but passed through a sympathetic ganglion without synapsing). These fibres convey visceral (slowly conducting) pain from thoracic, abdominal and upper pelvic viscera. Posterior roots from S2 to S4 convey visceral afferents from lower pelvic viscera.

Fig.9.27 Visceral afferent fibre paths


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Efferent fibres in anterior roots

Somatic efferent fibres comprise the vast majority of fibres in an anterior root of all spinal nerves.

Fig.9.28 Somatic efferent fibre paths

In addition, anterior roots from T1 to L2 convey (pre-ganglionic) sympathetic fibres and anterior roots from S2 to S4 convey (pre-ganglionic) parasympathetic fibres.

Fig.9.29 Visceral efferent fibre paths

Dural sleeve

The union of an anterior nerve root with a posterior nerve root forms a spinal nerve. Each spinal nerve emerges from an intervertebral foramen. Nerve roots are invested by thin pia mater, continuous with that surrounding the spinal cord. As anterior and posterior nerve roots pass into an intervertebral foramen they also receive an arachnoid lined extension of dura mater (a dural sleeve).

The dural sleeve merges with the epineurium of the spinal nerve.

Recurrent meningeal branch

The recurrent meningeal (or sinuvertebral) nerve is a

small branch that arises immediately from each spinal nerve and re-enters the associated intervertebral foramen.

Fig.9.30 Recurrent meningeal nerve and its branches

Each recurrent meningeal nerve receives sensation from the adjacent anterior wall of the vertebral canal (ligaments, periosteum and the periphery of intervertebral discs) and dura mater, including the dural sleeve of the associated nerve roots. These structures are highly sensitive to painful stimuli (in contrast to the spinal cord and the spinal nerve roots which are totally insensitive).

Posterior and anterior nerve rami

The spinal nerve proper is very short (only a few millimetres in length) because it divides almost immediately.

Every spinal nerve receives a connection from the sympathetic trunk and some spinal nerves (T1-L2) give a connection to the sympathetic trunk as well. These connections (rami communicantes) are located at, or just

distal to the division of a spinal nerve into its rami. Each spinal nerve divides into a posterior and an

anterior ramus (L. ‘branch’). The posterior ramus turns

sharply backwards, while the anterior ramus is a direct continuation of the spinal nerve.

Fig.9.31 Typical spinal nerve and its components

Posterior rami of spinal nerves directly supply skin, intrinsic back muscles and joints of the dorsal aspect of the trunk and neck.

Fig.9.32 A thoracic spinal nerve


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Anterior rami of thoracic spinal nerves typically supply the ventral aspect of the trunk directly, while anterior rami of cervical and lumbosacral spinal nerves supply all parts of the limbs and ventral aspect of the neck indirectly (via peripheral nerves derived from plexuses).

Segmental nerve distribution

Unlike the motor (anterior) or sensory (posterior) nerve roots, spinal nerves and rami are mixed (containing both motor and sensory fibres).

Each spinal nerve supplies a continuous strip of skin (its dermatome) via cutaneous branches of the posterior

ramus and of the anterior ramus (e.g. lateral and anterior cutaneous branches of an intercostal nerve).

A spinal nerve also supplies a mass of muscle (its myotome) via muscular branches from the posterior ramus

(segmentally to intrinsic back muscles) and from the anterior ramus (e.g. branches of an intercostal nerve to muscles of the associated intercostal space).

Thoracic spinal nerves are regarded as typical spinal nerves. The segmental pattern of distribution from a thoracic spinal nerve is retained in both its posterior and anterior rami.

Fig.9.33 Distribution of a thoracic spinal nerve

The segmental pattern of distribution from cervical, lumbar and sacral spinal nerves is disguised by their anterior rami linking to form plexuses.

Features of a segmental nerve lesion

A lesion of a spinal nerve produces a segmental pattern of loss of function. This is reflected in loss of power and muscle tone affecting the myotome, sensation affecting the dermatome (minus overlap from adjacent dermatomes), reflexes and (if long term) muscle wasting. A lesion of anterior nerve roots produces a segmental pattern of loss of motor function, while a lesion of posterior nerve roots produces a segmental pattern of loss of sensory function. However, a lesion to any component of its reflex arc will diminish a reflex.

CRANIAL NERVES AND FIBRE TYPES

Cranial nerves

All vertebrates have 12 pairs of cranial nerves numbered from rostral (L. ‘beak’) to caudal (L. ‘tail’). The

first two are atypical, being direct outgrowths from the brain. The remaining ten arise from the brain stem.

I olfactory (for smell) II optic (for vision) III oculomotor (for most eye movements and to pupil) IV trochlear (to one eye muscle) V trigeminal (for most head sensation and chewing)

VI abducent (to one eye muscle) VII facial (for facial expression, salivation and taste) VIII vestibulocochlear (for hearing and balance) IX glossopharyngeal (for sensation to throat) X vagus (to pharynx, larynx and multiple viscera) XI accessory (to two neck muscles) XII hypoglossal (for tongue movements)

I and II are continuous with the forebrain, while the others arise from the brain stem (midbrain, pons and medulla).

III and IV arise from the midbrain. V and VI arise from the pons. The trigeminal nerve is in

three divisions: ophthalmic (V1), maxillary (V2) and mandibular (V3).

VII, VIII, IX, X, XI (cranial part) and XII emerge from the medulla.

The spinal part of XI arises from upper cervical spinal cord segments. It passes up through the foramen magnum in the skull to join with the cranial part. Some cranial nerves are purely sensory (I, II and VIII), others purely motor (III, IV, VI, XI and XII) while the remainder (V, VII, IX and X) are mixed nerves with both motor and sensory fibres.

Fig.9.34 Levels of cranial nerve origin

Branchial arches and associated nerves

The mixed cranial nerves supply special muscles that possess certain properties of somatic and visceral muscle.

Fig.9.35 Branchial arches

Muscles of the jaw, face, pharynx and larynx are derived from the branchial (G. ‘gill’) arches (first, second,

third and fourth/sixth respectively). These are paired masses of mesoderm situated between gill clefts

(ectoderm depressions overlying endoderm pouches). Branchial arches may also be termed pharyngeal

arches as their associated pouches line the developing

pharynx.


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Remaining derivatives (bones, cartilages and muscles) of the (six) original branchial arches retain their nerve supply despite migration. The fifth branchial arch disappears early while the sixth can be included with the fourth. Each arch has a designated cranial nerve (V, VII, IX and X, respectively).

Each branchial arch is supplied by a mixed cranial nerve.

The cranial nerves with both motor and sensory fibres are peculiar in that their supply includes branchial arch muscles (as well as skin or mucosa of associated clefts and pouches) to pathways for a special set of reflexes.

Although structurally identical to skeletal muscles and capable of voluntary movement (e.g. in speech), branchial muscles develop from splanchnic mesoderm. These

muscles, which tend to be covered by mucous membrane or located near a mucocutaneous junction, are also functionally related to smooth muscle.

They are effectors for a special group of superficial reflexes (e.g. swallow and cough reflexes) arising from mucous membranes (of upper digestive and respiratory tracts) and often act in conjunction with visceral reflexes involving glandular secretion (e.g. salivation).

Fig.9.36 Protective airway reflex territories

Special sense organs and receptors

Certain cranial nerves (particularly those that are purely sensory) are involved with special senses. The special

senses are smell, vision, taste, hearing and balance. Receptors for the special senses are located at the peripheral end of each of the following cranial nerves:

I (smell) II (vision) VIII (hearing and balance) VII, IX and X (taste) Some special receptors create discrete organs or parts

of organs (e.g. retina of the eye as well as cochlea and vestibular apparatus of the inner ear) while others are microscopic collections on mucous membranes (e.g. taste buds of the tongue and olfactory receptors of the nose).

The optic nerves join in the midline at the optic chiasm (G. ‘lines that cross’) prior to diverging as optic tracts.

Fibres conveying impulses from the medial half of the retina in each eye (stimulated by light from the lateral half of both left and right visual fields) cross the midline at the optic chiasm. Fibres conveying impulses from the lateral

half of the retina in each eye (stimulated by light from the medial half of both visual fields) do not cross the midline. Each half of the visual field is thus represented on the cerebral cortex in the opposite side of the brain.

Fig.9.37 Visual pathways to left visual cortex

Importance of testing visual fields

The visual pathway travels from the front to the back of the brain (hence the importance of visual field examination for identifying the site of a lesion within the brain).

Columns of cranial nerve nuclei

The cell bodies of cranial nerves are clustered in groups termed nuclei.

Cranial nerve nuclei within the brain stem are arranged in columns associated with particular fibre types. Seven of

the cranial nerves (I, II, IV, VI, VIII, XI and XII) contain only one fibre type, while the remainder contains multiple fibre types. There are three groups of motor fibre types ‘special’ (branchial) efferents in addition to somatic and visceral. There are also three groups of sensory fibre types: ‘special’ afferents in addition to general and visceral.

Fig.9.38 Columns of cranial nerve nuclei in brainstem


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Each half of the brain stem contains a set of three columns of motor nuclei (medially located) and a set of three columns of sensory nuclei (laterally located).

NERVE GANGLIA

Posterior root ganglia

A nerve ganglion (G. ‘swelling’) is a localised

enlargement on a component of the PNS. It is due to a collection of cell bodies of neurons. These occupy much more space than axons. Nerve ganglia may be sensory or motor.

A ganglion, created by the collection of cell bodies of sensory neurons, is found on the posterior root of every spinal nerve.

The ganglion is located on the posterior root just before it unites with the anterior root (to form the spinal nerve).

Fig.9.39 Posterior root ganglia

Each posterior root ganglion resides in an intervertebral foramen, regardless of the length of the associated nerve root.

Fig.9.40 Site of posterior root ganglia in exit foramina

Cranial nerve ganglia

Fig.9.41 Sensory ganglia at exit foramina

Ganglia are located on cranial nerves conveying general sensation (V, VII, IX and X) and/or visceral sensation (VII, IX, X).

The sensory ganglia of cranial nerves are located in or near the associated foramina in the skull.

The location of sensory ganglia is similar to the sensory

ganglion of a spinal nerve in an intervertebral foramen.

Features of an autonomic ganglion

Motor ganglia are associated with autonomic nerves (not somatic nerves). An autonomic ganglion is a swelling

created by clusters of cell bodies of visceral motor neurons and is distinguished from a sensory ganglion by the presence of synapses. Each synapse receives the axonal termination of a ‘preganglionic’ neuron. Each cell body in an autonomic ganglion is of a ‘postganglionic’ neuron.

Fig.9.42 Sympathetic ganglion and associated fibres

Autonomic ganglia involve sympathetics and/or parasympathetics.

Preganglionic fibres are myelinated, while postganglionic fibres are nonmyelinated (and of smaller diameter). Additional fibres may pass through an autonomic ganglion without synapsing in it. This applies to all visceral afferent fibres and even applies to some visceral efferent fibres. The latter may be postganglionic (having already synapsed in a more proximal ganglion) or preganglionic (to synapse in a more distal ganglion).


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Paravertebral sympathetic ganglia

Fig.9.43 Ganglia on sympathetic trunk

More than 20 pairs of ganglia with synapses that are purely sympathetic are located on the sympathetic trunks. These are termed paravertebral ganglia as they are also

situated alongside the vertebral column. Sympathetic postganglionic fibres are much more

numerous than preganglionic fibres and are typically long, correlating with the widespread effects of sympathetic stimulation (enhanced and prolonged by adrenaline). In addition to supplying arteries, sympathetic postganglionic fibres may form a plexus around arteries and accompany

them to their peripheral destinations.

Prevertebral sympathetic ganglia

A series of ganglia, termed prevertebral ganglia, is

found in front of the vertebral column. They are located at the origins of major arteries arising from the abdominal aorta and have synapses that are primarily for sympathetics. Postganglionic fibres are distributed along the associated arterial branches to abdominal and pelvic viscera.

Fig.9.44 Types of sympathetic ganglia

Preganglionic parasympathetic fibres (from branches of the vagus nerve) also pass through these ganglia. They tend to synapse in (parasympathetic) ganglia located adjacent to the viscera.

Suprarenal medulla and paraganglia

The suprarenal (adrenal) medulla is a modified sympathetic ganglion (having also developed from the neural crest). It is supplied by (pre-ganglionic) sympathetic fibres and secretes adrenaline and noradrenaline directly into the blood stream. Clumps of (chromaffin) tissue

similar in composition to suprarenal medulla (and neural crest derived) may be found nearby, along the abdominal aorta. These paraganglia can also secrete adrenaline. The largest are known as the para-aortic bodies.

Parasympathetic ganglia

Parasympathetic ganglia tend to be smaller and

located more peripherally than sympathetic ganglia. They are typically situated adjacent to the viscera they supply.

Parasympathetic post-ganglionic fibres are short, correlating with the more localised effects of parasympathetic stimulation. The major autonomic ganglia in the head with synapses that are purely parasympathetic have specific names.

They are the ciliary ganglion (supplying the eye), the pterygopalatine ganglion (supplying lacrimal and nasal glands), the otic ganglion (supplying the parotid gland) and the submandibular ganglion (supplying submandibular

and sublingual glands).

Fig.9.45 Parasympathetic ganglion and associated fibres

SYMPATHETIC TRUNKS AND FIBRE PATHS

Sympathetic trunks are paired chains, linking a series

of sympathetic ganglia. These ganglia are primarily segmental, although some have fused. A sympathetic trunk runs vertically (along each side of the vertebral column) from the base of the skull to the tip of the coccyx.

Lateral column of spinal cord

Fig.9.46 Peripheral sympathetic path and its origin


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Central sympathetic pathways originate in the brain (from the hypothalamus and the medulla). They pass down the spinal cord to synapse primarily in the lateral column

of grey matter. The lateral column is present only in thoracic and upper lumbar spinal cord segments (T1-L2). The peripheral sympathetic pathway originates in the lateral column and consists of two neurons that synapse in a sympathetic ganglion.

White ramus communicans

Each spinal nerve from T1-L2 is connected to the sympathetic trunk by a white rami communicans.

A white ramus communicans contains pre-ganglionic

fibres that are myelinated (hence termed ‘white’). It is located at the origin of the anterior ramus of a spinal nerve. Some preganglionic fibres synapse immediately in the paravertebral ganglia they enter, while others pass up or down the sympathetic trunks and synapse in ganglia with further connections to all 31 pairs of spinal nerves.

Fig.9.47 White ramus communicans

Grey ramus communicans

Every spinal nerve is connected to a sympathetic trunk by a grey ramus communicans.

Fig.9.48 Grey ramus communicans

A grey ramus communicans contains post-ganglionic fibres that are not myelinated (hence termed ‘grey’). It typically connects with each spinal nerve at its division into posterior and anterior rami. Sympathetic postganglionic fibres subsequently pass into both rami. In this way vasomotor fibres are distributed to the limbs and trunk, and (together with sudomotor and pilomotor fibres) to the associated skin. A grey ramus communicans is located just proximal to a white ramus communicans (at the origin of its anterior ramus of a spinal nerve).

Sympathetic supply to head, neck, & thorax

Sympathetic supply to the head and neck emerges from the superior cervical ganglion of the sympathetic trunk at the base of the skull. It is then distributed via sympathetic perivascular plexuses, which follow the major arteries

and their branches. Thoracic viscera receive their sympathetic supply by cardiac, pulmonary and oesophageal branches from cervical and upper thoracic ganglia of the sympathetic trunk.

Splanchnic nerves to abdomen and pelvis

The splanchnic (G. ‘viscus’) nerves distribute

sympathetic fibres to abdominal and pelvic viscera.

Fig.9.49 Sympathetic trunk and connections

These fibres are preganglionic, synapsing in prevertebral ganglia well prior to the viscera. The thoracic splanchnic nerves (greater, lesser, least) arise from

paravertebral ganglia of the sympathetic trunk in the thorax. They pierce the diaphragm to join prevertebral ganglia (coeliac, aorticorenal and mesenteric) located in front of

the abdominal aorta, where their fibres synapse. Postganglionic fibres are distributed to abdominal (and

pelvic) viscera along the unpaired branches of the aorta. The lumbar splanchnic nerves arise from paravertebral

ganglia of the sympathetic trunk in the abdomen and join prevertebral ganglia (primarily inferior mesenteric) where

their fibres synapse. Postganglionic fibres are distributed to the pelvic viscera along arteries.

Visceral pain fibre paths

Pain is conveyed from thoracic, abdominal and upper pelvic viscera (between the thoracic pain line and the pelvic pain line) via visceral afferent fibres to spinal cord

segments T1-L2. Until they enter posterior nerve roots, these fibres transmitting visceral pain accompany sympathetic motor fibres along their peripheral paths. This includes passing through sympathetic ganglia (without synapsing).


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Fig.9.50 Thoracic and pelvic pain lines

In contrast, pain is conveyed from lower pelvic viscera (below the pelvic pain line) via visceral afferent fibres to spinal cord segments S2-4. Until they enter posterior nerve roots, these fibres transmitting visceral pain accompany parasympathetic motor fibres along their peripheral paths. This includes passing through parasympathetic ganglia (without synapsing).

NERVE PLEXUSES

A plexus (L. ‘braid’) is the linking together of nerves (or

of vessels). A nerve plexus involves the intermingling (but not joining) of axons from different nerves connected by continuous sheaths of fibrous tissue. In this rearrangement of bundles there is no mixing of electrical impulses (unlike vascular plexuses where the fluid contents mix within the interconnected lumens). The largest nerve plexuses are created from linking anterior rami of certain spinal nerves. In a similar way, although on a smaller scale, communicating branches can also occur between neighbouring nerves, particularly their cutaneous branches.

Somatic plexuses

Fig.9.51 The brachial plexus

Anterior rami of spinal nerves primarily consist of somatic nerve fibres (although they also contain postganglionic sympathetic fibres). The majority link up to form somatic plexuses from which many peripheral nerves arise. The somatic plexuses are cervical (derived from segments C1-C5), brachial (derived from segments C5-T1) and lumbosacral (derived from segments L1-S4).

Only anterior rami of spinal nerves take part in the formation of plexuses.

Each thoracic anterior ramus from T2-T12 does not

unite with any other anterior ramus (although T2 and T12 usually have a small connection to the brachial plexus and lumbosacral plexus, respectively). Typical intercostal nerves (the continuation of almost all thoracic anterior rami) retain their obvious segmental arrangement rather than arise from a nerve plexus.

Limb buds and associated plexuses

With the exception of the cervical plexus, somatic plexuses are associated with the development of a limb bud. The brachial plexus supplies the upper limb, while the

lumbosacral plexus supplies the lower limb.

Fig.9.52 Limb buds and supply from anterior rami

Limb buds arise only from the ventral aspect of the trunk (i.e. in front of the ‘coronal morphological plane’)

and are supplied only by anterior rami. Posterior rami are not involved in their associated nerve plexuses (or the cervical plexus) as posterior rami are distributed to the back (and the back of the neck) rather than the ventral aspect of the trunk.

Fig.9.53 Coronal morphological plane and supply from rami


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Anterior and posterior divisions of plexuses

The brachial and lumbosacral plexuses each form two divisions. Peripheral nerves derived from anterior divisions of a plexus are distributed to flexor compartments while those derived from posterior divisions are distributed to extensor compartments.

Fig.9.54 Divisions of plexus and supply of compartments

The anterior rami of the brachial plexus unite to form trunks, each dividing into two divisions, the anterior for distribution to flexor compartments and the posterior for distribution to extensor compartments. The divisions (after reuniting to form cords) provide the major peripheral

nerves to the limb as terminal branches of the plexus. Branches to proximal muscles (surrounding the plexus) may arise directly from components of the plexus (anterior rami, trunks or cords).

Pre-fixed and post-fixed plexus variants

Occasionally the segments of origin can vary by a cranial shift of one segment up (a ‘pre-fixed’ plexus) or by a caudal shift of one segment down (a ‘post-fixed’

plexus). This may be associated with bony variants (e.g. a ‘cranial shift’ or a ‘caudal shift’) of the vertebral column. Awareness of the possibility for anatomical variation is important in interpreting the findings from a neurological examination and may account for atypical patterns of nerve distribution.

Autonomic plexuses

Viscera are supplied by sympathetics and parasympathetics (including visceral afferent fibres accompanying either of them). These fibres tend to converge, forming plexuses near the viscera.

The major autonomic plexuses in the thorax (cardiac, pulmonary and oesophageal), the abdomen (coeliac and mesenteric) and the pelvis (hypogastric) contain all of the

above visceral nerve fibre types. In the abdomen and pelvis these plexuses surround major arteries supplying the viscera. In addition, purely sympathetic plexuses occur along arteries in the head, neck and limbs.

NERVE DISTRIBUTION AND BRANCHES

Peripheral nerve distribution

Peripheral nerves supply their target organs (e.g. territories of skin and muscle) by branches. The structures

supplied by branches of a particular nerve are regarded as its distribution.

The distribution of a peripheral nerve depends on the functional fibre types within it. There are generally three fibre types in a typical peripheral nerve.

Somatic afferent fibres are distributed to skin, bones,

joints and muscles. Those arising from receptors in the skin

are termed superficial somatic (cutaneous) afferents. Those arising from receptors in bones, joints and muscles are termed deep somatic (proprioceptive) afferents.

Fig.9.55 Nerve fibre types and their distribution

Somatic efferent fibres are distributed to skeletal muscles. Visceral efferent fibres are sympathetic and

primarily vasomotor. These fibres are distributed to blood vessels in muscles, bones, joints and skin. Sudomotor fibres (to sweat glands) and pilomotor fibres (to arrector pili muscles) are also distributed to skin.

Features of a peripheral nerve lesion

It is important to distinguish a peripheral pattern from a segmental pattern in deducing the site of a nerve lesion. A

lesion of a peripheral nerve may affect each of the functional fibre types in it. The features are primarily determined by the distribution of the particular nerve distal to the site of the lesion. A lesion of a peripheral nerve produces a peripheral (in contrast to a segmental) pattern of loss of function.

There is loss of power and tone of muscles supplied by motor branches arising distal to the lesion. This is coupled with loss of sensation of the skin supplied by sensory branches arising distal to the lesion (minus overlap from adjacent peripheral nerves). Reflexes (e.g. tendon jerks) may also be affected and (if long term) there may be associated muscle wasting.

Peripheral nerve branches

A typical peripheral nerve distributes its fibres via muscular, cutaneous, articular and vasomotor

branches.

Fig.9.56 Types of peripheral nerve branches

Muscular branches are generally the most important branches. They are mixed, containing both motor and sensory fibres.

Cutaneous branches are primarily sensory (except for some sympathetic fibres). The terminal branch of a peripheral nerve is typically cutaneous.


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Articular branches are sensory. They are variable and may arise indirectly from muscular branches.

Vasomotor branches are purely sympathetic postganglionic fibres. They are very fine, variable and may arise at multiple levels.

Fig.9.57 Branches of the musculocutaneous nerve

Hilton’s law

Peripheral nerves link somatic structures to create functional units. There is a relationship between

structures supplied by a particular nerve (according to ‘Hilton’s law’).

A nerve which supplies a muscle producing movement at a joint also supplies sensation to the joint and skin overlying (the insertion of) the muscle.

Branching sequence of nerves

There is generally a sequence in type of branches from a peripheral nerve. Each nerve of a particular compartment supplies muscular branches (to the associated flexor or extensor muscle group), then articular branches (for

sensation from that aspect of the joint underlying the muscles) and terminates as a cutaneous nerve (supplying

skin overlying the part that is moved).

Fig.9.58 Sequence of peripheral nerve branches

The musculocutaneous nerve in the arm supplies a muscular branch to biceps brachii (a flexor at the elbow), then an articular branch (to the flexor aspect of the elbow

joint capsule) and terminates as the lateral cutaneous nerve of the forearm (supplying skin over and beyond the insertion of biceps).

Variations of nerve branching

Branching patterns for peripheral nerves (as with plexuses) depend on how nerve fibres are bundled within connective tissue deep to the epineurial sheath. This provides plenty of scope for variations, which are common. Variation may be in number (with branches combining or separating) and in level (with branches arising more proximally or distally). Communications may occur between nerves anywhere along their course, including within the spinal canal, within a plexus or between their peripheral branches. As a result, there are multiple possible paths for a nerve fibre to reach its target.

Protective somatic reflexes

The shared nerve supply for muscles, their underlying joints and overlying skin may provide reflex arcs linking them. Protective reflexes for joints or skin utilise associated skeletal muscles. This may be regarded as an application of ‘Hilton’s Law’. Stretch on a joint capsule, or associated ligaments, tends to trigger a reflex contraction of the overlying muscle.

Fig.9.59 Protective withdrawal reflex and biceps jerk

This draws the bones together, reducing the degree of stretch and protecting the joint from injury. Stretch of the flexor aspect of the capsule (e.g. of elbow joint) from hyperextension elicits a reflex contraction of flexor muscles (e.g. biceps brachii) producing flexion at the joint. A painful stimulus to skin elicits muscle contraction that tends to move the associated part away from the threat. Such superficial somatic (cutaneous) reflexes may be involved in a generalised ‘withdrawal reflex’. Contact with a sharp

object (e.g. to skin on the sole of the foot) elicits reflex contraction of muscles (e.g. calf muscles) producing plantar flexion at the ankle joint, withdrawing the foot (particularly when accompanied by contraction of flexor muscles at the hip and knee joints).

Reflex muscle spasm

Reflex muscle spasm is a protective mechanism that

usually occurs after injury to deep structures (e.g. joints and ligaments). Continuous involuntary contraction (spasm) of overlying muscle, which has a common nerve

supply to the underlying structures, tends to protect them from further injury.

Injury to joints of the spine is associated with reflex muscle spasm of overlying back muscles (erector spinae), which have a common nerve supply (via dorsal rami of spinal nerves).

While reflex muscle spasm tends to be protective it may create further problems (e.g. via a positive feedback cycle with pain escalation). Reflex muscle spasm also needs to


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be overcome when reducing a joint dislocation. Pain from

other types of deep somatic structures, including meninges and the parietal layer of serous membranes (in addition to its referral to skin) is also associated with reflex muscle spasm.

Fig.9.60 Protective back muscle spasm

Inflammation of the meninges (meningitis) is

accompanied by neck stiffness due to reflex spasm involving extensor muscles of the cervical spine as well as referral of pain (headache). ‘Guarding’ and ‘rigidity’ of the

anterior abdominal wall protects underlying viscera when inflammation has spread to involve the parietal peritoneum (peritonitis).

Fig.9.61 Protective abdominal muscle rigidity

VASCULAR SUPPLY OF A NERVE

Vasa nervorum

Although nerve cells are derived from ectoderm, the surrounding connective tissue within a peripheral nerve is derived from mesoderm (as are vessels) and provides an avenue for a peripheral nerve to receive its blood supply. Peripheral nerves obtain their blood supply by a succession

of small branches termed vasa nervorum (L. ‘vessels of nerves’).

These involve both arteries (arteriae nervorum) and veins (venae nervorum). Arteriae nervorum typically arise

as direct branches from a major artery or one of its named branches. They may also arise indirectly from muscular or cutaneous branches. Arteriae nervorum are given off at multiple levels along the course of a nerve and form branches that run longitudinally in the epineurium as well as penetrating to create communicating plexuses around fibre bundles.

A major peripheral nerve is typically a part of more than one angiosome and (like the other components of these 3-dimensional territories) is supplied by ‘choke vessels’

across the boundaries of adjacent angiosomes.

Fig.9.62 Vasa nervorum

Blood-brain barrier

The CNS receives blood supply from its periphery.

The cerebral and the spinal arteries run on the surface of the brain and the spinal cord invested in pia mater. Their branches penetrate the white and grey matter, progressively getting smaller.

The deepest areas of the CNS tend to have the most precarious arterial supply (particularly as these branches do not link with each other).

Capillaries in the brain and spinal cord allow water to

pass freely across the endothelial membrane with ease. However, their endothelium is impermeable to many substances, while others cross slowly. In addition, these capillaries are peculiar in being almost completely surrounded by the footplates of astrocytes (star-shaped neuroglial cells). This unique permeability barrier of cerebral capillaries has been termed the ‘blood-brain barrier’ by physiologists and tends to protect the brain from

toxic substances. A few areas of the brain are outside the blood-brain

barrier. These small zones either have chemoreceptors

(monitoring chemical changes in the plasma) or secrete hormones. The blood-brain barrier does not fully develop until early childhood. It also may be affected by brain disease (e.g. infection or tumours) and certain drugs may enter it (e.g. a few antibiotics), while others do not.

There are no lymph vessels in the CNS.

Barrier to spread of brain tumours

The presence of the blood-brain barrier, coupled with the absence of lymph vessels, may explain why tumours arising within the CNS tend not to spread outside it.


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NERVE INJURIES AND NEUROGENIC PAIN

Types of nerve injuries

A peripheral nerve lesion impairs motor and sensory (including reflex) functions (and in the long term may lead to muscle wasting). Mild injury causes a transient loss of function. A peripheral nerve may be injured by laceration, traction or compression.

Compression may directly damage nerve fibres (e.g. from a crush injury), compromise the blood supply to the nerve (e.g. from entrapment) or both.

Large nerve fibres within a peripheral nerve are the most susceptible to pressure.

Grades of nerve injury

There are three major grades of peripheral nerve injury. The mildest injury is a temporary interruption of conduction without loss of continuity of axons. The intermediate grade

of injury involves loss of continuity of axons but without disruption of endoneurium. More severe injuries have

disruption of endoneurial tubes. In addition to loss of continuity of axons, there is loss of continuity of nerve fibres. If the perineurium is also disrupted in these injuries, there is loss of continuity of nerve bundles and if the epineurium is disrupted as well, the nerve as a whole is severed.

Fig.9.63 Complete nerve disruption

Axonal degeneration

Interruption of an axon results in degeneration of the entire axon distally (including associated degeneration of its myelin sheath) and (for motor neurons) is coupled with subsequent muscle atrophy. This is termed ‘antegrade’

degeneration. There is also a variable degree (depending on severity) of degeneration proximally. This ranges from a short distance of axon, to the cell body itself and (for severe injuries) may even include neurons that synapse with its cell body. This is termed ‘retrograde’ degeneration.

This is demonstrated in both antegrade and retrograde degeneration.

A neuron influences the vitality of its connections.

Axonal regeneration

Nerve cells are highly specialised and have lost the capacity to divide.

Although the cell bodies of neurons in the CNS or PNS may not be replaced, axons in peripheral nerves may regenerate. This is provided they have a track to regenerate along (created by the endoneurium). In contrast, axons in the CNS do not have tubes of connective tissue and do not tend to regenerate. In a peripheral nerve, gradual regeneration can occur (at a rate of approximately 2 mm/day) provided the endoneurium is intact.

Disruption of the endoneurium, with or without perineurium and epineurium (in addition to the axons), results in variable degrees of permanent impairment. Even

if the gap of damage is bridged and scar tissue negotiated, many axons may regenerate along functionally different endoneurial tubes. This may be minimised by careful realignment of nerve fibre bundles in the surgical repair of a severed nerve.

Pain from meninges and dural sleeves

Paradoxically, nerve tissue (including the brain and spinal cord) can be cut painlessly. However, the coverings of the brain and spinal cord, together with their extensions along nerve roots, are highly sensitive to painful stimuli. The meninges of the brain are richly innervated with pain fibres in meningeal branches of cranial and upper cervical nerves. Similarly, meninges of the spinal cord and extensions of them along nerve roots (dural sleeves) are also richly innervated by the recurrent meningeal branch of a spinal nerve.

Inflammation of the meninges (‘meningitis’) results in

severe headache as well as referred pain to all structures supplied by other branches of the same cranial and cervical nerves. It is also accompanied by neck stiffness (due to associated reflex muscle spasm).

Irritation of dural sleeves around nerve roots (e.g. from a prolapsed intervertebral disc) causes severe pain. It may be referred (via the recurrent meningeal branch of a spinal nerve) to the areas of the body supplied by the same spinal cord segment. It is also accompanied by back stiffness (due to associated reflex muscle spasm).

Neuralgia and phantom pain

Direct irritation of fibres within a peripheral nerve or a posterior nerve root may produce neurogenic pain (arising

from the nerve) and/or abnormal sensation along the distribution of that nerve. Neurogenic pain is also known as neuralgia (G. ’nerve’ + ‘pain’).

Herpes zoster (G. ‘creep’ + ‘girdle’) also known as ‘shingles’ is due to reactivation of Varicella zoster

(‘chicken pox’) virus from the sensory ganglion of a spinal or cranial nerve. This condition is characterised by pain (and vesicles) along the cutaneous distribution of the affected nerve.

Fig.9.64 Neuralgic pain

With limb amputation, inadvertent stimulation of fibres associated with the stump may result in pain (‘phantom pain’), and/or abnormal sensation (‘phantom limb),

attributed to the absent extremity The anatomical causes of pain (based on its possible

sources) are somatic, visceral and neurogenic. Somatic and visceral pain ‘directly’ arise from pain

receptors within their respective tissues, while neurogenic pain arises ‘indirectly’ from abnormal activity in nerve fibres that transmit pain.

Chapter 10: Arterial System and Arteries

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ARTERIAL SYSTEM

ARTERIES AND BRANCHES

ANASTOMOSES

END ARTERIES

NEUROVASCULAR SUPPLY OF A VESSEL

ARTERIAL SYSTEM

The arterial system arises from the ventricles of the

heart. It is divided into two separate systems.

Fig.10.1 Arterial system within trunk modules

The pulmonary arterial system arises from the right

ventricle of the heart and consists of the pulmonary trunk and pulmonary arteries.

Fig.10.2 Pulmonary arterial system

The systemic arterial system arises from the left

ventricle of the heart and consists of the aorta and its branches (and their branches in turn).

Fig.10.3 Systemic arterial system

ARTERIES AND BRANCHES

Arteries are vascular tubes that carry blood towards

the tissues. Their branches become progressively smaller, with the direction of flow away from the heart. The term artery (G. ‘air’ + ‘carry’) is a misnomer. Arteries were so named because, prior to Harvey’s concept of blood circulation, they were mistakenly thought to contain air.

Blood is made up of cells (red blood cells, white blood cells and platelets) and plasma. Arterial blood is typically

oxygenated (with oxygen transported principally by red blood cells).

Pulmonary arterial blood

Fig.10.4 Pulmonary arterial blood is deoxygenated


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Throughout postnatal life, blood in the pulmonary arteries (to the lungs) is deoxygenated. Blood becomes oxygenated in pulmonary capillaries by diffusion of oxygen from air in lung alveoli.

Foetal arterial blood

Prior to birth, foetal blood is oxygenated in the placenta and returns via the umbilical vein. This blood enters the foetal heart via the inferior vena cava (after bypassing the liver), mixing there with deoxygenated venous blood, and exits via the aorta (after bypassing the lungs).

Fig.10.5 Foetal umbilical artery blood is deoxygenated

Arteries throughout the foetus have deoxygenated blood mixed with oxygenated blood.

Structure of arteries

Arteries consist of a cylindrical wall surrounding a central channel, the lumen (L. ‘light’, as at the end of a tunnel). The inner layer (intima) of the wall is connective tissue lined by endothelium (G. ‘within’ + ‘nipple’ i.e. an inner surface lining). The middle layer (media) contains

concentrically arranged smooth muscle fibres and elastic fibres. The outer layer (adventitia) is primarily composed

of collagen fibres. The adventitia also contains vasomotor nerve fibres and even vasa vasorum (L. ‘vessels of vessels’).

Fig.10.6 Layers of arterial wall

Elastic and muscular arteries

Elastic arteries (e.g. the aorta) are closest to the heart

and have the largest diameter, with abundant elastic tissue (yellow in appearance) in the media.

Elastic arteries act as ‘conducting’ vessels. Their elastic recoil also prevents a sudden drop in blood

pressure during ventricular filling. Muscular arteries have a media predominantly of

smooth muscle and form the majority of named arteries. Muscular arteries act as ‘distribution’ vessels by

branching extensively and progressively reducing their calibre.

Fig.10.7 Arterial tree and changes in its dimensions

Arterioles

Arterioles are small branches that feed the capillary

bed. They have the largest ratio of wall thickness to lumen calibre, which is maintained by smooth muscle tone controlled by vasomotor nerves. Vasomotor nerves are

almost exclusively part of the sympathetic nervous system (although parasympathetic nerves also supply arterioles associated with erectile tissue).

Arterioles act as ‘resistance’ vessels. Control of changes in their calibre regulates blood flow and blood pressure.

The greatest drop in blood pressure occurs across arterioles.

Capillaries

The thinnest walled vessels are capillaries (L. ‘minute hairs’) consisting of a single layer of endothelial cells (plus basement membrane) permeable to water, electrolytes

and gases as well as cellular nutrients and wastes. Capillaries act as ‘exchange’ vessels, creating a

microcirculation. Water moves across capillary walls according to the

difference between hydrostatic pressure (decreasing from the arterial to the venous end of a capillary) and osmotic pressure. There is net water movement out of the arterial end and into the venous end of a capillary.

10. Arterial System and Arteries

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Fig.10.8 Capillary microcirculation

Sinusoids

Capillary permeability varies at different sites and in different organs. It is also dramatically increased as part of the inflammatory response to tissue injury (producing leakage of fluid into the tissues, with swelling).

Sites often subject to great hydrostatic pressure (e.g. the limbs) have less capillary permeability, minimising fluid leaking out of the vascular system.

Certain organs (e.g. endocrine glands), where the capillary membrane is involved in transport of large molecules (e.g. hormones), have greater capillary permeability. Sinusoids (L. ‘space-like’) are specialised

capillaries with a larger calibre and more sluggish flow. They are found particularly in components of organs with haemopoietic and defence function (i.e. liver, spleen and bone marrow). In these tissues, sinusoids have a modified endothelium with a discontinuous or absent basement membrane (and greater permeability) and phagocytes (G. ‘eat + cells’) that scavenge particles including old blood cells.

The presence of sinusoids enables newly formed red and white blood cells to pass into the vascular system (as well as certain white blood cells to pass out of the vascular system).

Rete mirabile

A rete mirabile (L. ‘net’ + ‘wonderful’) is a capillary bed

located between two arteries. In certain animals arteries may break up into capillary

beds, and then arise as arteries again. Retia mirabilia occur only at special sites (e.g. in the testes of marsupials or base of the brain in grazing animals) for special functions (e.g. temperature regulation influencing spermatogenesis or assisting venous return when the head is dependent, respectively).

In humans, retia mirabilia only occur as microscopic collections termed glomeruli (L. ‘little balls of thread’)

within the cortex of the kidney (between each pair of afferent and efferent arterioles). Their special function is the filtration of plasma. Unlike typical capillaries, those of a rete mirabile are not designed primarily for exchange of gases, cellular nutrients or wastes.

Avascular structures

Structures not derived from mesoderm are avascular (they do not have capillaries).

Tissues that do not possess capillaries include epidermis (ectoderm derived) and all other surface epithelia (primarily endoderm derived).

Capillaries are also absent from hyaline cartilage.

Although mesoderm derived, articular cartilage is subject to continuous compression.

Fig.10.9 Avascular tissues

Arterial branches

In the trunk, arterial branches are classified as parietal (to the body wall) or visceral (to viscera). Arteries often give many branches with a change in direction (and reduction in calibre) tending to occur where a large branch is given off.

Fig.10.10 Parietal and visceral branches of arteries in trunk

Where arteries divide into terminal branches, the larger branch tends to be more directly in line with the main trunk, with the smaller at a greater angle.

Arterial branches to somatic structures (e.g. in the limbs and body wall) may be regarded as cutaneous, muscular, arteriae nervorum (to nerves), nutrient (to long bones) and articular (around joints).


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Fig.10.11 Types of arterial branches

Potential and preferred channels

Vessels develop from mesoderm, commencing in the embryo as capillary networks with interconnecting lumens.

Fig.10.12 Development of arteries from networks

Many of these narrow, while others remain as the preferred channels and subsequently become the major

arterial pathways. However, the capacity to open narrower paths (e.g. after occlusion of the preferred channel) remains. There is also the capacity for considerable anatomical variation of arterial patterns as there is often more than one avenue that may become a preferred channel. Variations of arteries or their branches include origin, number, course and distribution.

Pulmonary and systemic arteries

The cardiovascular system is not only a closed system but also a double system with two distinct blood circulations.

The pulmonary (L. ‘lung’) circulation enables gaseous

exchange between air and blood (via the lungs), while the systemic circulation enables gaseous (and metabolic)

exchange between blood and all tissues of the body.

The arterial component of the pulmonary circulation is made up of the pulmonary trunk (arising from the right ventricle of the heart), right and left pulmonary arteries and their branches in the lungs. Pulmonary arteries transport deoxygenated blood. The blood pressure in pulmonary capillaries is much lower than in systemic capillaries preventing fluid escape into the lung alveoli with impairment of gaseous exchange (particularly as oxygen has very poor solubility in water). The arterial component of the systemic circulation is made up of the aorta (arising from the left ventricle of the heart) together with all of its branches.

Fig.10.13 Systemic pressure is higher than pulmonary

Almost all the arteries in the body (other than pulmonary arteries) are derived from the aorta (and even include bronchial arteries supplying walls of the airways, lymph nodes and visceral pleura).

Systemic arteries transport oxygenated blood. The lung has two circulations: pulmonary (via

pulmonary arteries) and systemic (via bronchial arteries). Systemic arterial blood pressure is higher than pulmonary arterial blood pressure (and much higher than venous blood pressure).

Arterial blood pressure

Blood pressure is the pressure exerted radially on the vessel wall by the contained blood.

Arterial blood pressure is primarily produced by the pressure wave created by contraction of the heart. However, hydrostatic pressure (of the column of blood) due to gravity adds to it. This hydrostatic component (particularly in the lower limbs) is accentuated by standing upright as it increases the length of the column of blood above the specified level. There are two elements to arterial blood pressure: ‘systolic’ and ‘diastolic’. Systolic

(G. ‘with + contract’) pressure is produced by forward expulsion of blood during ventricular contraction. Diastolic (‘between + contract’) pressure is produced by forward expulsion of blood by elastic arteries (between ventricular contractions) while the heart refills with blood from venous return.


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Measurement of blood pressure

Systolic and diastolic blood pressure can both be measured clinically (utilising a sphygmomanometer and cuff) by auscultation (with a stethoscope). The cuff is wrapped around the arm to overlie and (when pumped up) compress the brachial artery. This site is selected because it is at the approximate level of the heart (thus without additional hydrostatic pressure). The diaphragm of the stethoscope is placed over the brachial artery near its termination. Tapping sounds are produced when flow becomes intermittent (between systolic and diastolic blood pressures) as pressure in the cuff is gradually released.

Fig.10.14 Mechanism of flow during systole and diastole

Pulsatile arterial blood flow

Blood flows down a pressure gradient. Blood flow in elastic and muscular arteries is pulsatile (reflecting systole

and diastole) as well as being at high pressure relative to that in other types of vessels. This may be contrasted with the continuous low-pressure blood flow in capillaries. Pulsation of arteries is termed expansile pulsation (and

occurs in all directions). Pulsation through a structure overlying an artery is termed transmitted pulsation.

Features of the arterial pulse include pulse rate and rhythm as well as pulse pressure (the difference between systolic and diastolic pressures).

Clinical examination of the pulse

Pulse rate and rhythm may be detected clinically by palpation of any accessible artery. The radial artery at the wrist is usually chosen because at this site it is easily felt between skin and bone (the distal end of the radius). Pulse volume and character may be detected clinically by palpation of the common carotid artery in the neck against the carotid tubercle (on the transverse process of C6). Palpation should not be performed near the carotid sinus (at the level of C3/4) where compression of baroreceptors may cause reflex bradycardia and subsequent hypotension).

Arteriosclerosis

Changes tend to occur in arteries as they age. Arteriosclerosis (G. ‘artery’ + ‘hardness’) is hardening of

the arteries due to increased fibrous tissue and even calcification in the wall. Aging is accompanied by a

decrease in arterial elastic tissue. They tend to become less compliant (reflected by increased systolic blood pressure). Arteries also tend to become tortuous in old age.

Atherosclerosis and arterial aneurysm

In contrast to the normal variation of arteriosclerosis due to aging, arteries may also undergo pathological changes of atherosclerosis. In atherosclerosis (G. ‘gruel + hardness’) fatty deposits are distributed irregularly along the wall of elastic and muscular arteries, in addition to fibrosis and calcification in the wall. An aneurysm (G.

‘widening’) is a localised dilatation of an artery. It is due to a weakness in the arterial wall (e.g. due to atherosclerosis). Aneurysms tend to increase in size and may rupture, resulting in massive haemorrhage and death.

Haemorrhage

Haemorrhage (G. ‘blood + gush’) is loss of blood from a blood vessel. Haemorrhage may be arterial, capillary or venous and is typically due to external injury of the vessel

wall. It also occurs with rupture of a weakened vascular wall. Vessels constrict and, if possible, plug (by platelet aggregation) the wall defect, minimising blood loss. Blood clotting or platelet defects predispose a patient to haemorrhage, as does increased blood pressure (which also accentuates bleeding).

Fig.10.15 Contrast between arterial and venous bleeding

Haemorrhage can be external (and visible) or internal (hidden in a body cavity, compartment or space).Although blood tends to spurt from elastic and muscular arteries due to the pulsatile flow at higher pressure, large thin-walled veins oozing at lower pressure can cause equally severe blood loss.

Blood loss and drop in blood pressure may cause symptoms (e.g. fainting). Internal haemorrhage may also cause pain due to pressure in the surrounding compartment. However, large compartments (e.g. muscle compartments of the thigh) or body cavities can accumulate dangerous volumes of blood without a significant rise in local pressure (and pain).

First aid management of haemorrhage

RICE is an acronym for the first aid management of haemorrhage. Rest (e.g. immobilizing the part and the

patient) minimizes the rise in systolic blood pressure which


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otherwise occurs with movement and with anxiety. Ice enhances vasoconstriction. Compression over the vessel

is the most important factor in first aid management as it directly arrests bleeding.

Fig.10.16 External and internal haemorrhage

Compression may need to be applied upstream from the damaged vessel at strategic sites (e.g. where it runs over bone and can be compressed). Elevation, when

possible (e.g. of a limb) will reduce hydrostatic pressure. The definitive treatment of a damaged vessel (particularly a large artery or vein) may involve ligation or surgical repair. In severe haemorrhage, fluid replacement or blood transfusion (via a major vein) may also be required to prevent shock (inadequate perfusion of tissues) and maintain blood pressure.

Fig.10.17 Rest, ice, compression and elevation

ANASTOMOSES

An anastomosis (G. ‘through’ + ‘mouth’) is a linking of

tubular structures, such as blood vessels (lumen to lumen, i.e. mouth to mouth). When it occurs without an intervening capillary bed, it offers an alternative (collateral) route.

Anastomosis is the term used for links between arteries or between arterioles (although a special type of anastomosis, between arterioles and venules, occur in certain regions). Links between veins or between lymph vessels are generally termed communications.

Fig.10.18 Arterial and A-V anastomoses in the hand

True anastomoses

Links directly between branches of muscular arteries are typically of large calibre. These are termed ‘true’

anastomoses.

Fig.10.19 True anastomosis

Adjacent (branches of) arteries tend to anastomose with each other.

The more branches the greater the potential for

anastomoses. The labial branches of the facial artery are continuous

with their counterparts from the other side of the body, forming a true anastomosis around the lips. A midline incision through the lips tends to result in arterial blood spurting from both sides.

Other examples of anastomoses include the cerebral arterial circle (of Willis) at the base of the brain, the palmar arches in the hand and the plantar arch in the foot.

Potential anastomoses

Links directly between arterioles are typically of small calibre. These are termed ‘potential’ anastomoses as they

have the capacity to enlarge their calibre.


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Fig.10.20 Potential anastomosis

Anastomoses in continuous organs

Arteriolar anastomoses are extensively located throughout most organs and regions of the body.

Skeletal muscles receive the most arterial branches and contain the majority of anastomoses.

Continuous arterial arcades (via anastomoses between contributing arteries) supply ‘continuous organs’ (e.g. muscles, bones and skin). These structures are in continuity via their connective tissue framework (which like vessels is mesoderm-derived).

Fig.10.21 Anastomoses within a muscle

Anastomoses around joints

Fig.10.22 Alternate pathways open when artery is kinked

Anastomoses occur around joints but are only significant within muscle bellies that cross the joint.

Branches from the main artery provide alternative pathways of flow when the artery is kinked by joint flexion. The anastomoses around joints occur primarily within the surrounding muscles rather than as true anastomoses independent of the muscles.

Arteriovenous anastomoses

Arteriovenous (‘AV’) anastomoses are direct

communications between small arteries and veins without an intervening capillary bed.

The wall is thickened and the lumen diameter can be varied (via neural control of the smooth muscle tone). They are located in areas where there is intermittent blood flow. Arteriovenous anastomoses occur in exposed parts, including the skin of the nose, lips and ears.

Fig.10.23 AV anastomoses and temperature regulation

Arteriovenous anastomoses are involved in temperature regulation, enabling blood to be shunted between superficial and deeper cutaneous vessels.

Glomus tissue

At special sites arteriovenous anastomoses occur in the form of tiny clusters of interconnecting vessels termed glomera (L. ‘balls of thread’). They are most numerous in

the skin of the fingers and toes, particularly digital pads and nail beds. A localised collection of glomus tissue also occurs at the origin of the internal carotid artery (carotid body), at the origin of the internal jugular vein (jugular body) and at the tip of the coccyx (coccygeal body). The carotid body has a high blood flow enabling it to function as a chemoreceptor monitoring the partial pressure of dissolved gases in the plasma. The roles, if any, of the other two are unknown.

Erectile tissue

Arteriovenous anastomoses also occur in erectile (cavernous) tissue, where they are associated with vascular spaces (venous sinuses) arranged like a honeycomb and capable of expansion. Erectile tissue is present in the nasal mucosa, where it warms and humidifies inspired air. Erectile tissue is especially prominent in the penis and clitoris. A tube of dense connective tissue (tunica albuginea) surrounds a mesh of

venous sinuses in the corpora cavernosa of the penis.


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Arterial vasodilatation (of dorsal and deep arteries of the penis, stimulated by parasympathetic nerve fibres) coupled with restriction of venous drainage (via the deep dorsal vein, compressed against the penile fascia) may create considerable turgor. The penis thus elongates and becomes rigid prior to coitus.

Fig.10.24 AV anastomoses in erectile tissue of penis

END ARTERIES

An end artery is an artery isolated from others. Its

branches do not appear to link with those of another artery. The territory it supplies is dependent on that single vessel.

Anatomical end artery

An anatomical end artery is a single artery, which

does not form anastomoses with another artery. The entire territory of distribution for an anatomical end artery is compromised by its occlusion. Branches of cerebral arteries are anatomical end arteries. These supply the brain by pushing into its substance from the exterior.

Arterial branches to individual segments of the kidneys and of the liver are also anatomical end arteries.

The retina may be regarded as an outgrowth from the brain. The sole arterial supply to the critical layers of the retina is from its central artery, a long slender vessel that is the classic example of an anatomical end artery. Its branches radiate from the optic disc.

Fig.10.25 The anatomical end artery to the retina

Effect of central retinal artery occlusion

Occlusion of the central retinal artery causes total blindness of the affected eye.

Fig.10.26 Retina with central artery branches and optic disc

Functional end arteries

Functional end arteries take part in potential (small

calibre) anastomoses with each other at the arteriolar level. Part of their territory of distribution will not remain viable if one is suddenly occluded. However, gradual occlusion may enable time for existing anastomotic branches (collaterals) to dilate. This creates a collateral avenue of supply (without development of new branches). Coronary arteries are functional end arteries. Although terminal branches of the right and left coronary arteries anastomose with each other, these are only at the arteriolar level.

Fig.10.27 Coronary arteries are functional end-arteries

Effect of sudden coronary artery occlusion

Because occlusion of a coronary artery is usually sudden, the majority of its territory of distribution typically undergoes infarction unless the occlusion is cleared rapidly.

End organs

An ‘end organ’ is a body part or organ that is isolated

from others. It tends to be supplied by a single artery or at least via a single avenue of arterial supply.

End organs are particularly vulnerable to having their arterial supply cut off.

Arteries to terminal body parts

Although terminal body parts (e.g. fingers, toes, penis and tip of the nose) each receive more than one artery, these tend to be via a single avenue and may (collectively) be regarded as end arteries.


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Fig.10.28 Terminal body parts are supplied by end-arteries

Vulnerability to vasoconstrictors

Vasoconstrictor drugs (e.g. adrenaline) should not be injected into the digits or penis. These drugs can produce intense arterial spasm resulting in death of tissue in these terminal body parts.

Fig.10.29 Contraindicated sites for vasoconstrictors

Arteries to blind ending organs

Blind ending organs typically project into or are suspended within a cavity.

The appendix and the gall bladder are hollow viscera protruding into the abdominal cavity. Each is an outpouching from the gastrointestinal and the biliary tract, respectively and is supplied by an end artery (unlike other hollow viscera of these tracts). The appendicular artery runs to the tip of the appendix and the cystic artery runs to the fundus of the gall bladder.

Fig.10.30 The vermiform appendix and its end-artery

The spleen is a solid organ suspended within the abdominal cavity by its attachment at the splenic hilum where it receives the splenic artery. The brain and spinal cord are suspended within the cranial cavity and vertebral canal, respectively, while the heart is suspended within the

pericardial cavity. Although each receives more than one artery of supply they may collectively be regarded as end arteries, because there are only arteriolar anastomoses between branches within their substance. The retina, as an outpouching of the brain, is an ‘end organ’s end organ’.

Organs with avascular barriers

The arteries supplying the epiphyses and metaphyses of growing long bones are end arteries. Epiphysial and metaphysial arteries are end arteries because the cartilaginous epiphysial (growth) plate is avascular and forms a barrier preventing communication between them (until epiphysial fusion occurs).

Fig.10.31 End arteries in a developing long bone

Arteries to visceral segments

The branches of arteries supplying solid viscera that are divided into separate vascular segments (e.g. kidney and

liver) are typically end arteries. The lungs are also divided into separate

(bronchopulmonary) segments, although each receives a dual arterial supply (bronchial and pulmonary).

Danger of ligating a segmental artery

Ligation of a segmental artery compromises the

arterial supply to that segment. A triangular wedge of dead tissue with its apex towards the hilum typically results.

Fig.10.32 Segments of the kidney and their end arteries

End tissue

Within an organ, the furthest area from its arterial source may be regarded as ‘end tissue’ as it tends to be supplied by terminal arterial branches.

End tissues within end organs are most vulnerable to having their arterial supply interrupted.


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The periphery of solid organs (e.g. spleen and kidney) receiving supply via branches that penetrate from a central hilum, may be regarded as end tissue. This will even include the capsule and/or serosa for those solid organs (e.g. spleen) suspended within a cavity as they do not receive any additional external supply. However, central parts of the brain and spinal cord are most vulnerable because their arterial supply penetrates from their exterior. Similarly, the coronary arteries pass around the external (epicardial) surface of the heart, with branches penetrating the myocardium. The deepest part of the wall of a heart chamber is the endocardium lining its internal surface. The endocardium itself is bathed in the blood inside the chamber. However, subendocardial myocardium may be regarded as end tissue and is therefore particularly vulnerable to ischaemia in coronary disease.

Within hollow viscera, internal (mucosal) surfaces are furthest from arteries that penetrate from the external (serosal) surface. Epithelia are avascular (despite being highly metabolic) relying on diffusion via capillaries derived from terminal arterioles in the underlying connective tissue (lamina propria) of the mucosa. The mucosal lining of a hollow viscus is particularly endangered by interruption of its arterial supply.

Types of arterial occlusion

Arterial occlusion may be classified into three types based on the location of its source relative to the wall. Internal (Intraluminal) occlusion is from within the lumen of an artery (e.g. by a thrombus or an embolus). Intramural

occlusion is from within the wall (e.g. thickening of the intima by atherosclerosis or spasm of smooth muscle in the media). External (extramural) occlusion may occur from

compression or ligation.

Effects of anatomical end artery occlusion

Arterial occlusion to an end artery produces potentially serious adverse effects. These range from decreased blood supply, termed ischaemia (G. ‘keep back + blood’)

to tissue death from complete loss of supply, termed infarction (L. ‘stuffing’).

Sudden occlusion of an anatomical end artery compromises its entire territory of supply. Even gradual occlusion of an anatomical end artery affects its entire territory of supply. This occurs because there is no other avenue (new vessels not being created). Occlusion is particularly significant for anatomical end arteries that supply the whole organ. Central retinal artery occlusion causes total blindness of the affected eye.

Effects of functional end artery occlusion

Sudden occlusion of a functional end artery compromises part of its territory of supply. Not all of the area is affected because there are peripheral anastomoses (although these are limited to arterioles). Gradual occlusion of a functional end artery may allow time for an adequate collateral circulation to develop. This can only occur by dilation of existing avenues (potential anastomoses which increase their calibre) and not by formation of new vessels.

Arterial occlusion to vital areas

Occlusion of end arteries supplying an organ with specific functions corresponding to an anatomical location (e.g. the brain and the heart) is potentially most serious. Occlusion of even a relatively small branch of a cerebral or of a coronary artery may produce tissue death in a vital area, resulting in profound effects (e.g. hemiplegia or cardiac arrhythmia, respectively).

In contrast, although segmental arteries to the kidney or liver are end arteries, the functional reserve of the rest of the organ may compensate for death of one or more segments.

Inadvertent ligation or injection

Care must be taken during surgery to avoid inadvertent ligation of an end artery (e.g. of a posterior intercostal artery that may supply part of the spinal cord or of accessory renal arteries that may supply segments of the kidney). Care must be taken during general anaesthesia to avoid inadvertent intra-arterial injection of drugs that produce intense vasoconstriction or during local

anaesthesia to avoid injecting vasoconstrictor drugs into terminal parts.

Thrombosis and embolism

A blood clot formed in the vascular system of a living person is termed a thrombus (G. ‘clot’). This is in contrast

to a post mortem clot (occurring after death) or a haematoma (occupying tissues outside the vascular system). Thrombosis may be due to endothelial damage, decreased blood flow or abnormal blood constituents.

A substance transmitted by the blood stream that lodges in a vessel is an embolus (G. ‘plug’). A thrombus,

or part of one, that dislodges (and is transmitted by the blood stream to a distant site) becomes a thromboembolus.

An embolus tends to occlude arteries because they progressively narrow by branching.

An embolus within an artery tends to lodge immediately distal to a branch point, where the main artery narrows.

An embolus is particularly dangerous if an end artery is

occluded.

Fig.10.33 Emboli tend to lodge at branch points

Pulmonary embolus

Emboli originating from systemic veins (e.g. deep veins

in the calf) pass through the right atrium and ventricle, then into the pulmonary arterial system, to occlude a pulmonary artery or branches of it in the lung. Pulmonary emboli may also arise from the right side of the heart.


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Fig.10.34 Paths of pulmonary emboli from a systemic vein

Pulmonary emboli are life threatening if large, occluding a major branch of a pulmonary artery, or multiple, involving many branches. Pulmonary emboli tend to cause breathlessness and chest pain (due to the associated area of pulmonary infarction). However, symptoms range from none to sudden death.

An area of lung affected from a large embolus typically appears red (a haemorrhagic infarct) due to the

continued arterial supply from bronchial arteries, which bleed because they are not able to keep the lung tissue alive on their own. Typical infarcts elsewhere are white (a pale infarct) due to absence of blood.

Fig.10.35 Paths of emboli through and from right atrium

Systemic arterial embolus

Emboli originating from the left atrium or ventricle (or associated valves) may occlude systemic arteries (e.g. a

cerebral artery via the common carotid artery and a femoral artery via the descending aorta).

Emboli may also arise from within systemic arteries (e.g. from a plaque of atheroma at the origin of internal carotid artery). Its effects are potentially much more harmful if they involve end-arteries (e.g. even a small embolus in the central artery of the retina may produce sudden blindness in one eye).

Fig.10.36 Paths of systemic emboli from left side of heart

NEUROVASCULAR SUPPLY OF A VESSEL

Vessels have both a nerve supply and a vascular supply.

Vascular tone

Vascular smooth muscle tends to be in a state of continuous partial contraction (tone), modulated by (visceral) motor nerves. This is most significant in arterioles, the vessels primarily involved in the regulation of blood flow and blood pressure.

Vasomotor nerve fibres

Motor nerve fibres supplying blood vessels are termed vasomotor nerve fibres. These are almost exclusively part

of the sympathetic nervous system (although arterioles associated with erectile tissue also receive fibres from the parasympathetic nervous system that result in vasodilatation).

Fig.10.37 Vasomotor fibres to media of a blood vessel

Sympathetic (postganglionic) fibres enter the ventral ramus of each spinal nerve (via a grey ramus communicans from the sympathetic trunk).

Many of these fibres then pass through the major limb plexuses to be distributed (via associated peripheral nerves) to arteries in the limbs. These arteries receive their vasomotor nerve fibres at multiple levels as tiny branches from the neighbouring peripheral nerves. In addition to


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supplying arteries, sympathetic postganglionic fibres may also form plexuses along certain arteries (e.g. arteries to the head and to abdominal viscera) and are distributed with their branches.

Vasa vasorum

Fig.10.38 Vasa vasorum of aorta

The endothelium and intima receive nutrition directly by diffusion from blood in the lumen of a vessel.

However, the media and adventitia (particularly of elastic and muscular arteries) require a blood supply from vessels of their own, termed vasa vasorum (L. ‘vessels of vessels’). Vasa vasorum include arteriae vasorum as well as venae vasorum.

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VENOUS SYSTEM VEINS AND TRIBUTARIES

VENOUS VALVES AND VENAE COMITANTES

VENOUS SINUSES AND COMMUNICATIONS

VENOUS SYSTEM

The venous system comprises three separate major systems.

Fig.11.1 Venous system within major trunk modules

The pulmonary venous system drains into the left

atrium of the heart and is made up of pulmonary veins.

Fig. 11.2 Components of pulmonary venous system

The systemic venous system drains into the right

atrium of the heart. It has four sub-systems: superior vena caval, inferior vena caval, azygos and vertebral.

The azygos vein drains into the superior vena cava

(SVC). It also links the superior vena caval system and the inferior vena caval system, although its connection to the latter is usually minor. The azygos system drains the vertebral system (via lumbar and posterior intercostal

veins).

Fig. 11.3 Components of systemic venous system

The portal venous system drains blood from the gut

into the liver where it branches into a capillary bed, then forms the hepatic veins. These veins, in turn, drain into the inferior vena cava (IVC).

Fig. 11.4 Components of portal venous system

VEINS AND TRIBUTARIES

Veins are vascular tubes that carry blood away from

the tissues. This occurs by a progression of larger tributaries, with the direction of flow towards the heart. Venous blood is typically deoxygenated (oxygen transported by red blood cells having been mostly taken up by the tissues after diffusing through capillaries).


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Fig.11.5 Venous tree and changes in its dimensions

Pulmonary venous blood

Throughout postnatal life, blood in the pulmonary veins (from the lungs) is oxygenated. This process occurs in pulmonary capillaries by diffusion of oxygen from air in lung alveoli.

Fig.11.6 Pulmonary venous blood is oxygenated

However, blood in pulmonary veins (and subsequently systemic arteries) is not quite fully saturated with oxygen, as tributaries of bronchial veins do not extend to the peripheral parts of the bronchial tree, lung parenchyma and visceral pleura. Venous blood from these areas drains into tributaries of the pulmonary veins. This phenomenon has been termed ‘the physiological shunt’.

Blood in foetal umbilical vein

Prior to birth, a foetus receives oxygenated blood via the umbilical vein (from the placenta). This blood enters the inferior vena cava (after bypassing the liver) where it becomes mixed with deoxygenated blood. It then enters the heart and exits via the aorta (after bypassing the lungs).

Fig.11.7 Foetal umbilical vein blood is oxygenated

Structure of veins

Veins are more numerous than arteries. However, like arteries, veins consist of a tubular wall surrounding a central channel, termed the lumen. The wall comprises an (inner) intima lined by endothelium, a media with smooth muscle and an (outer) adventitia.

Fig.11.8 Layers of venous wall

Veins have thinner walls and larger lumens than arteries, correlating with their lower pressure (and slower rate of flow) and greater volume.

Veins act as ‘capacitance’ vessels. They contain most of the blood volume.

Venules and venous tributaries

The smallest venous tributaries are termed venules.

These vessels directly drain the capillary bed. They correspond to arterioles (which directly supply the capillary bed) but have much thinner walls and, in particular, much less smooth muscle. Unlike arterioles, the direction of flow in venules is from smaller to larger vessels.

Fig.11.9 Superficial abdominal veins and tributaries

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Venous tributaries correspond to arterial branches. However, as well as having thinner walls, they are more numerous (and more variable than branches of arteries) with many tributaries remaining un-named. This correlates with their development from even more extensive networks (creating more opportunity for variation).

Managing venous bleeding in surgery

Numerous veins are encountered during surgical procedures. Venous tributaries can usually be diathermied, clamped or ligated without adverse effect. There are many alternative pathways for venous return.

Pulmonary venous system

The venous component of the pulmonary circulation comprises tributaries of the pulmonary veins (in the lungs). These drain into two right and two left pulmonary veins that empty directly into the left atrium of the heart.

Pulmonary veins transport oxygenated blood (which is almost fully saturated with oxygen) towards the heart.

Systemic venous system

The venous component of the systemic circulation comprises tributaries of the superior vena cava and inferior vena cava. These empty deoxygenated blood directly into the right atrium of the heart.

Fig.11.10 Systemic venous systems

The azygos (G. ‘not yoked’ i.e. unpaired) vein is a

partial bypass between the superior and inferior vena cava (helping equilibrate pressure between them). It is primarily located in the thorax. In addition to draining the posterior abdominal and thoracic wall, azygos vein tributaries receive vertebral veins. Vertebral veins include internal and external vertebral venous plexuses (networks located

inside and outside the vertebral canal). Blood cells produced in the vertebral column enter the circulation via them.

Hepatic portal venous system

A portal venous system begins as well as ends in capillaries.

A portal system of veins links two capillary beds at low pressure.

The hepatic portal venous system is regarded as ‘the’ portal system. It consists of the portal vein that enters the liver at the porta hepatis (G. ‘gateway’ + ‘liver’), together

with tributaries of the portal vein (prior to its formation) and branches of the portal vein (within the liver).

Fig.11.11 Hepatic portal venous system

The portal system drains the gastrointestinal tract, pancreas and spleen. Although portal venous blood is deoxygenated, it transports absorbed nutrients from the small intestine and hormones (secreted from the pancreas) to the liver. The liver also filters absorbed toxins. Veins of the hepatic portal system are characterised by their branching and subsequent termination in another capillary bed (the hepatic sinusoids). Tributaries of hepatic veins

(belonging to the inferior vena caval system), in turn, drain the hepatic sinusoids.

Hypophyseal portal venous system

The hypophyseal portal system consists of tiny veins

linking capillary beds between the hypothalamus (at the base of the brain) and the hypophysis (G. ‘under +

growth’, i.e. the pituitary gland suspended from the brain).

Fig.11.12 Hypophyseal portal venous system


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These veins transport special hormones (releasing factors from nerve endings) to regulate anterior pituitary hormone production.

VENOUS VALVES AND VENAE COMITANTES

Role and structure of venous valves

Venous valves (L. ‘flaps’) are folds of endothelium lining veins, typically with a pair of cusps. They allow blood

to flow in one direction only, thus directing venous return towards the heart.

Fig.11.13 Venous valves allow flow in one direction only

Sites of venous valves

A valve in a vein is also often located just distal to the entry of a major tributary (as well as at the termination of the tributary)

A valve is typically located at the termination of a vein.

Fig.11.14 Valves located near entry of a major tributary

Valves in long veins of limbs

Valves in the superficial and deep veins of the limbs direct flow from distal to proximal (generally against gravity) to prevent pooling of blood distally.

Fig.11.15 Numerous valves in long veins of limbs

Valves are particularly numerous in long veins. They break up what would otherwise be a continuous column of blood into shorter units. Each valve in the series takes a share of the hydrostatic pressure (which is considerable when standing upright). Valves are also present in veins (termed perforating veins) connecting superficial and deep systems. Their valves direct flow from the superficial veins to the deep veins and are most important in the calf where blood is pumped upwards against gravity by muscles surrounding the deep veins.

Venae comitantes

Venae comitantes (L. ‘veins’ + ‘accompanying’) are a

pair of companion veins wrapped around an artery. These veins also intercommunicate.

Fig.11.16 Venae comitantes


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These veins conserve heat (by transfer of heat from warm arterial blood to cool venous blood returning from the periphery). Venae comitantes are primarily located in the limbs, particularly distally. Their arrangement around the artery also assists venous return.

Fig.11.17 Heat conservation by venae comitantes

Venous flow

Venous return is directed towards the atria of the heart (via systemic veins to the right atrium and via pulmonary veins to the left atrium). Venous flow is due to blood pressure, contraction of adjacent skeletal muscles and the oscillation of intra-thoracic pressure with respiration.

Vascular venous pump

The arrangement of venae comitantes (coupled with the presence of valves) aids venous flow in the periphery.

Fig.11.18 Vascular venous pump

The connective tissue around the vascular bundle tends to resist the expansion associated with each arterial pulsation, compressing blood within the pair of veins.

Venous flow is directed proximally due to the presence of venous valves (even though the arterial flow is in the opposite direction).

Muscular venous pump

The muscular venous pump is the main factor

responsible for flow from peripheral veins. In the limbs, fascial sheets and septa (e.g. of the leg) subdivide muscle compartments. Contraction of the belly of a skeletal

muscle compresses (deep) veins within the associated compartment.

Fig.11.19 Muscular venous pump

Thoracic venous pump

Venous return in the trunk is via a double pump

mechanism that is coupled to respiration.

Fig.11.20 Thoracic venous pump

Descent of the diaphragm during inspiration, in addition to creating a negative intrathoracic pressure, shortens the inferior vena cava emptying it (into the right atrium of the heart from below) while the superior vena cava lengthens and fills. During expiration the converse occurs as the superior vena cava shortens and empties (into the right atrium from above), while the inferior vena cava lengthens and fills. The thoracic pump therefore augments venous

return from both caval systems (if not from below in inspiration, then from above in expiration).


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Valveless veins of the trunk

Venous blood is trapped within the trunk by major valves above, at the junction of the thorax with the neck (in the terminations of the subclavian and internal jugular veins), and below, at the junction of the abdomen with the lower limbs (in the termination of the femoral veins).

The veins of the vena caval systems traversing body cavities of the trunk, together with the entire vertebral and azygos systems of veins, are valveless.

This means that flow may occur in either direction within

these systems. Flow may also occur in either direction between these systems (due to valveless communicating veins).

Fig.11.21 Blood trapped in trunk by valves above and below

Thus, raising intrathoracic and intra-abdominal pressure (coughing, straining) shunts blood into the vertebral system (via the azygos system from the caval systems). Conversely, inspiration creates a suction that shunts blood from the vertebral system to the caval systems.

Varicose veins and haemorrhoids

A varicose vein (or varix) is an abnormal dilatation of a

vein, which also may become elongated and tortuous.

Fig.11.22 Features of a varicose vein

Varicose veins tend to become more prominent with prolonged elevation of venous pressure. Oesophageal varices are dilated veins under the surface lining of the lower oesophagus, associated with elevation of venous pressure in the portal system (portal hypertension).

They tend to bulge into the lumen and may rupture producing catastrophic bleeding.

Haemorrhoids are dilated veins under the surface

lining of the anal canal, associated with chronic straining (e.g. from weightlifting, constipation or coughing) or pregnancy (e.g. from the foetal head compressing pelvic veins). They tend to bulge into the lumen and may bleed, or if large, prolapse beyond the external anal sphincter. Haemorrhoids may even thrombose.

Fig.11.23 Haemorrhoids and prolapse with straining

Venous valve incompetence

Structural damage to valve cusps (e.g. from venous thrombosis) may result in venous valve incompetence.

The cusps fail to close adequately allowing flow in the reverse direction.

Fig.11.24 An incompetent venous valve and effect on flow


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Incompetent valves of perforating veins in the leg are particularly significant as the muscular venous pump shunts blood back under pressure from deep veins (surrounded by the calf muscles) to superficial veins (not surrounded by muscles) where the blood pools. Circulation to the skin is impaired by the high venous blood pressure and the skin may ulcerate and heal very poorly.

Fig.11.25 Effect of incompetent valve in perforating vein

A dilatation of a vein at the site of a venous valve may also result in impairment of valve function with the potential for a domino effect further along the vein. This occurs particularly with long veins subjected to periods of high hydrostatic pressure (e.g. superficial veins of the lower limb in prolonged standing). Varicose veins may therefore be both cause and effect of valve incompetence.

VENOUS SINUSES AND COMMUNICATIONS

Venous sinuses (L. ‘hollow’) are normal dilatations of

veins. These occur in special sites, including the mesh of minute venous sinuses capable of expansion in cavernous (erectile) tissue.

Fig.11.26 Venous sinuses and deep vein in calf

Calf muscle venous sinuses

Extensive venous sinuses occur in the soleus muscle of the calf. Blood tends to pool in these soleal venous sinuses with gravity (e.g. from prolonged standing) and without muscular activity (e.g. prolonged bed best).

Fig.11.27 Calf muscle venous pump

Repeated contraction of the calf muscles (e.g. by raising the heel and flexing the toes) promotes venous return by enlisting the muscular venous pump and minimises pooling of blood.

Dural venous sinuses

Dural venous sinuses are endothelial-lined spaces

within the cranial cavity between the outer and inner layers of dura mater (L. ‘tough mother’) surrounding the brain.

Fig.11.28 Venous sinus between the two layers of dura

These venous sinuses are unique in that they are enclosed by dense connective tissue. Tension in the fibrous walls keeps the sinuses open despite the negative intracranial venous pressure, which would otherwise collapse them when standing upright.

Perforating veins

Links between veins are generally termed venous communications. Communications between superficial and deep veins in the lower limbs are termed perforating veins. They perforate the deep fascia (via openings in it). Valves are present in perforating veins (of particular importance in the calf) directing flow from superficial to deep.


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Fig.11.29 Perforators link superficial to deep veins

Emissary veins

Communications between intracranial and extracranial veins are termed emissary (L. ‘out + send’) veins. They

arise from the dural venous sinuses and exit via foramina in the skull.

Fig.11.30 Emissary veins pass via foramina in the skull

Azygos venous system

The azygos system of veins may be regarded as a large set of venous communications.

Fig.11.31 Valveless azygos venous system

The azygos system links the inferior vena caval system with the superior vena caval system, drains the vertebral system and communicates with the portal system. The azygos venous system is valveless so flow may occur in either direction and through its communications with the other systems equilibrates pressure between them.

Portal-systemic anastomoses

Communications between the portal system and the systemic system have the special term portal-systemic anastomoses. The portal venous system is valveless so

flow may occur in either direction.

Fig.11.32 Major site of portal systemic anastomosis

The two major sites of portal-systemic anastomosis are at the lower end of the oesophagus (with the azygos system) and at the anal canal (with the inferior vena caval system).

Venous plexuses

Just as venous tributaries tend to be more numerous than arterial branches, they communicate more freely (and have even more capacity for variation). Tributaries of veins tend to form intercommunicating networks where they are particularly numerous. These networks are termed venous plexuses.

Fig.11.33 Venous plexus in spinal canal

Many venous plexuses are found around pelvic viscera (e.g. bladder, rectum and vagina) accommodating to changes in shape of these organs. They are also extensive as a cushion in the sole of the foot and within the vertebral canal (where they help cushion the spinal cord).


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Deep vein thrombosis in the calf

The deep veins of the calf are predisposed to thrombosis if surrounding muscles are not contracting

regularly (particularly in postoperative, postpartum or bedridden patients and from long aeroplane flights).

Fig.11.34 Deep vein thrombosis in calf from stasis

Stasis and pooling of blood in soleal venous sinuses occurs if not emptied by regular contraction of the calf muscles.

A thrombus in a deep calf vein, especially one that

propagates proximally, may dislodge (or part of it may break off) to become a thromboembolus. Pulmonary

thromboembolism (occlusion of a pulmonary artery or major branch of it in the lung by a thromboembolus) is a potential consequence of deep vein thrombosis originating in the calf and is life-threatening.

Thromboemboli are carried via the inferior vena cava and the right side of the heart into the pulmonary arterial system. One or more arteries subsequently become occluded, as they become progressively narrower by branching.

Thromboemboli are more common in veins than arteries because of the more sluggish flow. Usually these are small and are filtered by the lungs without damage. Organs supplied by systemic arteries are protected as all venous blood passes through the pulmonary capillary bed before proceeding to systemic arteries.

Alternative routes of venous return

There are numerous communications between tributaries of neighbouring veins. Venous occlusion, unless very extensive or of a major large vein, is usually not a problem as blood may return via many possible venous routes.

Unlike certain arteries (end arteries) or their branches, it is generally safe to ligate a vein or tributary.

Venous spread of tumours and infections

Tumours and infections in organs can spread via the veins that drain them. Venous blood with cancer cells or microbes may be carried to larger veins within the associated system and to veins of a communicating system. Ultimately they may pass to the liver (via the portal venous system) or to the lungs (via the vena caval systems and right side of the heart). The liver and lungs are common sites of tumour metastases. Proliferation of microbes within the blood stream (septicaemia) and

passage through pulmonary capillaries results in spread of infection throughout the body via the arterial system.

Prostate cancer commonly spreads via veins to

vertebral bodies. Communications between the prostatic venous plexus and the internal vertebral venous plexus provide the pathway, while the absence of valves allows retrograde flow to the vertebral column.

A potential avenue of spread for infections of the face is to venous sinuses in the cranial cavity via (valveless) emissary veins that communicate with them. This may lead to a septic thrombosis (e.g. cavernous sinus thrombosis).

Venous congestion and oedema

Increased venous pressure creates an abnormal pooling of blood in veins within organs, termed venous congestion (L. ‘bring together’).

Water moves across capillary walls according to the difference between hydrostatic pressure (decreasing from the arterial to the venous end of a capillary) and osmotic pressure. Affecting this equilibrium (e.g. by increased venous pressure, increased capillary permeability or decreased plasma protein osmotic pressure) causes an abnormal accumulation of tissue (interstitial) fluid, termed oedema (G. ‘swelling’).

Back pressure from a failing right ventricle of the heart (‘right heart failure’) leads to congestion in systemic veins and peripheral oedema. Oedema around the ankles tends

to occur from standing, while oedema over the sacrum tends to occur from lying supine. Back pressure from a failing left ventricle of the heart (‘left heart failure’) leads to congestion in the lungs. This may progress to accumulation of fluid there (pulmonary oedema).

Increased pressure in the portal venous system tends to produce venous congestion and abnormal accumulation of fluid within the peritoneal cavity, termed ascites (G. ‘bag’). Increased venous pressure (e.g. from portal hypertension) may also produce dilated (varicose) veins.

Eventually varices tend to develop at sites of portal-systemic anastomosis (particularly the lower end of the oesophagus). Rupture of oesophageal varices produces

severe haemorrhage, often resulting in death.

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LYMPHATIC AND HAEMOPOIETIC SYSTEMS

LYMPH VESSELS

LYMPH RETURN

LYMPH NODES

LYMPHOID ORGANS AND TISSUES

LYMPHATIC AND HAEMOPOIETIC SYSTEMS

Fig.12.1 Lymphatic and haemopoietic systems in modules

Fig.12.2 Components of lymphatic & haemopoietic systems

The lymphatic system consists of lymph vessels, lymph nodes, lymphoid organs and lymphoid tissues

(in other organs). The haemopoietic system consists of haemopoietic tissue in bone marrow.

Roles of lymphatic system

The roles of the lymphatic system are fluid return and defence.

Lymph (L. ‘clear fluid’) is fluid within lymph vessels.

Lymph vessels return lymph to the circulation (via the venous system). More than five litres of fluid per day (containing plasma protein), escapes from arteriovenous capillaries. This would otherwise accumulate in the interstitial fluid compartment (between intracellular and intravascular fluid compartments).

Fig.12.3 Interstitial fluid filtered via lymph nodes

The volume and the protein content of lymph vary from site to site. They are particularly high where capillaries are most permeable (e.g. from sinusoids of the liver). Lymph carrying foreign material is transmitted via lymph vessels to lymph nodes, where it is filtered and brought in contact with defence cells.

Roles of haemopoietic tissue

Active bone marrow is red. It is the site where red blood cells, certain white blood cells and platelets are produced. Red marrow remains in the axial skeleton, but in the limbs becomes yellow marrow during adolescence. Yellow marrow has the potential to revert to red marrow in certain circumstances (particularly after major haemorrhage).

LYMPH VESSELS

Lymph capillaries

There are two types of lymph capillaries. Superficial (initial) lymph capillaries are located directly under an epithelium. In the skin, they are found in the papillary layer of the dermis. Deep lymph capillaries are located in the reticular layer of the dermis.

Initial lymph capillaries have a blind origin. This distinguishes them from other capillaries, although the wall of both lymph capillaries and arteriovenous capillaries is made up of a single endothelial layer. Tiny filaments (of fibrillin), between the endothelial cells and the surrounding extracellular matrix, produce temporary intercellular gaps when interstitial fluid volume increases.

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Fluid enters the lymph capillary until interstitial volume reduces, slackening the filaments with closure of the gaps. Initial lymph capillaries are saccular and have no basement membranes.

Deep lymph capillaries are transitional in structure, between initial capillaries and lymphatics. There are occasional valves, an intermittent basement membrane and patches of smooth muscle cells in the surrounding wall.

Where abundant, lymph capillaries link freely to form communicating networks.

Fig.12.4 Features of initial lymph capillaries

Lymph capillary plexuses

Lymph capillaries are most numerous beneath surface epithelia.

Skin and mucous membranes, being the surface of the body, are its first line of defence. Lymph capillaries are particularly abundant in dermis (the subepidermal layer of the skin) and lamina propria (the subepithelial layer of mucous membranes).

Fig.12.5 Lymph capillary plexuses in dermis

Tissues without lymph capillaries

Lymph capillaries are present only in tissues derived from mesoderm.

Tissues that do not possess lymph capillaries include epidermis (ectoderm derived) and other surface epithelia

(primarily endoderm derived). Lymph capillaries are absent from the central nervous system (ectoderm derived).

Although abundant just deep to surface epithelia, lymph capillaries are absent from the epithelia themselves. Lymph capillaries are also absent from hyaline articular cartilage. Although mesoderm-derived, articular cartilage has a solid matrix and is subject to continuous compression which would collapse any lymph (or blood) capillaries if present.

Fig.12.6 Tissues where lymph capillaries are absent

Lymphatics

Lymph capillaries drain into progressively larger tributaries termed lymphatics. Those from the skin drain

into lymphatics located in the subcutaneous tissue. Although these vessels have thicker walls than lymph capillaries, they are still at low lumenal pressure (and are therefore easily compressed). They possess a basement membrane, circumferential smooth muscle cells and pacemaker cells (producing spontaneous rhythmic contractions). Lymphatics resemble veins and venous tributaries. However, as well as having thinner walls, they are more numerous (and more variable than veins and their tributaries). This correlates with their development from even more extensive networks (creating more opportunity for variation). Lymphatics have valves (formed by infolding of the endothelium) for one-way flow. The flow is directed ultimately to the venous system.

Fig.12.7 A lymph node and associated lymphatics


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Lymph from a particular organ or body area normally drains through at least one set of lymph nodes before reaching the venous system.

Lymph nodes lie in the course of lymph vessels. Afferent lymphatics enter lymph nodes, while efferent

lymphatics leave them. Afferent lymphatics tend to be multiple (several entering a lymph node around its periphery), while efferent lymphatics are typically single (leaving a lymph node via its hilum).

Fig.12.8 Excised inguinal lymph node with its lymphatics

Lymphatic pathways

As well as having a similar structure to veins, lymphatics have a common direction of flow with them. Lymphatics also tend to accompany veins.

Fig.12.9 Relationship between lymphatics and veins

Although superficial lymphatics accompany superficial veins, deep lymphatics do not always accompany deep veins.

In the abdominal cavity, lymphatics accompany arteries to their origins from the front of the aorta (rather than accompany the portal vein to the liver).

Lacteals

Intestinal lymphatics are termed lacteals (L. ‘milk’). This is because they contain chyle (L. ‘juice’), lymph rich in lipid

molecules absorbed after a meal. Lacteals do not accompany veins (as intestinal veins are part of the portal venous system to the liver). Instead, they accompany the artery of the midgut (an unpaired branch of the aorta). As a result, large lipid molecules (primarily triglycerides in the form of chylomicrons) are conveyed to systemic veins via

a large lymph collecting duct. The other nutrients, in contrast, are absorbed from the small intestine directly into portal venous blood.

Lymph trunks

Lymphatics are tributaries of lymph trunks. These larger lymph vessels typically accompany major blood vessels.

The paired jugular, subclavian and bronchomediastinal lymph trunks collect lymph from the

head and neck, upper limb, and thorax, respectively. The jugular lymph trunk accompanies the internal jugular vein and the subclavian lymph trunk accompanies the subclavian vein. The bronchomediastinal lymph trunk is atypical in that it runs independently of blood vessels. The unpaired intestinal lymph trunk and paired lumbar lymph

trunks drain the abdomen, pelvis and lower limb. These lymph trunks accompany the aorta or its branches.

Fig.12.10 Relationship of lymph vessels to blood vessels

Thoracic duct and right lymphatic duct

Lymph trunks typically drain into a lymph duct. On the

left, jugular, subclavian and bronchomediastinal lymph trunks typically enter the termination of the thoracic duct.

On the right they typically unite to form a short common channel, termed the right lymphatic duct. However, the

terminations of these lymph trunks are variable and each often enters the venous system independently. When this occurs on the right, the right lymphatic duct is absent.

The thoracic duct is the largest single avenue of lymph return to the venous system.

The thoracic duct traverses the entire length of the thorax (from aortic opening in the diaphragm to the root of the neck). The thoracic duct has thin walls, with valves, that give it a beaded appearance. Its flow is dependent on the thoracic pump (variations in intrathoracic pressure with phases of respiration).


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Fig.12.11 Lymph ducts drain into the venous system

Cisterna chyli

The thoracic duct typically originates in the abdominal cavity from a small sac termed the cisterna chyli (L. ‘reservoir’ + ‘juice’), lying adjacent to the aortic opening of

the diaphragm. The cisterna chyli receives the intestinal lymph trunk plus the lumbar lymph trunks. However, the thoracic duct often arises directly from a confluence of these lymph ducts without the presence of a cisterna chyli.

Territory drained by thoracic duct

Fig.12.12 Quadrants of body drained by thoracic duct

At its origin the thoracic duct drains lymph from almost the entire body below the diaphragm (from intestinal and lumbar lymph trunks via cisterna chyli).

At its termination it typically receives lymph from the left upper quadrant (after collecting the associated jugular, subclavian and bronchomediastinal lymph trunks). The thoracic duct (or the origin of the left brachiocephalic vein) is therefore the major direct pathway for lymph from three quadrants of the body. The right lymphatic duct (or the origin of the right brachiocephalic vein) is the major direct pathway for the right upper quadrant, of the body.

LYMPH RETURN

Mechanisms of lymph flow

The pressure within the lymphatic system is much lower than that of the cardiovascular system and for many lymph vessels throughout the body lymph flow is often against gravity. Lymph flow is dependent on three potential pumps coupled with the presence of one-way valves. The vascular lymph pump is provided by rhythmic contraction

of the smooth muscle wall of lymph vessels, intimate contact with veins and the common direction of flow (milking effect). The muscular lymph pump is from contraction of adjacent muscles (squeezing effect). The thoracic lymph pump is due to the oscillation of intrathoracic pressure with respiration (sucking effect).

Lymph return to venous system

All lymph is normally returned to the venous system

Fig.12.13 Major direct path of lymph to the venous system

Lymph is directly returned to the venous system via the thoracic duct (on the left) and the right lymphatic duct. These empty into large central veins (typically the origin of the brachiocephalic veins) at the junction of the neck and thorax.

Fig.12.14 Sites of zero pressure in erect or supine posture


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The termination of lymph ducts occurs where the venous pressure is about zero, whether upright or supine.

This site is located near the front of the thoracic inlet just above the level of the heart. Lymph therefore tends to flow freely into the central venous system without backflow of blood into the lymphatic system. The thoracic and right lymphatic ducts also have valves and arch up into the root of the neck just prior to their entry into the venous system (this helps prevent blood inadvertently entering them). Between two to three litres of lymph per day is returned via the thoracic duct. The thoracic duct is of sufficient diameter to carry cells (e.g. lymphocytes) as well as large molecules (e.g. plasma proteins and chylomicrons).

Lymphovenous communications

There are numerous small communications between the lymphatic system and the venous system. These are via peripheral connections between some lymphatics and veins, as well as via veins that emerge from the hila of lymph nodes. There are also numerous communications between the thoracic duct and tributaries of the azygos veins in the thorax. A significant proportion of fluid return is by these routes.

Fig.12.15 Paths of lymph return to venous system

Effect of thoracic duct laceration

Although the thoracic duct can be ligated without significantly impeding fluid return, laceration causes profuse lymph leakage. The subsequent accumulation of this lymph in the thoracic cavity is termed chylothorax.

Lymphatic spread

Tumours and infections can spread by lymphatics, particularly as tumour cells and microbes tend to be carried along with the lymph. However, since lymph drained from any particular organ tends to pass through at least one set of lymph nodes (prior to reaching the venous system), tumour cells or microbes carried in it are exposed to defence cells at these sites.

Lymph nodes tend to enlarge in response and may also become tender (particularly with infection) or firmer (particularly with tumour involvement).

Fig.12.16 Stages of lymphatic spread

Predicting the path of tumour cells carried by lymphatics is complicated by the possibility of:

1. Occlusion of some lymphatics by tumour cells (altering the direction of spread)

2. Variation in, as well as overlap of, lymph drainage territories

3. Posture and external compression influencing lymph flow (being at low pressure)

4. Lymphovenous communications via direct peripheral connections and via veins emerging from lymph node hila (providing potential avenues for short circuit)

5. Certain tumours producing growth factors generating new lymph capillaries which subsequently link with existing capillaries

First aid for venomous bites

Venom from a bite tends to be located initially in tissue

fluid (unless the bite is directly into a blood vessel) and is taken up by local lymph capillaries (rather than by blood capillaries). However, venom carried in the lymph may ultimately reach the venous system with potentially serious consequences. First-aid management for most venomous bites involves applying a compression bandage to collapse lymph vessels. A tourniquet is contraindicated, as it may totally occlude arterial (and venous) flow, compromising the viability of the body part distal to it. Dissemination of venom is particularly accelerated by muscle contraction. Immobilising an affected part (e.g. by splinting the appropriate limb) retards venom carriage in the lymph.

Lymphoedema

Lymph capillaries normally take up fluid that has leaked from blood capillaries, which would otherwise accumulate in the interstitial compartment (between intravascular and intracellular fluid compartments).

Although there are many lymphatic and lymphatico-venous communications, with multiple avenues of lymph return to the venous system, extensive lymphatic obstruction may prevent sufficient lymph return.

The abnormal accumulation of tissue fluid (oedema) due to this mechanism is termed lymphoedema. Causes

include surgical removal of lymphatics (e.g. from a radical mastectomy for breast cancer) and parasitic occlusion of lymphatics (e.g. elephantiasis of the lower limb and external genital organs from filarial worm infestation and subsequent inflammation).

LYMPH NODES

Lymph nodes (L. ‘knots’) are multiple, discrete,

encapsulated collections of lymphoid tissue lying along the course of lymph vessels.


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Lymphoid tissue contains lymphocytes and associated cells on a supporting framework of reticular (L. ‘little net’)

fibres. Lymph nodes resemble small glands and are typically kidney shaped with a hilum. Several afferent lymphatics enter lymph nodes around their periphery, while a single efferent lymphatic emerges from the hilum (where an artery and vein also enter and leave, respectively).

Fig.12.17 Lymph nodes

Lymph filter and antigen response roles

Lymph nodes function as mechanical filters of lymph. Those in the lung appear black due to the presence of inhaled carbon particles.

Antigens (G. ‘produce against’) are generally foreign

substances that trigger the formation of antibodies specifically acting against them. Antigens carried in lymph from the tissues to lymph nodes (via afferent lymphatics) are brought in contact with certain defence cells, which respond to them. Lymph nodes also release lymphocytes

into the venous system (directly via veins that drain them and indirectly in lymph via efferent lymphatics).

Sentinel lymph nodes

Lymph from each part of the body normally drains through at least one set of lymph nodes before reaching the venous system. The first lymph nodes encountered in the path of lymph drainage from a particular organ or area of the body may be regarded as sentinel nodes, ‘guarding’

the rest of the body from dissemination of tumour cells or microbes.

Sentinel nodes in tumour spread

Sentinel lymph node involvement is particularly significant not only as the first potential barrier to lymphatic spread of tumours, but also in their clinical assessment. ‘Staging’ of tumour spread influences treatment and indicates prognosis.

Signal node near end of thoracic duct

The lymph node (‘Virchow’s’ or signal node) adjacent

to the termination of the thoracic duct, which has communications with the duct, may be regarded as the final lymph node, ‘guarding’ entry into the venous system.

Significance of signal node enlargement

Enlargement of this left supraclavicular lymph node may signal lymph spread of cancer from a structure within the territory drained by the thoracic duct. It may even be the first (although late) sign of cancer in a thoracic organ (e.g. lung) or abdominal organ (e.g. stomach or testis), since the thoracic and abdominal lymph nodes are all deeply located and none are readily palpable.

Palpation of both left and right supraclavicular (groups of cervical) lymph nodes should be performed in the routine examination of the thorax. Palpation of the left supraclavicular lymph nodes should be performed in the routine examination of the abdomen (and is mandatory if there is suspicion of cancer in an abdominal organ).

Fig.12.18 The final sentinel lymph node

Major lymph node groups

The major lymph node groups of the body (cervical, axillary and inguinal) are located at the gateways of the

trunk. These key sites are at the junctions of the trunk with the head, upper limb and lower limb (in the neck, armpits and groins, respectively). The cervical, axillary and inguinal lymph nodes are readily palpable in a physical examination.

Superficial and deep groups of nodes

Fig.12.19 Major palpable lymph node groups

The major palpable lymph node groups are typically subdivided into superficial and deep groups, each located adjacent to a major vein.

Fig.12.20 Drainage of lymph from superficial to deep

Lymph drains from superficial nodes to deep nodes.


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Superficial cervical nodes (e.g. along the external jugular vein), drain to deep cervical nodes (along the internal jugular vein). Superficial inguinal nodes (e.g. along the great saphenous vein) drain to deep inguinal nodes (along the femoral vein). The majority of axillary nodes (e.g. along the axillary vein or its major tributaries) may be regarded as superficial nodes and drain to deep (apical) nodes (along the termination of the axillary vein at the apex of the axilla).

LYMPHOID ORGANS AND TISSUES

In addition to multiple, discrete, collections of lymphoid tissue (lymph nodes), there are single discrete organs of lymphoid tissue (lymphoid organs) and collections of lymphoid tissue within other organs (lymphoid tissues).

Thymus

Fig.12.21 Thymus with its lobes

The thymus is a lymphoid organ, which reaches its

greatest absolute size at puberty (although its greatest relative size is at birth). It is located primarily in the thorax (extending up into the neck) and receives a blood supply from neighbouring vessels. Special immature lymphocytes (T cell precursors) from the bone marrow are transported

via the blood stream to the thymus where they develop and differentiate. Those that would otherwise attack host cells are recognised and removed there. The particular lymphocytes (T cells) released from the thymus into the

circulation are involved in mobilising certain defence cells against a foreign agent (cell mediated immunity).

After puberty, the thymus in particular (together with lymphoid tissue in general) involutes with age.

Spleen

Fig.12.22 Spleen with its blood vessels

The spleen has a rich blood supply, large vascular

spaces and sinusoids. The spleen filters blood picking up antigens, responds to them and releases lymphocytes into the blood stream. The spleen also removes old red blood cells from the circulation and is a store of red blood cells.

Breakdown products of red blood cells are taken (via the portal venous system) to the liver and excreted in the bile.

Accessory spleens and splenectomy effect

Additional discrete anatomical variants, termed accessory spleens, are sometimes found along the

course of the splenic artery. These may enlarge after splenectomy.

Mucosa Associated Lymphoid Tissue

Lymphoid tissue is present diffusely in the mucous membranes of the digestive, respiratory and urinary tracts. This Mucosa Associated Lymphoid Tissue ('MALT') is

subepithelial (in the lamina propria) where it is well placed to respond to antigens that penetrate the delicate surface lining.

Tonsils

Fig.12.23 Waldeyer's ring (of tonsillar tissue)

The (palatine) tonsils plus other collections of tonsillar

tissue (nasopharyngeal and lingual) encircle the pharynx (as ‘Waldeyer’s ring’) in the mucosa at the junction of the upper digestive and respiratory tracts. Lymph vessels drain them to deep cervical lymph nodes (particularly the tonsillar lymph node, palpable just below the angle of the mandible).

Peyer’s patches

Lymphoid tissue in the terminal small intestine forms aggregates termed ‘Peyer’s patches’. These tend to be

arranged longitudinally in the ileum (along the anti-mesenteric border), while the lymphatics draining them pass transversely.

Fig.12.24 Peyer's patches in small intestine

.

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Section III

BODY REGIONS AND ORGAN POSITION

Introduction: 'Everything is somewhere'

Chapter 13: Regions of the Human Body

Chapter 14: Arrangement of Body Regions

Chapter 15: Body Compartments and Fascial Planes

Chapter 16: Body Wall and Cavities

Chapter 17: Neurovascular Pathways

Introduction: ‘Everything is somewhere’

134

Organ position

Organs occupying a common location are regarded as belonging to a particular region (L. ‘area’). An organ is

therefore simultaneously the structural (and functional) unit of a body system as well as an occupant of a region.

Regional anatomy is concerned with the situational

(extrinsic) properties of an organ – its position and relations.

Position refers to spatial relationships of an organ to

the body as a whole, while relations are those to its

immediate neighbours.

Body regions

A cluster of neighbouring regions may be grouped into a common module.

The human body can be conceptualised as being made up of (or divided into) 8 modules containing a grand total of 72 regions.

Head and neck modules:

- head 15 regions - neck 6 regions Total = 21

Trunk modules:

- back 3 regions - thorax 8 regions - abdomen 4 regions - pelvis 6 regions Total = 21

Limb modules:

- upper limb 15 regions - lower limb 15 regions

Total = 30

The majority of regions are paired, including all regions

of the limbs with some additional regions of the trunk, head and neck. The remaining regions being along the midline of the body are unpaired.

From a geographic perspective, a module is like a country while regions are like its states.

Although arbitrary, regions (with clearly defined borders) are necessary for precise localisation of any specific organ. Regional anatomy also enables accurate description of the course for a structure (e.g. a nerve or a vessel) passing from one region to another, as well as relations at any point along a pathway.

The first step in a clinical diagnosis is to determine the (anatomical) site of a lesion.

This is typically expressed in terms of the region it is

situated in and its proximity to a key bony or soft tissue

landmark contributing to a boundary of the region. Landmarks can also be apertures (through or across

boundaries) allowing pathways between one region and another.

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REGIONS OF HEAD

REGIONS OF NECK

REGIONS OF BACK

REGIONS OF THORAX

REGIONS OF ABDOMEN

REGIONS OF PELVIS

REGIONS OF UPPER LIMB REGIONS OF LOWER LIMB

REGIONS OF HEAD

The head may be divided into 15 regions arranged in three groups of five.

The cranial regions surround or are enclosed by cranial

bones of the skull. The facial regions cover the facial skeleton. The upper airway regions are the proximal parts of the

respiratory and digestive tracts. The pharynx and larynx are regarded as regions of the

head, although they extend into the neck (covered by the anterior triangle of the neck).

Fig.13.1 Regions of head

1. - scalp 2. - temporal region 3, - cranial cavity 4. - orbit 5. - ear

Fig.13.2 Cranial regions of head

6. - face 7. - parotid region 8. - deep styloid region 9. - infratemporal region 10. - pterygopalatine fossa

Fig.13.3 Facial regions of head

11. - mouth 12. - tongue 13. - nose 14. - pharynx 15. - larynx

Fig.13.4 Upper airway regions of head


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REGIONS OF NECK

The neck may be divided into 6 regions, arranged in (2) anterior and (4) posterior groups.

1. - anterior triangle of neck 2. - root of neck 3. - sternomastoid region 4. - vertebral region of neck 5. - posterior triangle of neck 6. - back of neck

Fig.13.5 Regions of neck

The anterior triangle surrounds the larynx and lower

part of the pharynx, which although extending into the neck are regarded as regions of the head.

Fig.13.6 Triangles of neck

Their continuations (trachea and oesophagus, respectively) are located in the root of the neck. The root of the neck is covered by the lower third of sternomastoid muscle while the sternomastoid region is covered by its upper two thirds.

The vertebral region of the neck includes the cervical vertebral column as well as the enclosed vertebral canal (and associated pairs of intervertebral foramina).

Fig.13.7 Regions of neck and modules adjacent to them

REGIONS OF BACK

The back may be divided into 3 regions, which span its entire length from the 1st thoracic vertebra to the tip of the coccyx. These may be arranged into (2) muscle compartments and the vertebral column (with its

enclosed vertebral canal). 1. - superficial compartment of back 2. - deep compartment of back 3. - vertebral region of back

Fig.13.8 Regions of back

The superficial compartment includes the skin of the back and extends further laterally than the deep compartment, which in turn overlaps the much narrower vertebral region. The superficial compartment contains extrinsic back muscles while the deep compartment contains intrinsic back muscles.

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The vertebral region of the back includes the thoracolumbar and sacrococcygeal parts of the vertebral column (with associated intervertebral joints and discs), as well as the enclosed vertebral canal (with associated intervertebral foramina). The compartments of the back are continuous above with the back of the neck region. The vertebral region of the back is continuous with the corresponding region of the neck.

Fig.13.9 Deep compartment of back and vertebral region

REGIONS OF THORAX

The thorax may be divided into 8 regions arranged as (3) thoracic wall regions and (5) thoracic cavity regions.

Fig.13.10 Regions of thorax

1. - anterior thoracic wall 2. - posterior thoracic wall 3. - the diaphragm 4. - pleural sacs 5. - anterior mediastinum 6. - middle mediastinum 7. - posterior mediastinum 8. - superior mediastinum

Fig.13.11 Modules overlapping thorax

The anterior thoracic wall is covered by skin of the pectoral region (of the upper limb). The posterior thoracic wall is directly in front of the thoracic vertebral column (classified as part of the back).

The thoracic walls have superior and inferior apertures. The inferior aperture is filled by the diaphragm, which in turn contains major and minor openings.

Fig.13.12 Thoracic wall regions

The thoracic cavity, made up of paired pleural sacs with the mediastinum between them, contains the thoracic viscera.

Fig. 13.13 Subdivisions of mediastinum

The pericardial sac occupies most of the middle mediastinum. A lung is located in each pleural sac while the heart is in the pericardial sac.

Fig.13.14 Pleural sacs

REGIONS OF ABDOMEN

The abdomen may be divided into 4 regions arranged as (3) abdominal wall regions and the abdominal cavity.

The abdominal wall regions are primarily the large muscular and posterior abdominal walls. The anterior abdominal wall includes overlying skin while the posterior abdominal wall is directly in front of the lumbar vertebral column and hipbones (classified as part of the back and lower limb, respectively). The inguinal canal is at the lower


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part of the anterior abdominal wall and, in the male, it is continuous with the scrotum.

The abdominal cavity is enclosed by the abdominal walls. Its contents include the abdominal viscera and the peritoneal cavity.

Fig.13.15 Regions of abdomen

1. - posterior abdominal wall 2. - anterior abdominal wall 3. - inguinal canal (and scrotum) 4. - abdominal cavity.

Fig.13.16 Position of abdominal cavity within trunk

The abdominal cavity is separated from the thoracic cavity by the diaphragm but is continuous with the pelvic cavity below the pelvic brim.

REGIONS OF PELVIS

The pelvis (L. ‘basin’) may be divided into 6 regions arranged into pelvic walls, the pelvic cavity and the two triangles of the perineum (L. ’discharge’).

The pelvic wall regions are primarily the lateral and posterior pelvic walls (formed by the lesser pelvis). The posterior pelvic wall is directly in front of the sacrum and coccyx (classified as part of the back).

The pelvic cavity is enclosed by the pelvic walls and located above the pelvic floor. Its contents include pelvic viscera and the peritoneal cavity. The pelvic cavity is continuous with the abdominal cavity above the pelvic brim, but is separated from the perineum by the pelvic floor, which in turn contains openings for certain viscera.

Fig.13.17 Regions of pelvis

1. - posterior pelvic wall 2. - lateral pelvic wall 3. - pelvic floor 4. - pelvic cavity 5. - anal triangle of perineum 6. - urogenital triangle of perineum

Fig.13.18 Position of pelvic floor

Fig.13.19 Pelvic wall regions

Fig.13.20 Subdivisions of perineum

The perineum is covered by skin with cutaneous orifices for the urogenital tract and for the (lower) digestive tract.

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REGIONS OF UPPER LIMB

The upper limb may be divided into 15 regions, (all covered by skin) arranged in (8) anterior regions and (7) posterior regions.

Fig.13.21 Regions of upper limb

1. - pectoral region 2. - axilla 3. - anterior compartment of arm 4. - cubital fossa 5. - anterior compartment of forearm 6. - carpal tunnel 7. - palm of hand 8. - palmar aspect of digits 9. - scapular region 10. - deltoid region 11. - posterior compartment of arm 12. - posterior compartment of forearm 13. - anatomical snuffbox 14. - dorsum of hand 15. - dorsal aspect of digits

Fig.13.22 Major parts of upper limb

REGIONS OF LOWER LIMB

The lower limb can also be divided into 15 regions, (covered by skin) arranged in (8) anterior regions and (7) posterior regions.

Fig.13.23 Regions of lower limb

1. - femoral triangle 2. - sub sartorial canal 3. - anterior compartment of thigh 4. - medial compartment of thigh 5. - anterior compartment of leg 6. - lateral compartment of leg 7. - dorsum of foot 8. - dorsal aspect of digits 9. - gluteal region 10. - posterior compartment of thigh 11. - popliteal fossa 12. - posterior compartment of leg 13. - tarsal tunnel 14. - sole of foot 15. - plantar aspect of toes

Fig.13.24 Major parts of lower limb

Chapter 14: Arrangement of Body Regions

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UNPAIRED REGIONS & MIDLINE OF BODY

PAIRED REGIONS & BILATERAL SYMMETRY

FLEXOR AND EXTENSOR REGIONS

BOUNDARIES OF REGIONS

APERTURES BETWEEN REGIONS

UNPAIRED REGIONS & MIDLINE OF BODY

The mid-sagittal plane is the most important reference plane. It represents the midline of the body.

30 regions are unpaired (while the remaining 42 are

paired). Unpaired regions are located in the midline although they may be divided into two halves by the midline. These regions are confined to the head, neck and trunk.

Ventral and dorsal cavities of body

Fig.14.1 Major body cavities in a mid-sagittal section

The body contains two main cavities, both of which are in the midline. The two main cavities in the body are the ventral cavity and the dorsal cavity.

The ventral cavity is partitioned by the diaphragm into: -.a thoracic cavity, above it - an abdomino-pelvic cavity, below it

These are not empty spaces, being occupied mainly by thoracic and abdomino-pelvic viscera, respectively (as well as their surrounding membranes and fluid).

The dorsal cavity is divided into: - a cranial part (the cranial cavity, in the head), - a vertebral part (the vertebral canal, in the neck and

the back – continuous with the cranial part)

These also are not empty spaces, being occupied mainly by the brain and spinal cord, respectively (as well as their surrounding membranes and fluid).

PAIRED REGIONS & BILATERAL SYMMETRY

42 regions of the body are paired (while the remaining

30 are unpaired). Paired regions include those (12) regions of the trunk not in the midline, together with all (30) regions of the limbs. The latter are further from the midline.

Bilateral symmetry

Animals, being capable of independent movement, tend to have bilateral symmetry (in contrast to the myriad of

forms evident in plants). This is particularly important in humans to maintain balance in (bipedal) gait and locomotion.

Symmetry facilitates movement and is exhibited by the skeleton and its associated muscles, especially in the limbs.

Fig.14.2 Bilateral symmetry in a coronal section

Even within the head, neck and trunk unpaired regions have bilateral symmetry, being divided into two halves by the midline.

Asymmetrical regions and structures

The major exceptions to bilateral symmetry are serous sacs within the thoracic and abdomino-pelvic cavities, together with their contained unpaired organs (heart, digestive system and spleen) and associated unpaired vessels.

The branching patterns of vessels tend to be asymmetrical resembling the branching of a tree.

Even paired viscera are not perfectly symmetrical. Left and right lungs are of slightly different sizes and are

shaped differently by adjacent structures. The same applies to the kidneys and suprarenal glands.

One side of the face is not a mirror image of the other, limbs may not be exactly the same length and even certain individual limb muscles may be larger on the dominant side of the body.

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141

FLEXOR AND EXTENSOR REGIONS

Certain characteristics of the ventral aspect of the body are different to those on the dorsal aspect (which, in quadrupeds, is more exposed to the elements).

Flexor muscles with a richer nerve supply (for fine control of movements) tend to occupy compartments on the ventral aspect of the body and are covered by delicate skin with a correspondingly richer nerve supply (for fine sensory discrimination).

Fig.14.3 Flexor and extensor regions in the trunk and limbs

Course antigravity extensor muscles tend to occupy compartments on the dorsal aspect covered by hairier skin with tougher dermis.

Coronal morphological plane

The coronal morphological plane of the trunk separates flexor from extensor territory. It passes through the vertebral column and its transverse processes, bisecting each intervertebral foramen (passing between rami of each emerging spinal nerve).

Fig.14.4 The coronal morphological plane

Rami of spinal nerves

Posterior rami of spinal nerves directly supply the dorsal aspect of the trunk (and also of the neck) with their associated extensor regions containing skin, joints and (deeply located) intrinsic muscles.

Fig.14.5 Territories within a segment of the trunk

The exception is the (superficially located) extrinsic muscles. They have migrated onto the dorsal aspect (and retain their nerve supply from ventral rami).

Fig.14.6 Spinal nerve rami

Anterior rami of thoracic spinal nerves directly supply

the ventral aspect of the trunk with its associated regions (regarded as flexor regions).

Fig.14.7 Supply of limb buds from anterior rami

Anterior rami of cervical and lumbosacral spinal nerves supply the upper and lower limbs, respectively (although indirectly via peripheral nerves derived from plexuses).


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Divisions of limb plexuses

During development, the upper limb buds and the lower limb buds rotate through 90 degrees in opposite directions

to each other when viewed in the anatomical position.

Fig.14.8 Supply of musculature within a limb bud

Flexor and extensor regions therefore become situated on opposite aspects of the respective limbs, (flexor regions are anterior in the upper limb but posterior in the lower limb)

The nerve supply for both flexor and extensor regions in the limbs are derived from anterior rami of spinal nerves.

A limb plexus divides into anterior and posterior divisions, with their nerve fibres distributed (via associated peripheral nerves) to flexor regions and extensor regions, respectively.

In view of the rotation of the limb buds, anterior divisions of the lumbosacral plexus supply posterior compartments of the lower limb and posterior divisions supply anterior compartments.

BOUNDARIES OF REGIONS

Regions are demarcated from each other by their boundaries. These may be bony or soft tissue or a

combination of both.

Bony boundaries

Fig.14.9 Key bony boundaries in upper limb

Bony boundaries of regions may be bony features, prominences or borders. For example the apex of the

axilla (bounded by the medial border of the first rib, the clavicle and the superior border of the scapula) demarcates the upper limb from the neck.

Bony boundaries may also include imaginary lines

between them. For example an imaginary line between the medial and lateral humeral epicondyles demarcates the base of the cubital fossa.

Bony boundaries are often expressed in terms of surface markings or vertebral levels that can be determined on living bodies.

Fig.14.10 Imaginary lines between bony borders in thorax

Soft tissue boundaries

Soft tissue boundaries of regions may include borders of muscles. The distal border of teres major muscle demarcates the base of the axilla. Pronator teres muscle is the boundary between the cubital fossa and the anterior compartment of the forearm.

Fig.14.11 Key soft tissue boundaries in upper limb

Soft tissue boundaries of regions may also include borders of connective tissue thickenings (e.g. intermuscular septa, retinacula, tendons, ligaments). Medial and lateral intermuscular septa separate the anterior from the posterior compartment of the arm. The flexor retinaculum demarcates the carpal tunnel (between the anterior compartment of forearm and the palm of the hand).

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APERTURES BETWEEN REGIONS

Major and minor apertures

Pathways between regions are through gaps in their boundaries termed apertures.

Fig.14.12 Key structures exiting from spinal apertures

Apertures may be major or minor. Major apertures are

clearly defined openings (e.g. between or within bones), while minor apertures are avenues across boundaries (e.g. over or under a muscle) or through a boundary (e.g. by piercing a muscle) where an opening is not readily apparent. Many structures may pass through a major aperture

Fig.14.13 Major apertures from neck

For example, the superior aperture of the thorax transmits many viscera, vessels and nerves to and from the root of the neck. Generally only one or two structures pass through a minor aperture. For example, the radial artery, median nerve and ulnar artery each leave the cubital fossa to enter the forearm via a different avenue (passing over, through or under pronator teres muscle, respectively).

Fig.14.14 Minor apertures associated with a muscle

Pathways between regions may be via both major and minor apertures. For example, the diaphragm has major apertures (centrally) providing pathways to and from the abdominal cavity and minor apertures (around the periphery) providing pathways to and from the abdominal walls.

Fig.14.15 Major and minor apertures in the diaphragm

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COMPARTMENTS AND LAYERS

MOBILE AND FIXED FASCIAL PLANES

COMPARTMENTS AND LAYERS

Regions are typically in the form of compartments with clearly defined boundaries.

Fig.15.1 Compartments and layers of leg

Compartments have boundaries (composed of walls, a roof and a floor) and contents.

Boundaries and contents of a compartment

Compartment walls are typically fascial (e.g. inter-muscular septa and retinacula) but may also be bony or muscular. A roof typically includes overlying skin, subcutaneous tissue and deep fascia. A floor typically includes underlying bone (covered by periosteum) or a joint (demarcated by its fibrous capsule). If there is a pair of bones (e.g. radius and ulna or tibia and fibula) the floor may also include an associated interosseus membrane (separating anterior from posterior compartments of the forearm or leg).

Apertures in certain boundaries allow passage of structures between a compartment and neighbouring regions.

The contents of a compartment may be grouped into those structures that enter or exit the region (e.g. tendons,

ducts, nerves and vessels) and those structures that reside solely in the region (e.g. muscles and glands).

Remaining space within a compartment tends to be occupied by fat.

Layers of structures within a compartment

Compartments tend to be in layers.

Fig.15.2 Layers within compartments of arm

The roof of a compartment is composed of superficial structures (skin, subcutaneous tissue and deep fascia) that can be in the form of concentric layers (e.g. around a limb). The floor is composed of deep structures (bones and joints). The contents of compartments (between the roof and the floor) provide the intermediate group of structures and may also be in layers. This particularly applies to muscle compartments.

Muscles in a superficial layer tend to be prime movers.

They tend to span greater distances between their attachments (moving levers through greater arcs) or at least exert greater leverage.

Muscles in a deep layer are usually shorter and tend to be fixators. Their role is more for stabilising rather than for

generating power and range of movement. The key supply lines (major nerves and vessels) tend to run in the intermediate layer. Branches from them are well placed to pass superficially (cutaneous branches), deeply (articular branches) and to adjacent structures (muscular branches).

Flexor and extensor compartments

Muscle compartments are either flexor or extensor,

typically being situated on ventral or dorsal aspects of the body, respectively.

Fig.15.3 Plexus divisions and type of compartment supplied

The deep muscle compartment (for intrinsic back muscles) on the dorsal aspect of the trunk, is an extensor compartment, and is supplied by posterior rami of spinal nerves. In the limbs, flexor compartment muscles are located anteriorly in the upper limb and posteriorly in the lower limb. They are supplied by anterior divisions of the associated nerve plexuses. Conversely, extensor compartment muscles are located posteriorly in the upper limb and anteriorly in the lower limb. They are supplied by posterior divisions of the associated nerve plexuses.

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Flexible and rigid compartments

At least one of the walls surrounding a compartment (particularly if transmitting a major vessel) is generally flexible, or at least has a sufficiently large aperture, to allow for expansion. However, unyielding walls may almost completely surround certain compartments.

These compartments may be absolutely rigid bony cavities (e.g. cranial cavity) or relatively rigid fibro-osseous tunnels, canals and foramina (e.g. carpal tunnel, vertebral canal, intervertebral foramina). The contents of a rigid compartment may be cushioned by fat (e.g. around the dural sac in the vertebral canal, within the median nerve in the carpal tunnel) or fluid (e.g. cerebrospinal fluid within the dural sac in the cranial cavity and vertebral canal).

Fig.15.4 Rigid compartments in the spine

Major vessels tend not to run through a rigid compartment (e.g. carpal tunnel).

Compartment syndrome

A compartment with rigid walls is a confined space and a potential site for compartment syndrome.

Fig.15.5 Anterior compartment syndrome in leg

The anterior compartment of the leg is particularly prone to this condition (termed 'anterior compartment syndrome').

Fig.15.6 Disc bulging into intervertebral foramen

Swelling in rigid compartments increases the pressure and ultimately leads to compression of its contents. Even a small degree of swelling tends to give considerable pain. Further increase of compartment pressure compresses vessels (initially veins then arteries) and nerves.

An emergency surgical operation to decompress the compartment may need to be performed (e.g. a laminectomy, to relieve spinal cord or spinal nerve root

compression). Swelling (in 'mini-compartments') where skin is bound

down to underlying structures (e.g. palm, nail bed, ear, nose and anus) may also be extremely painful. Drainage (e.g. by an appropriate incision) suddenly releases the pressure and typically brings rapid relief.

Fig.15.7 Bruised nail bed

MOBILE AND FIXED FASCIAL PLANES

Fascial planes (L. ‘flat’) are created by flat layers of fascia and may be mobile or fixed.

Mobile fascial planes

Mobile planes occur between parallel sheets of fascia and tend not to be pierced by vessels or nerves.


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Fig.15.8 Mobile plane between fascial sheets

While major vessels and nerves may course along them, few cross mobile fascial planes as they would otherwise overstretch or have their own mobility restricted.

Fig.15.9 Vessels parallel to a mobile fascial plane

Vessels and nerves may course parallel to mobile planes for long distances (e.g. where fascial coverings of muscles slide against each other and where subcutaneous tissue glides over deep fascia or periosteum).

Mobile fascial planes are exploited in anatomical dissection and in surgical operations as they are easier to separate and there is less chance of damaging vessels or nerves.

Fixed fascial planes

Fixed planes occur within fibrous septa. They tend to

transmit vessels and nerves from deep to superficial. Arteries travel with connective tissue particularly via the fascia associated with muscles. Arteries (being derived from mesoderm) are retained within the connective tissue mesh that develops from mesoderm.

Vessels tend to cross planes at sites of fusion, where connective tissue is anchored.

This occurs particularly at the periphery of muscles, over intermuscular septa, under flexure lines (and skin creases) and where deep fascia attaches to bone. Nerves may also pass from deep to superficial via canals within bones (e.g. cranial nerves).

Courses from fixed to mobile areas

Vessels and nerves course from fixed to mobile areas.

This particularly applies to the scalp where vessels and nerves (e.g... supratrochlear, supraorbital, superficial temporal, posterior auricular and occipital) course from peripherally (arising deep to sites where fascia is fixed), converging on the vertex. The scalp is arranged in layers (skin, subcutaneous tissue, epicranial aponeurosis, loose areolar tissue layer and pericranium). The epicranial

aponeurosis is fixed by muscles (frontalis and occipitalis) to the supraorbital margin and superior nuchal line, at the front and back of the skull, respectively. The loose connective tissue between the epicranial aponeurosis and the periosteum of the skull is a mobile fascial plane. Vessels and nerves tend not to cross this plane. Similarly, on the side of the head (in the temporal region) the subcutaneous tissue glides over the tough deep (temporal) fascia.

Fig.15.10 Paths of vessels and nerves in fascial planes

Vessels and nerves also course from fixed (concave) areas to mobile (convex) areas across the face. The facial artery enters the face from the neck by passing across the lower border of the body of the mandible (to which the investing deep fascia of the neck attaches). Branches of the facial nerve emerge from within the parotid gland (fixed by the parotid deep fascia which encloses it) prior to radiating across the face (in the subcutaneous tissue).

Potential paths of tracking and direct spread

Fluids (including blood and pus) tend to track along mobile fascial planes as they provide paths of least resistance.

Fig.15.11 Tracking of blood along a mobile fascial plane

Bruising may appear or an abscess may discharge considerable distances from their source.

Direct spread of infection or tumours may also occur more easily along mobile planes, while fixed fascial planes provide a barrier.

Chapter 16: Body Walls and Cavities

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BODY WALL AND PARIETAL STRUCTURES

SEROUS SACS AND BODY CAVITIES

BODY WALL AND PARIETAL STRUCTURES

The body wall is made of parietal (L. 'wall') structures.

Fig.16.1 Lateral and posterior walls of thorax & abdomen

The trunk includes large cavities surrounded by walls. Each wall and cavity is regarded as a discrete region.

Layers of body wall in trunk

Fig.16.2 Layers of anterior abdominal wall

The body wall in the trunk is arranged in layers. The anterior thoracic and abdominal walls have skin on

a layer of subcutaneous tissue, in turn, on layers of muscles or aponeuroses (rather than an unyielding layer of deep fascia). The anterior thoracic wall even has bones (sternum and ribs) and associated joints, in addition to muscles (the intercostals).

The parietal layer of a serous membrane lines the interior of the body wall. Viscera are located internal to the body wall where they are more protected, occupying the cavity enclosed by the wall.

Somatic nerves and parietal vessels

The key supply lines to the walls are somatic nerves and parietal vessels. They arise from their sources (e.g.

spinal cord and aorta) posteriorly and run forwards around the body wall on each side to supply the parietal structures particularly skin, muscles and parietal layer of the serous membrane (e.g. parietal pleura or peritoneum).

Fig.16.3 Supply lines to abdominal wall and cavity

In contrast, visceral nerves and vessels supply the

viscera (and are directed towards them in the cavity). They also supply the visceral layer of an associated serous membrane (e.g... visceral pleura or peritoneum).

Apertures in body wall

Apertures occur in the body wall at certain sites to allow passage of structures from one region to another (e.g. the spermatic cord through the inguinal rings in the anterior abdominal wall).

Hernia

A hernia (L. ‘rupture’) is an abnormal protrusion of an

anatomical structure through an opening, defect or area of weakness (in its containing walls).

Fig.16.4 Avenues of herniation


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Herniation may occur through a normal opening (e.g.

of stomach through oesophageal opening in diaphragm, intestine through deep inguinal ring) a defect (e.g. nucleus

of intervertebral disc through a split in its peripheral part) or a weakness (e.g. intestine directly pushing through posterior wall of inguinal canal). Herniation is commonly precipitated by raised pressure inside a cavity (e.g. raised intra-abdominal pressure through coughing or straining).

Fig.16.5 Herniation from straining

Rarely, but importantly, a sudden change in pressure gradient by performing a lumbar puncture in raised intra-cranial pressure may cause (potentially fatal) herniation of part of the brain through the (rigid) foramen magnum into the spinal canal.

Serious complications from herniation include direct compression (of a vital structure), obstruction (of a hollow viscus) and strangulation (choking of vascular

supply).

Fig.16.6 Iatrogenic herniation of brain stem

Fig.16.7 Herniation through a rigid opening

Strangulation is more likely to occur if the opening has rigid edges.

Fig.16.8 Strangulation of a herniated loop of gut

SEROUS SACS WITH BODY CAVITIES

A serous (L. ‘serum’ a watery fluid) sac consists of a

serous membrane (L. ‘thin skin’), an enclosed body cavity and the structures surrounded by the serous sac.

Fig.16.9 Types of surface linings

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Mesothelium

A serous membrane is composed of connective tissue covered by mesothelium.

Fig.16.10 Peritoneum and peritoneal cavity

A mesothelium consists of a (mesoderm-derived) single layer of flat cells that secrete a small amount of lubricating fluid (into an enclosed potential space) minimising friction. A mesothelium may be contrasted with an endothelium (also a single layer of mesoderm-derived cells but lining the interior of vessels) and with an epithelium (ectoderm-derived epidermis or endoderm-derived lining of a mucous membrane).

A serous membrane has a parietal layer lining the wall of the sac and a visceral layer covering structures

contained in the sac. These two layers are continuous via a mesentery (L.

‘middle + carry’) that suspends the structures projecting into the sac.

Fig.16.11 Parietal & visceral layers of a serous membrane

Closed body cavity

Serous sacs contain structures that have considerable mobility or motility, in particular many viscera within the

large ventral cavities of the trunk.

Serous sacs are also associated with joints (synovial cavities and bursae) and synovial tendon sheaths. A serous sac is a closed body cavity. A potential space (normally occupied by only a thin film of fluid) is between parietal and visceral layers of the serous sac.

With the exception of the peritoneal cavity in females (through the abdominal opening of the uterine tube) a body cavity does not communicate with any other type of cavity nor does the mesothelium become continuous with any other type of surface. However, such a cavity may be divided into compartments with a communication between them. The mesothelium lining one cavity is continuous with that of another (e.g. where certain bursae communicate with a synovial joint cavity).

With the exception of articular cartilage, discs or menisci in a joint cavity, the serous fluid secreted into a body cavity does not come in contact directly with structures enclosed within the serous sac (they are covered by the serous membrane).

Drainage of accumulations in a body cavity

Fluid, air, blood or pus may track along and accumulate within a body cavity as a result of trauma or disease. This may need drainage through the body wall by needle aspiration or by insertion of a drain tube (which usually remains present for a number of days).

Pleural, pericardial and peritoneal sacs

The diaphragm partitions the large ventral cavity of the

trunk into a thoracic cavity above and an abdomino-pelvic cavity below. The superior aperture of the pelvis, in turn,

divides the abdomino-pelvic cavity into abdominal cavity and pelvic cavity. These ‘cavities’ are not empty spaces as they are fully occupied by their contents (e.g. viscera, nerves and vessels). They include serous sacs with potential spaces (pleural, pericardial and peritoneal cavities) located between parietal and visceral layers of the serous membrane.

Fig.16.12 Subdivisions of the ventral cavity of the body

The two pleural cavities and the pericardial cavity are

separate closed cavities within the pleural sacs and pericardial sac, respectively. The peritoneal cavity is a single cavity within the peritoneal sac of the abdomino-

pelvic 'cavity' (providing continuity between the abdominal 'cavity' and the pelvic 'cavity'). The peritoneal sac is subdivided into greater and lesser sacs (communicating

via the omental foramen).


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Fig.16.13 Pleural and pericardial sacs

In males, the scrotal cavities are extensions of the

peritoneal cavity (via the inguinal canal on each side) that normally become separate from it just prior to birth (by closure of the processus vaginalis).

Prolapse

A prolapse (L. ‘falling’) is the descent of an organ from

its normal position. Organs affected include those supported within a large

body cavity (e.g. uterus, rectum). Prolapse of an organ is due to weakened supports (e.g... from stretching during childbirth or from aging) coupled with gravity and aggravated by straining.

Fig.16.14 Prolapse of uterus into vagina

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NEUROVASCULAR BUNDLE

COURSE THROUGH A REGION

RELATIONS WITHIN A REGION

NEUROVASCULAR BUNDLE

Nerves and vessels tend to accompany each other as components of a neurovascular bundle.

Fascial sheath

Large vessels and nerves are typically enclosed by connective tissue as a discrete fascial sheath forming a

tube around them.

Fig.17.1 Components of a neurovascular bundle

Smaller vessels and nerves also tend to be surrounded by connective tissue, although this may not be in the form of a tubular sheath when passing via fixed fascial planes (e.g. within intermuscular septa).

Fig.17.2 Neurovascular bundle in the calf

Within a neurovascular bundle, the vein and lymph vessels are located more peripherally.

In addition, the fascial sheath of a neurovascular bundle

tends to be thin or absent around the vein and lymph vessels (or have a vacant compartment next to it) allowing room for expansion.

Fig.17.3 The major neurovascular bundle of the neck

Venae comitantes in limbs

In smaller neurovascular bundles, a single vein is often replaced by venae comitantes. These are a pair of intercommunicating veins wrapped around an artery. They conserve heat by its transfer from warm arterial blood to cool venous blood returning from the periphery.

Fig.17.4 Venae comitantes and heat conservation

Venae comitantes are located in the limbs, particularly distally.

Fig.17.5 Venae comitantes in the calf

COURSE THROUGH A REGION

Nerves and vessels, being the supply lines for anatomical structures, tend to traverse many regions on the way to their destinations. However, a particular nerve or vessel along a path may change its name according to the region in which it is situated.


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Components to a course include those between regions (e.g. through an aperture) and those within a region (which may also be divided into parts).

Fig.17.6 Course of popliteal artery behind knee

A course is described from origin to termination according to convention. Thus, arteries and their branches course from proximal to distal, while veins and their tributaries course from distal to proximal. Nerves and their branches course from proximal to distal (even those that contain only afferent fibres).

Position of major arteries relative to joints

The major limb arteries tend to run through flexor regions and are generally located on the flexor aspect of joints.

Fig.17.7 Position of major arteries adjacent to joints

A limb bud develops initially with an axial artery located

along the line of least tension. The line is altered during subsequent growth and development (including rotation) with accompanying changes in the arterial pattern. Preferred channels enlarge while others regress. This is reflected in the final path of a major limb artery.

In addition to being minimally stretched by movement, major arteries, being deeply located on the flexor aspect of a joint, are less vulnerable to injury.

The femoral artery runs on the flexor aspect of the hip joint (which is anterior). Its continuation (as it passes through the hiatus in adductor magnus muscle) is the popliteal artery. The course in the popliteal fossa (divided into 3 parts by the components of the floor; bony, ligamentous and muscular) is on the flexor aspect of the knee joint (even though this is posterior).

Course of superficial veins in limbs

Major arteries develop along the axis of a limb and do not run superficially for long distances, thus conserving heat. In contrast, the major superficial veins of limbs follow the pre-axial and post-axial borders. Cutaneous nerves (which also run along these borders) and lymph vessels accompany the superficial veins within the subcutaneous tissue and make up the other components of the major superficial neurovascular bundles.

Fig.17.8 Veins coursing along axial borders of limbs

Changed course of a nerve

Generally nerves and vessels take a direct course within a region. However, certain nerves change direction or even loop around a structure (e.g. left recurrent laryngeal nerve around the ligamentum arteriosum under the aortic arch) due to migration of the organ of supply during development.

The nerve supply to a structure remains constant even if the structure has migrated.

In contrast, vessels, having the capacity for alternative channels, may be acquired along the way and take more direct paths (e.g. renal arteries usually arise from the aorta

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at the level where the kidney completes its ascent). Nerves may even temporarily change compartments (e.g. radial and ulnar nerves in the arm) if the septa bordering them have shifted in development.

Tortuous arteries

Many arteries are tortuous and accommodate

movement (e.g. facial, splenic), protrusion (lingual) or expansion (uterine) of the organs supplied.

Fig.17.9 Arterial tortuosity from mobility of face

Fig.17.10 Tortuosity of uterine and lingual arteries

However, any artery may ultimately become tortuous because of loss of elasticity through aging.

Convergence of paths at neurovascular hila

Nerves and vessels tend to enter their organ of supply (e.g... a muscle or a viscus) at a common neurovascular hilum.

RELATIONS WITHIN A REGION

Regions have boundaries (typically walls, roof and floor) with apertures. The contents of a region may be

classified into those that course to or from another region (via apertures in the boundaries) and those that are situated in that region only.

Relations to boundaries of a region

Structures forming the boundaries of a region include skin, subcutaneous tissue, deep fascia, retinacula, septa, muscles, tendons, ligaments, joint capsules and bones. Gaps between them create apertures in the boundaries.

Fig.17.11 Boundaries and key contents of femoral triangle

The boundaries of a (3-dimensional) region have the following pairs of relations to its contents:

- anterior/posterior - superior/inferior - medial/lateral In typical regions, one pair of boundaries form the roof

and floor, the remainder form the walls. However, boundaries may receive different terms for regions of a different shape. The femoral triangle has a (anterior) roof and (posterior) floor, a (superior) base and (inferior) apex, medial and lateral borders.

Relations of contents within a region

The following types of contents within a region may be related to each other:

- muscles (and tendons) - fascia (and fibrous tendon sheaths) - glands and hollow viscera (including ducts) - serous membranes (including synovial membrane, bursae and synovial tendon sheaths) with associated body cavities - nerves (and branches) - arteries (and branches) - veins (and tributaries) - lymph nodes (and lymph vessels). The most important relationships (‘direct relations’)

are where structures are in direct contact.

This is applicable to nerves and vessels where they course alongside each other or where they intersect. A gland is often enclosed by fascia that splits to form a sheath around it. A tendon (surrounded by a synovial tendon sheath) may occupy a fibrous tendon sheath. Major nerves and vessels typically accompany each other in a neurovascular bundle enclosed by a fascial sheath.

Detecting arterial pulsation

Arterial pulsation is best detected by palpation at a site where an artery is closely related to both skin and bone.


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The usual site for clinical examination of an arterial pulse is where the radial artery lies on the distal end of the radius just deep to skin of the wrist.

Fig.17.12 Palpating the radial artery against bone

Predicting vascular endangerment

Vessels closely related to skin are prone to be severed by lacerations, resulting in external haemorrhage. Vessels

closely related to bones or joints may be severed by fracture or dislocation, respectively and result in internal haemorrhage.

This is particularly significant when hidden in large compartments or body cavities, which may accumulate dangerous volumes of blood without significant initial symptoms.

Fig.17.13 Endangerment of vessels near skin or bone

Predicting nerve endangerment

Nerves closely related to skin are prone to be severed by lacerations. Those closely related to bones or joints may be injured by fracture and dislocation, respectively.

Nerves are endangered by compression from entrapment in confined spaces with rigid walls (e.g. an

intervertebral foramen, carpal tunnel). They are also endangered by compression from entrapment where they pierce certain muscles or dense fascia.

Fig.17.14 Vulnerability to nerve severance or entrapment

In addition, nerves may be endangered by external compression from tight or inappropriate splints and casts.

This is applicable to nerves closely related to both skin and bone (e.g... common fibular nerve around neck of fibula).

Although compression may directly damage nerve fibres (e.g... from a crush injury) it primarily compromises blood supply to the nerve.

Fig.17.15 Vulnerability to external nerve compression

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Section IV

HUMAN DEVELOPMENT AND VARIATION

Introduction: 'Derivation determines destiny'

Chapter 18: Growth and Development

Chapter 19: Normal Variation

Chapter 20: Anatomical Variation in Structure

Chapter 21: Anatomical Variation in Position

Chapter 22: Pathological Changes

Introduction: ‘Derivation determines destiny’

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Even in so-called identical twins, no human body is

exactly the same as another. Bodies vary within a wide range of normality (as well as beyond that range) resulting in observed differences from the typical case described in anatomy textbooks.

Normal variations are atypical (G. ‘not’ + ‘type’) in

that they do not conform to a standard model (e.g. an adult male of medium build) but their structure and function are both within the normal range.

Normal variation for somatic structures is primarily in external form due to constitutional factors (age, sex and body build) and there is no alteration of structure other than the normal stages of development (e.g. epiphyses in growing bones). Viscera may also vary in size or shape (for expansible organs) and in position (for mobile organs) due to physiological factors (e.g. posture, phase of respiration and pregnancy).

Anatomical variations have a significant structural or positional modification that is abnormal (L. ‘away’ + ‘rule’),

meaning a deviation away from the norm (i.e. beyond the range of normal). However, normal function is retained. An anatomical variant may also be termed an anomaly (G. ‘irregular’). Anomalies tend to have a developmental basis and may reflect features from ancestral life forms.

Some are also associated with the presence of other

anomalies. Anatomical variation is not uncommon, although for certain anomalies there is a different incidence across different population groups (e.g. sex, race). Careful dissection of an entire body on average uncovers about 50 anomalies. With closer examination (e.g. by tracing fine branches of vessels) the number detected is potentially much greater (e.g. as vascular patterns, like fingerprints, are unique for each individual). Thus, each human body should be regarded as special and assessed on its own merits.

Anatomical variants may be partial or complete, single or multiple and, if affecting a paired structure, unilateral or bilateral. If multiple, they may be reciprocal and even compensatory.

Anomalies found on physical examination or by imaging may be of clinical significance per se or when misdiagnosed as being pathological.

Although anomalies are not normal they are not diseased. However, certain anomalies may have a decreased functional reserve, or a predisposition to disease. Others may impinge on or compress neighbouring structures.

Encountering anomalies, particularly when not anticipated, can pose problems during invasive procedures or surgical operations.

Pathological (G. ‘disease’) changes have impaired function in addition to abnormal structure. Pathological variation may be congenital or acquired. Congenital

malformations have a genetic and/or developmental basis, while acquired disorders are more likely to be caused primarily by environmental factors (e.g. physical, chemical, organisms), although genetic factors often contribute. Either way there is abnormal structure with abnormal function that is not healthy (health being the state of optimal physical wellbeing with the absence of disease).

It is vital for a clinician to distinguish typical from atypical, normal from abnormal, and health from disease.

Chapter 18: Growth and Development

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PRENATAL GROWTH AND DEVELOPMENT

POSTNATAL GROWTH AND DEVELOPMENT

PRENATAL GROWTH AND DEVELOPMENT

From an anatomical perspective, growth is increase in physical size while development refers to other structural

changes that occur until maturity. Prenatal growth and development occur from

conception to birth, typically lasting for about 38 weeks. This corresponds to about 40 weeks from the first day of the last menstrual period (as ovulation usually occurs two weeks later, just prior to fertilization of the ovum).

The embryonic (G. ‘in’ + ‘grow’) phase is the first eight weeks from conception while the foetal phase continues

until full term.

Implantation and bilaminar germ disc

The fertilized ovum is termed a zygote (G. ‘yolk’). This

single cell undergoes a series of divisions (cleavage) producing a ball of cells termed a morula (L. ‘mulberry’).

Fig.18.1 Development in 1st week

A fluid-filled cavity develops, creating a blastocyst (G. ‘germ’ + ‘bladder’). This has an inner cell mass, the embryoblast and an outer cell mass, the trophoblast (G. ‘nutrition’ + ‘germ’). The latter implants into the uterine wall at the end of the first week.

During the second week a bilaminar germ disc, the dorsal lamina becoming ectoderm and ventral lamina becoming endoderm, develops from the inner cell mass.

Ectoderm subsequently forms epidermis (and skin appendages) and nerve cells. Endoderm forms the epithelial lining of the digestive tract (gut) and of the respiratory tract (which buds out of the foregut).

Fig.18.2 Two-layered germ disc in 2nd

week

Trilaminar disc and organ development

During the third week, a trilaminar germ disc is formed with the development of mesoderm between the ectoderm

and the endoderm.

Fig.18.3 Three-layered germ disc in 3rd week

Mesoderm forms all connective tissues (including bone, muscle, fascia, dermis and the sheaths of peripheral nerves). Mesoderm also forms vessels (only mesoderm-derived structures are vascular).

Fig.18.4 Longitudinal folding due to neural tube

In the 3rd

to 8th weeks, all the major organ systems start to

develop (organogenesis). During this period of dramatic

structural change, birth defects tend to occur. The embryo folds both longitudinally and transversely. Cephalo-caudal folding occurs due to development of the neural tube while lateral folding occurs due to development of somites.

During the early embryonic phase, features appear from more primitive ancestors.

Early human embryos appear almost identical to those of other vertebrates. Further modifications, including disappearance of certain features, occur progressively with the embryo becoming more recognizably human by the end of the embryonic phase.


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Fig.18.5 Transverse folding due to somite development

Development of the upper lip, nostrils, external ears and eyelids make the face more human looking. The limbs have elongated and fully formed hands and feet are present. Primary centres of ossification appear in long bones and during this time the limbs rotate to the foetal position (elbows pointing backwards, knees forward).

Fig.18.6 Embryo at 6 weeks

Features of a foetus

Fig.18.7 Embryo at 5 weeks (7mm) and foetus at 6 months

At the beginning of the third month a foetus is about 3 grams in weight and 3 cm in crown-rump length (CRL). In

the foetal phase there is further growth and maturation of the organ systems created during the embryonic phase, with dramatic increase in size. The foetus is bathed in amniotic fluid with the lungs being un-inflated and

receiving minimal pulmonary circulation. The lungs are not developed sufficiently to enable

survival of a premature baby prior to about the 28th week.

The rib cage is relatively small with the ribs and heart horizontal. The large thymus (which attains its maximal relative size at birth) extends out of the thoracic cavity and into the neck. The abdomen is large, primarily due to the size of the liver. The spleen and suprarenal glands are also large and the kidneys are lobulated. The umbilicus is prominent and transmits the umbilical vessels and the urachus (a projection from the apex of the bladder).

The pelvis is relatively small with the bladder extending beyond the pelvic brim into the abdomen. In a male the scrotum is empty until about the 8

th month. The testes are

located in the abdominal cavity and gradually descend towards the scrotum guided by a fibrous band, the gubernaculum (L. ‘rudder’). The external genital organs

are large enough at about the 12th

week for sex to be determined on ultrasound examination. The spine of a foetus is C-shaped. In the foetal position the spine and limbs are all flexed.

Fig.18.8 Position of limbs and spine in a foetus

Moulding of cranium during birth

At birth a full term foetus is about 3 kg in weight and 50 cm in crown-heel length (CHL).

Fig.18.9 Least dimensions presented by skull at birth

The skull has the largest circumference of any part of the foetus and presents the major obstacle to passage along the birth canal. The foetal skull is elongated in an antero-posterior direction. During childbirth the foetal head normally rotates and flexes to present its smallest diameters as it negotiates the changing dimensions of the maternal bony pelvis between its inlet and outlet. The

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cranial bones also slide over each other allowing moulding of the cranium as the foetus passes along the birth canal.

Foetal circulation

The placenta, forming part of the internal lining of the uterus, is the site of exchange between maternal and foetal blood vessels, providing oxygen and nutrition (while removing carbon dioxide and wastes) throughout prenatal life. The umbilical cord is the connection to the placenta.

Fig.18.10 Umbilical vessels and bypass channels

Before birth, oxygenated blood is received from the placenta via the umbilical vein and deoxygenated blood

returned to it via the umbilical arteries. The immature liver and lungs are bypassed by

temporary vascular channels (ductus venosus and ductus arteriosus, respectively). Blood is also shunted

from the right atrium of the heart to the left via a temporary opening (foramen ovale) between them.

Circulatory changes at birth

Fig.18.11 Remnants after closure of vascular channels

At birth, respiration via the lungs occurs with the very first breath. Also at birth (with oxygen supplied by respiration from the lungs), the foramen ovale closes and the bypass channels are obliterated, becoming ligamentous remnants (ligamentum venosum and ligamentum arteriosum, respectively). The umbilical vein and arteries

are also obliterated, becoming ligamentous remnants (ligamentum teres and medial umbilical ligaments,

respectively).

POSTNATAL GROWTH AND DEVELOPMENT

General features of a neonate

The postnatal period of growth and development occurs until maturity. Infancy is the first year (including the neonatal phase for the first four weeks after birth).

The neonate (L. ‘new’ + ‘birth’) is a full-term infant,

delivered between 37 and 42 weeks. Neonates delivered before 37 weeks are ‘pre-term’ (or premature) while those

delivered after 42 weeks are ‘post-term’. The neonatal phase is associated with rapid maturation and growth of all organ systems.

Fig.18.12 Changes in body dimensions

A neonate has a relatively larger head and shorter lower limbs than an adult. This is also reflected in surface area. The head is about 18% of a neonate’s surface area while only 9% of an adult's.

Calculating fluid loss from burns in neonates

Fluid loss from burns depends on surface area and may be calculated from the ‘rule of 9’s’.

Fig.18.13 Changes in body surface area


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Neonatal head and neck

At birth the brain is large relative to the rest of the body and so is the skull accommodating it (the head represents approximately 30% of the newborn body mass). The bones of the cranial vault are ossified in membrane and at birth are separated by fontanelles (L. ‘small fountains’), gaps

filled with fibrous tissue. The anterior fontanelle is the largest (about 2.5 cm across).

Fig.18.14 Fontanelles in a newborn’s skull

The cranial bones become united at fibrous joints termed sutures. The frontal bone is in two halves, joined in

the midline at the frontal suture. The external auditory meatus consists of only a

cartilaginous part. The tympanic membrane is superficial and prone to be damaged unless care is taken during examination with an otoscope. The mastoid process is not developed, exposing the facial nerve, which is endangered in a forceps delivery. The mandible is small, with two halves joined in the midline at the mandibular symphysis (mental suture). Generally no teeth are present at birth.

Fig.18.15 High larynx in a neonate

The tongue is short and broad and is located entirely in the oral cavity. The larynx lies much higher than in the adult (enabling the newborn to simultaneously swallow while continuing to breathe).

Neonatal trunk and limbs

The bony thorax and pelvis are small relative to the size of the abdominal viscera. The liver extends well below the costal margin and even the spleen may be palpable. The suprarenal glands (primarily due to the cortex) are also large. Pelvic viscera (particularly the bladder) project upwards into the abdomen beyond the pelvic brim.

Primary curvatures of the vertebral column (thoracic

and sacral) develop first (with the heart and lungs and pelvic viscera, respectively).

Primary (diaphysial) centres of ossification are present in all limb long bones. One or two secondary (epiphysial) centres have appeared at the knee. A secondary centre in the distal end of the femur is the key forensic index of foetal maturity.

At birth the head of the femur is much larger than the acetabulum (predisposing it to dislocation).

The sacrum is more upright than in the adult, as is the iliac bone, contributing to the small, funnel-shape of the pelvis.

Changes to head and neck during infancy

The fontanelles of the cranium commence closure during infancy. The frontal and mental sutures begin to

disappear (resulting in a single frontal bone and a single mandible).

At approximately 6 months the primary (deciduous) dentition begins to appear. Lower central incisors erupt

first and by the end of the first year both upper and lower central and lateral incisors have usually erupted.

The neck lengthens and the larynx (with its epiglottis) descends. This elongates the pharynx, creating a region (the oropharynx) between the soft palate and the larynx enabling phonation. However, the capacity to simultaneously breathe and swallow is lost. Weaning normally occurs during the first year, when the infant accepts foods other than milk.

Changes to trunk and limbs during infancy

Secondary curvatures of the spine form during

infancy. The cervical curvature appears when the head is held erect and the lumbar curvature when walking commences. At birth, the bones of the pelvis and lower limb are less advanced than those of the pectoral girdle and upper limb but catch up by growing at a faster rate during infancy. The concavity of the sacrum increases as the infant begins to crawl, the bones of the pelvis become stronger and the acetabulum deepens. The feet are inverted and appear to lack arches (due to the presence of a large fat pad).

The infant’s high centre of gravity (at the level of the umbilicus) accentuates instability when the first attempts are made to walk.

Features of a child

Childhood may be divided into two phases, early childhood (years1-6) and late childhood (about years 7-13).

In childhood the remaining secondary centres of ossification appear, as well as primary centres in short bones (of the hand and foot).

Fig.18.16 Changes to skeleton from birth to childhood

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The skull vault almost reaches adult size by mid-childhood. The skull base (with bones formed in cartilage rather than membrane) continues to grow enabling the face to move anteriorly. This accommodates future development of the secondary (permanent) dentition. The bones of

the face and mandible grow as new teeth appear. At about 6 years the first permanent molar appears, followed soon after by the lower, then upper, incisors.

Fig.18.17 Body of mandible sectioned in a child

Paranasal sinuses start to develop at the beginning of the second year keeping pace with progressive eruption of the maxillary teeth, contributing to changes in the shape and size of the face.

Development of the pelvis allows the bladder and intestines to sink into it. This is associated with a flattening of the anterior abdominal wall.

Growth of nervous system

In the newborn the brain is relatively large although the cerebral cortex is only about half its adult thickness.

The neonatal cord terminates at about the level of the third lumbar vertebra. The adult level is higher (between L1 and L2) due to the differential growth between the nervous system and skeletal system. Myelination of nerve tracts is incomplete at birth, but occurs rapidly from about 6 months.

A number of reflexes are present at birth, the most important (for survival) including sucking and swallowing.

Fig.18.18 Nervous system starts maturing first

Growth of lymphoid organs

The thymus reaches its largest relative size at birth and increases in absolute size until puberty (when it starts to regress). Lymphoid tissue in general and the thymus in particular are very active during childhood, releasing lymphocytes into the circulation to establish the immune response.

Tonsillar tissue reaches its maximum development at about 6 years (and normally begins to involute afterwards).

Puberty and adolescence

Adolescence is the phase between the end of childhood (about year 13) and the beginning of adulthood

(about year 20).

Fig.18.19 Lymphoid organs reach maturity first

Growth of genital organs

The external and internal genital organs are immature in both male and female children. They enlarge dramatically following the end of childhood and in females there is also mammary gland development.

Fig.18.20 Genital organs start maturing last

Changes at puberty include ossification of the

remaining secondary centres and maturation of secondary sexual characteristics, including the external genitalia. Sex differences in the skeleton become more apparent. In males there is increased growth in width of the shoulders while in females the pelvis widens. In the male, onset of secondary sexual characteristics is associated with changes in the larynx (and deepening of the voice), in the skin and distribution of hair on the body. In females the menarche (G. ‘month’ + ‘beginning’) marks the time of the

first menstrual period. Adolescence is associated with a sudden and rapid

increase in growth in height. The ‘adolescent growth spurt’ is earlier in girls than boys, but in both sexes lasts

for approximately 2 years, after which increases in height continue, although at a slower rate. In girls, most growth in height is completed by the age of 18 years and in boys by 20 years. Typically, the earlier puberty begins the more rapidly it develops and the earlier it finishes.

Epiphysial closure and adulthood

The end of the adolescence is associated with adulthood (L. ‘grown up’) when the individual is physically mature. Growth in height ceases and all epiphyses close. The centre of gravity of the body shifts downwards to about the level of the pubic symphysis (due to accelerated growth in the length of the trunk during adolescence).

Forensic determination of age

In forensic determination of age or gauging maturity the stage of reproductive, skeletal and dental development can be assessed from a combination of external features, X-rays and dental records (providing reproductive, skeletal and dental age, respectively).

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AGE DIFFERENCES AND AGING

SEX AND BODY BUILD DIFFERENCES

FUNCTIONAL DIFFERENCES

AGE DIFFERENCES AND AGING

Fig.19.1 Age changes in the mandible

Normal variation due to age differences parallels the

stages of normal (prenatal and postnatal) growth and development until maturity (which for most organ systems

is reached by adulthood). Humans tend to survive beyond adulthood unlike wild animals, which are often killed by younger and healthier ones before effects of aging have time to develop.

Reduction in organ size termed involution (L. ‘to wrap up’) may occur prior to the onset of old age.

Involution of lymphoid organs

The lymphoid organs are the first organs to involute.

Fig.19.2 Lymphoid organs involute first

This occurs even before maturity is reached by many

other organs. The thymus is largely replaced by fibrous tissue before the end of adulthood (having reached its maximum size by puberty).

Lymphoid tissue in the spleen begins to decrease at puberty and lymph nodes and tonsillar tissue also regress.

Changes to bone marrow in adolescence

During adolescence red marrow in spongy bone of the limbs is gradually replaced by fat, becoming yellow marrow (except for the upper ends of the humerus and femur).

From adulthood red marrow is almost exclusively confined to the axial skeleton (although yellow marrow can revert to red marrow in response to severe blood loss).

Fig.19.3 Distribution of bone marrow types in an adult

Menopause

The menopause (G. ‘month’ + ‘pause’) is the cessation

of ovarian and uterine cycles. Although occurring at a variable time in different women, a normal ovarian cycle is rare beyond the age of 50. Diminution in the secretion of oestrogen leads to atrophy of the ovaries, shrinkage of the

uterus and atrophy of the breasts. A reduction in fat surrounding the breast causes the skin to wrinkle and sag.

Post-menopausal osteoporosis occurs rapidly due to

lower oestrogen levels, which results in bone removal exceeding bone deposition. At a critical level, post-menopausal osteoporosis is associated with a high risk of fracture.

Prostatic enlargement

The prostate gland tends to enlarge with age. Eventually, elderly men tend to have associated enlargement of the bladder due to increased thickness of its muscle wall (hypertrophy). This is a consequence of

urethral obstruction (distal to the bladder neck) caused by enlargement of the surrounding prostate.

There is no abrupt cessation of spermatogenesis,

although there is a gradual reduction in hormone production from the testes. The stage of decreasing

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androgens in men may be termed the andropause (G. ‘man’ + ‘pause’).

Senescence

Senescence (L. ‘growing old’) refers to the changes

that take place in the elderly. Adulthood may last for 40 years (from about age 20 to 60 years) while old age is from about 60 years to death. Normal life expectancy in western societies is approximately 80 years (females slightly greater than males).

Normal aging processes may merge with pathological changes (especially degenerative disorders). Bone mass decreases gradually with loss of collagen and of calcium. Osteoporosis (G. ‘bone’ + ‘porous’) results in thinning of

bony trabeculae with an increase in susceptibility to fracture and delayed healing.

Thinning of articular cartilage exposes underlying bone to increased stress while loss of water in the nucleus pulposus of intervertebral discs produces narrowing of disc spaces and loss of height. Soft tissues tend to calcify and skeletal muscles atrophy.

Sutures of the skull begin to fuse. Teeth deteriorate through gum disease (gingivitis) and tend to fall out.

Alveolar bone is exposed and the body of the mandible is resorbed (especially in the edentulous).

Skin loses its elasticity and pigmentation. Hair of the head tends to become grey and may fall out. This occurs particularly in males although coarse hair appears, especially in the nostrils and external ear. There is loss of elasticity throughout the cardiovascular system, including arteries, which also become harder (arteriosclerosis) and

more tortuous. The weight of the brain tends to decrease especially the

frontal lobes with fissures becoming deeper and wider than in the young.

SEX AND BODY BUILD DIFFERENCES

Aside from reproductive organs, there are certain differences between typical males and typical females elsewhere, particularly in the musculoskeletal system. However, there is also a range of variation that blurs the distinction between the sexes.

Forensic determination of sex

. In forensic anthropology, identification from individual bones is much less certain and often inconclusive. However, sex can usually be confidently identified from examination of complete adult skeletal remains. The part of the skeleton that best distinguishes males from females is the bony pelvis.

Typical male or female pelvis

There are identifiable differences in the shape and dimensions of the typical male bony pelvis in comparison to the typical female bony pelvis although the considerable range of variation may obscure this. The pelvic type in the male is typically android and in addition, the true pelvis is

small relative to the false pelvis. The inlet is heart shaped and the mid-pelvis funnels to the small pelvic outlet, which has a narrow sub-pubic angle. The pelvic type in the female is typically gynaecoid, and in addition the false

pelvis is small relative to the true pelvis (which accentuates its curve). The inlet is oval (wider transversely) and the mid-pelvis remains wide to the large pelvic outlet, which has a wide sub-pubic angle.

Fig.19.4 Sex differences in the pelvis

Obstetric assessment of pelvic dimensions

Dimensions of the birth canal indicate the probability of obstetric complications due to bony limitations.

Heavy, medium and light build

Due to genetic, hormonal and environmental factors

there is considerable normal variation in body size (both height and weight) and body build. This also applies to dimensions of particular body parts and even individual organs. Body build may be regarded as heavy, medium or light.

Fig.19.5 Sex and body build differences in the humerus.


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In general, males tend to be larger and of a heavier build than females. However, there is considerable variation between the sexes, as well as between, and within, different racial groups, let alone age groups.

Vulnerability to fractures from a fall

Lightly built elderly females are particularly vulnerable to bone fractures from a fall. In this group, the surgical neck of the humerus or lower end of the radius is particularly endangered from a fall on the outstretched hand and the neck of the femur is endangered from a fall on the hip

Prominent bone markings

Bones of heavily built males tend to be large and have prominent bony markings produced by the pull of correspondingly large muscles, tendons and ligaments. This may in part be related to occupation. A manual worker or an athlete is likely to have general or local muscular hypertrophy (associated with larger bones).

Central and peripheral fat deposits

Body build is not only dependent on musculoskeletal size but also on the amount of adipose (L. ‘fatty’) tissue.

While fat tends to be deposited in the subcutaneous tissue there are also certain areas of preferential deposition and these vary according to sex.

In males fat tends to be deposited within and around the abdomen (producing a lemon-shaped body form) while in females fat tends to be deposited around the buttocks and thighs (producing a pear-shaped body form).

Fat distribution and cardiovascular risk

The central distribution of fat typically seen in males is more likely to be associated with cardiovascular risk than the peripheral distribution, typically seen in females.

Somatotyping

Somatotyping (G. ‘body’ + ‘form’) is a method of

describing adult physique. Mesomorphs are muscular with little subcutaneous fat,

while endomorphs and ectomorphs are at opposite ends

of a continuum possessing greater to lesser amounts of fat. Excess body fat is termed obesity. It is common in many

western societies.

Body Mass Index

A measure of obesity may be derived from the ‘Body Mass Index’ (BMI).

BMI = weight (kg)/stature (m2).

A person with a BMI of 30 or more is regarded as obese. A person with a BMI of 25-30 is regarded as overweight.

FUNCTIONAL DIFFERENCES

Normal variation in size, shape or position of organs may occur due to functional differences.

The major physiological factors influencing anatomy are posture, phase of respiration and pregnancy. These particularly apply to mobile or expansile viscera. Other factors include exercise (e.g. on skeletal muscles and the cardiovascular system), presence of contents (e.g. food and fluid in the gastrointestinal tract) or activation (e.g. of

erectile tissue).

Normal variation with posture

When standing, due to gravity, all abdominal viscera descend, particularly those that are more mobile.

The most mobile viscera are those suspended by a mesentery.

The stomach and transverse colon are especially mobile, with mesenteries of considerable length. In certain individuals the stomach or transverse colon may even descend into the pelvis.

Fig.19.6 Postural variation in position of organs

The surface markings and vertebral levels for organs based on anatomical descriptions of a recumbent cadaver may be vastly different to those in a living person standing upright.

The curvatures of the spine and the arches of the foot are affected by lying, sitting or standing. Other effects of posture include distension and pooling of blood in veins.

Normal variation with respiration

Fig.19.7 Movement of organs during breathing

During inspiration the lungs expand and viscera directly below the diaphragm, particularly the liver (and gall bladder), spleen and kidneys, are pushed downwards as it descends.

Palpating abdominal organs on inspiration

Physical examination of abdominal organs includes attempting to palpate them on full inspiration.

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Normal variation with pregnancy

In pregnancy, there is enlargement of the uterus (which

rises above the pelvic inlet after the first trimester). Abdominal viscera (in particular those that are more mobile) tend to be displaced upwards (e.g. the appendix rises from its typical position in the right iliac fossa). Breast enlargement (due to mammary gland proliferation) occurs during pregnancy and beyond (until weaning or no longer required).

Fig.19.8 Functional differences in a woman near full term

Fig.19.9 Bloody Norm made me put this in when I was totally fucked

Fig.19.10 This one I just did because I’m a prick

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SIZE OR SHAPE

FEATURES OR ATTACHMENT

PRESENCE OR PERSISTENCE

ABSENCE AND DISAPPEARANCE

FUSION OR SEPARATION

NUMBER OR DUPLICATION

SIZE OR SHAPE

Abnormally long or short organs

The appendix in humans, although vestigial, is highly

variable in size (as well as orientation). In certain animals (e.g. ruminants that can digest cellulose) it can be even comparable in size to the large intestine.

Fig.20.1 Abnormally long appendix

The size of muscle bellies relative to their associated tendons is also variable. Palmaris longus has evolved a small belly in conjunction with a long tendon. It is now vestigial, with considerable variation in the degree of disappearance of the belly (and even its position along the tendon). Another example of a regressive variation is plantaris in the leg, which also has a small belly relative to the length of the tendon. Peroneus tertius is becoming more developed in humans (a progressive variation). Length of a duct (e.g. the cystic duct) is also variable. The cystic duct may be longer than normal (joining the common hepatic duct at a lower level) or shorter than normal. The 12th rib may be abnormally long or short.

An abnormally long appendix produces no functional impairment. However, it may be of clinical significance when inflamed, making diagnosis more difficult (especially if it comes to lie adjacent to other structures and irritates them).

Large or small vascular branches

Variations in size often affect branches of vessels and tend to be reciprocal. Thus if one branch is larger (and supplies a greater territory) another branch is reciprocally smaller (and supplies a correspondingly lesser territory). During development a vessel ultimately becomes the preferred channel from a number of possible alternative pathways. The vertebral arteries may vary in size, with one being larger than the other. The ‘circle of Willis’, an arterial anastomosis at the base of the brain, may have only small posterior communicating arteries connecting the internal carotid and vertebral systems.

Fig.20.2 Variation in size of arterial branches

Abnormally shaped viscera

The kidney is normally a paired organ that commences development in the pelvis and migrates into the abdominal cavity during prenatal life. A horseshoe kidney (incidence: about 0.2%) is the product of two kidneys that have united at their lower poles during migration, resulting in a characteristic horseshoe-shape. Although without overt functional impairment (hence regarded as an anatomical anomaly rather than a congenital malformation), functional reserve is decreased.

Fig.20.3 Horseshoe kidney

A horseshoe kidney may be endangered, particularly if encountered unexpectedly during surgical procedures.

Part of the head of the pancreas may encircle the duodenum, creating an annular (L. ‘ring’) pancreas.

Fig.20.4 Annular pancreas

Pelvic type in females

The shape of the pelvis in a female can vary from the classic gynaecoid type (about 50%).

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Fig.20.5 Gynaecoid pelvic type and variants in females

The android (about 30%), anthropoid (about 20%) and platypelloid (about 2%) types, have obstetric significance. Being of different internal dimensions, labour can be delayed or obstructed.

FEATURES OR ATTACHMENT

Abnormal processes and foramina

Fig.20.6 Anomalous bony features

Although rib elements are present in cervical and lumbar vertebrae, ribs normally occur only in the thoracic spine.

A cervical rib (about 1%) may arise from the (costal element of the) transverse process of the seventh cervical vertebra. This anomaly is associated with a higher

incidence of other spinal anomalies (‘cranial shift’ of the vertebral column) including a short 12

th rib. A lumbar rib

may form from the (costal element of the) transverse process of the 1st lumbar vertebra. This anomaly is also associated with a higher incidence of other spinal anomalies (‘caudal shift’ of the vertebral column) including a long 12

th rib. These anomalies may be unilateral or

bilateral, complete or partial. A supracondylar process or spur (about 0.8%) and a

supratrochlear foramen (about 4%) are anatomical variants in humans, although normal in certain animals. A spur may be associated with a fibrous band (ligament of Struthers) and even with other anomalies (e.g. muscular slips from biceps and coracobrachialis, or high division of the brachial artery).

The sternum ossifies from multiple centres on each side. Occasionally a gap may remain between adjacent centres leaving a sternal foramen (which may be confused with a bullet hole).

Abnormal features of viscera

Abnormal features of viscera include those that appear during development but usually disappear before birth. An ileal (Meckel’s) diverticulum (about 2%) may remain as an outpouching at the midpoint of the midgut. Duodenal diverticuli may also occur. Foetal lobulation of the kidney may persist as surface features demarcating each lobe. A developmental remnant associated with the thyroglossal duct is the pyramidal lobe of the thyroid gland (about 50%). Extra or absent fissures on the surface of a lung may result in three lobes on the left or two on the right (a reversal of the normal arrangement).

The arch of the azygos vein may segregate part of the lung, creating a lobe of the azygos vein (incidence: about 1%) although it is a normal feature of quadrupeds. This can create a shadow on X-ray.

Fig.20.7 Azygos lobe of right lung

Variations in the lobes of the liver also occur (a multi-lobed liver is normal in many animals). The uterus can be bi-cornuate may also occur (multiple horns are normal in the uterus of animals with litters). Deep notches of the spleen (creating multiple lobes) are associated with early branching of the splenic artery.

Abnormal muscle attachments

Attachments of certain muscles or tendons also vary. Pectoralis minor may have a slip of origin up to the second


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rib or down to the sixth rib (incidence: about 15%) from its normal attachment to the third, fourth and fifth ribs. The insertion of the tendon of peroneus longus may fall short in its migration across the foot, attaching to the base of the second metatarsal instead of to the first. Additional slips or heads of muscles are common (e.g. three heads of biceps brachii or three insertions of coracobrachialis). These normally occur in certain primates and are generally curiosities in man, rather than of clinical significance. There appears to be no functional disadvantage (they may even provide some advantage).

Abnormal ligamentous attachments

A partial cervical rib tends to be completed by a fibrous band (invisible on a plain radiograph). Although there is no functional impairment, it is of clinical significance if the cervical rib (or associated fibrous band) impinges on or compresses neighbouring structures (subclavian artery and/or lower trunk of brachial plexus).

Although a supracondylar spur produces no functional impairment, it tends to be attached by a fibrous band, termed the ‘ligament of Struthers’ (to the medial epicondyle). A ligament of Struthers may encircle and compress neighbouring structures (brachial artery and/or median nerve).

PRESENCE OR PERSISTENCE

Muscles not normally present

An extensor digitorum brevis manus is rare. It has no functional significance but may have clinical significance if it is misdiagnosed as a tumour.

Fig.20.8 Presence of extensor digitorum brevis manus

The axillary arch (about 7%) across the base of the axilla, sternalis (about 5%) on the front of the sternum and rectalis of the anterior abdominal wall are all examples of muscles not normally present in humans but present in some other animals. Rarely a remnant of the tail may be present in addition to the coccyx. Vestigial muscular tissue on the pelvic floor represents the levator and depressor caudae of tailed animals.

Scalenus minimus is an additional muscle of the neck (to scalenus anterior, medius and posterior) attaching to the first rib.

Bones not normally present

Abnormal presence of sesamoid bones may occur. The fabella (about 20%) occurs in the tendon of origin

of the lateral head of gastrocnemius and can be mistaken for a loose body in the knee joint.

Fig.20.9 Presence of fabella in a radiograph of the knee

A sesamoid bone may also be present in the tendon of peroneus longus (about 25%) and in the tendon of tibialis posterior (about 22%).

The persistent frontal (metopic) suture (about 8%, although more common in certain races) may be complete or partial. It is normally present during development (generally disappearing by the age of 8 years) and has no functional significance (but may have clinical significance if confused with a fracture).

Fig.20.10 Persistent frontal suture

Arteries not normally present

The thyroid ima artery (about 3%) arises from the arch of the aorta and passes in front of the trachea, where it is endangered by tracheotomy.

A persistent superficial brachial artery (about 5%) may remain from an earlier stage of development, in addition to the normal brachial artery. The median artery (about 8%) remains on the median nerve. The lateral costal artery (about 25%) arises from the internal thoracic artery (and links to the upper six intercostal arteries).

Multiple branches arising close to each other can have a common stem.

This particularly applies to the arch of the aorta. A

common trunk for the left common carotid artery and brachiocephalic trunk (about 22%) may occur. A common trunk for the left subclavian and left common carotid arteries (about 1%) may also occur. A common trunk from the external carotid artery for the lingual and facial arteries (about 20%) may occur.


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Veins not normally present

A persistent left inferior vena cava may remain between the left common iliac veins and the left renal vein, in addition to the normal inferior vena cava.

Variations in venous patterns are extremely common as veins develop from numerous endothelial channels.

These are too common to be considered anomalies. Many veins (and their tributaries) are un-named.

ABSENCE OR DISAPPEARANCE

Fig.20.11 Absent palmaris longus (on right)

Absent muscles and tendons

Complete absence of one or both palmaris longus muscles and associated tendons is common (about 15%). This has no functional significance but the otherwise underlying median nerve may be confused with this tendon. It is also more vulnerable to laceration, being directly under the skin.

Complete absence of plantaris is less common (about 6%). The sternocostal head of pectoralis major may also be absent. Peroneus tertius may be absent (about 6%).

Absent vascular trunks

An arterial trunk arsing from a main artery and subsequently dividing can be absent, with its branches arising independently.

The brachiocephalic trunk may be absent (about 2%) with the right subclavian and right common carotid arteries arising independently from the aortic arch.

The common interosseous artery is an arterial trunk that may be absent (about 10%). When this occurs the anterior and posterior interosseous arteries arise independently from the ulnar artery.

The right bronchomediastinal, subclavian and jugular lymph trunks may join to form the right lymphatic duct, which is the classic anatomical description. However, the right lymphatic duct is much more commonly absent (about 80%) and the lymph trunks enter the venous system independently.

Absent vessels

A large anastomosing branch of a neighbouring artery may replace an artery and take over its territory.

The dorsalis pedis artery may be absent (about 10%) with its territory taken over by the perforating branch of the peroneal artery. The lateral thoracic artery may be absent and its territory taken over by neighbouring branches of the axillary artery. Posterior communicating branches of the arterial circle of Willis (at the base of the brain) may be absent. The median cubital vein (linking the cephalic with the basilic vein) may be absent it is replaced by a median cephalic and a median basilic vein, which join forming a ‘V’.

Fig. 20.12 Absent median cubital vein (on left)

Unilateral agenesis of an organ

Agenesis (G. ‘absent’ + ‘formation’) of an organ is

regarded as an anomaly rather than a congenital malformation, provided there is no functional impairment. This applies only to certain viscera that are normally paired, as absence of an unpaired viscus cannot be compensated.

One kidney may be absent (about 0.2%) without functional impairment, although there is diminished functional reserve despite compensatory enlargement.

Before a paired organ is surgically removed the presence of its counterpart on the opposite side should be confirmed. Bilateral renal agenesis is a congenital malformation that is incompatible with life (at least soon after birth). Unilateral testicular or ovarian agenesis may also occur.

FUSION OR SEPARATION

Fused bones

Fig.20.13 Partial sacralisation of L5


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Abnormal fusion of vertebral elements tends to occur at transitional regions

Partial or complete fusion of the fifth lumbar vertebra to the sacrum (about 5%) is termed sacralisation of L5. If complete, there is a reduction of lumbar vertebrae from five to four (about 1%).

Occipitalisation of the atlas may occur, where the first cervical vertebra is incorporated into the occipital bone of the skull.

The lunate of the wrist may fuse with the adjacent triquetrum. Fusion of phalanges may occur, particularly in the little toe. A fractured biphalangeal little toe may be overlooked.

Incomplete fusion of bones

Fig.20.14 Sacralisation of L5 and lumbarisation of S1

Partial or complete separation of the first sacral vertebra from the rest of the sacrum (about 5%) is termed lumbarisation of S1. If complete, there is an additional lumbar vertebra (from five to six).

Accessory bones

Accessory bones are created by failure of a centre of ossification to fuse with the rest of the bone.

The os acromiale (about 8%) is an atavistic (L.

‘ancestor’) epiphysis. This is a phylogenetic remnant of a discrete bone in certain animals. The epiphysis for the tip of the acromion remains separate from the rest of the scapula.

Fig. 20.15 Os acromiale

Accessory bones in the foot include the os trigonum (about 8%), a separate lateral tubercle of the talus bone and the os tibiale externum (about 4%), a separate tuberosity of the navicular bone. The os trigonum is the homologue of the lunate bone in the hand (representing the phylogenetic ‘os intermedium’ seen in certain animals).

Fig.20.16 Os tibiale externum in a radiograph of the foot

The patella develops from multiple centres of ossification. One of these may remain unfused, creating a bipartite patella. Small accessory bones may occur within the sutures of the skull. These are termed sutural (or wormian) bones. The zygomatic bone may ossify in two parts, creating the os japonium (about 0.2%). The occipital bone may also ossify in two parts, creating the inter-parietal bone or os incae. These accessory bones are more common in certain races. The os odontoideum is a separate tip of the odontoid process of the 2

nd cervical

vertebra. Accessory bones cause no functional impairment but

have clinical significance if mistaken for a fracture on X-ray. As they may be bilateral in about one in three individuals with the anomaly, X-ray of the corresponding part on the opposite side provides confirmation in such cases.

Cranial or caudal shift of spinal elements

Genetic variation can produce changes in segmentation, which relies upon the differential expression of sets of genes in the long axis of the body about the fourth week of development.

Anomalies of bony fusion and non-fusion may create a domino effect along the spine.

Fig.20.17 Clusters of associated spinal anomalies


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In ‘cranial shift’, sacralisation of L5 is associated with

non-fusion of the fifth piece of the sacrum (and fusion of S5 to the coccyx), a short twelfth rib (resembling a transverse process) and the presence of a rib on the seventh cervical vertebra (cervical rib). In ‘caudal shift’, lumbarisation of the

sacrum is associated with non-fusion of the first coccygeal vertebra (and fusion of it to the sacrum), a long twelfth rib and the presence of a rib on the first lumbar vertebra (lumbar rib). An alteration in the number of vertebrae may also be associated with anomalies in the corresponding contributions of spinal nerves to limb plexuses (e.g. resulting in pre-fixed or post-fixed plexuses).

NUMBER OR DUPLICATION

Supernumerary and accessory arteries

Supernumerary (L. ‘above’ + ‘number’) arteries arise

when one or more additional arteries branch from the same arterial stem and they are equivalent in size. With supernumerary arteries it is difficult to distinguish which is the normal one. An accessory artery is the artery that is

clearly additional to the normal one. It may even start from a different arterial stem (an aberrant accessory artery). Supernumerary and accessory arteries occur when more than one of the multiple arterial channels that appear during development is retained. A succession of renal arteries arises from the aorta (and normally disappears) as the kidneys migrate upwards from the pelvis to their final position in the abdomen. Supernumerary or accessory renal arteries (about 25%) result if these intermediary arteries do not disappear.

Fig.20.18 Accessory renal arteries

Accessory nerves

An accessory phrenic nerve can occur in addition to the phrenic nerve (which arises from the cervical plexus). It is a small nerve that arises from the nerve to subclavius and may accompany the phrenic nerve and even join it. The accessory obturator nerve (about 0.8%) is an additional branch from the lumbar plexus.

Accessory organs and tissue

Supernumerary nipples (about 1%) may occur anywhere along the milk line (between the axilla and the thigh). They may even be associated with breast tissue. Many other animals, particularly those that produce litters, have multiple breasts (bilaterally) along each milk line.

Accessory spleens (about 10%) termed splenunculi, are aggregations of splenic tissue along the course of the splenic artery. They enlarge after splenectomy.

Accessory hepatic ducts (about 7%) can arise from the liver and join the common hepatic duct, or the cystic duct. They are endangered in gall bladder surgery.

Accessory suprarenal tissue (about 22%) can lie in the kidney, testis or scattered on the posterior abdominal wall.

Fig.20.19 Potential sites of supernumerary nipples

Double and bifid structures

Duplication of a ureter or the pelvis of the kidney, (about 1%), may be present unilaterally or bilaterally. Partial duplication creates a bifid ureter or a bifid renal pelvis (about 1% each). Such anomalies may be associated with recurrent urinary tract infection.

Fig.20.20 Complete and partial duplication of ureters

Bifid ribs (about 1%) can arise by duplication of part of the rib body. A double aortic arch encircling the oesophagus and trachea is a rare anomaly caused by retention of part of the original embryonic arterial pattern (six pairs of arches associated with the primitive aorta). It is a normal feature in certain other animals (e.g. frogs). A double vagina (or vaginal septum) and/or double uterus (about 0.1%) are uncommon human variants. However, multiple uterine horns occur in mammals that produce litters.

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SITE OR ORIENTATION

SIDE OR COMMUNICATION

ORIGIN OR BRANCHING

COURSE OR RELATIONSHIP

DIVISION OR DEPTH

ENDING OR DISTRIBUTION

SITE OR ORIENTATION

Incomplete ascent or descent

During development migration may occasionally fall short of the normal site.

The kidney ascends from the pelvic cavity during development and migrates to its normal position on the posterior abdominal wall, adjacent to the suprarenal glands (which develop separately). Its vascular supply changes as it ascends by progressively receiving (and losing) vessels from the nearest major arteries and veins. A pelvic kidney (about 0.1%) falls well short in its ascent. Its (segmental) arteries arise primarily from the adjacent part of the aorta (or common iliac arteries) and its veins drain primarily to the adjacent part of the inferior vena cava (or common iliac veins). In contrast, the suprarenal gland remains in the normal position.

Fig.21.1 Failed ascent of left kidney

The thyroid gland sometimes does not descend into the neck from its origin (the foramen caecum) on the dorsum of the tongue, but remains as a lingual thyroid (about 0.3%).

This has clinical significance if not recognised as

ectopic thyroid tissue (particularly if this is the only site of thyroid tissue) and is surgically removed.

Ectopic organs and tissue

During development migration may occasionally overshoot the normal site or deviate to an abnormal site.

Anatomical variation in position of parathyroid glands may be due to migration overshooting its descent to the normal site adjacent to the thyroid gland. This has clinical significance in thyroid surgery if an ectopic parathyroid gland is inadvertently removed.

Ectopic pancreatic tissue (about 2%) may be found in a persistent ileal diverticulum, as may ectopic gastric tissue.

Abnormally mobile viscera

The position of certain abdominal viscera can vary considerably due to due to excessive mobility from an

abnormally long mesentery. In such cases the stomach or large intestine may even lie in the pelvis.

The retention of a mesentery that normally disappears can create an excessively mobile caecum (about 10%), predisposed to twisting (caecal volvulus). This may lead to strangulation and subsequent gangrene.

Abnormally oriented viscera

The orientation of the appendix is highly variable. Although the position of its base at the caecum is constant the rest of the appendix may point in any direction. The

normal orientation is retrocaecal (about 64%). The most common anomaly is a pelvic appendix (about 32%).

Fig. 21.2 Variations in orientation of appendix

Other anomalies include transverse (about 2.5%), pre-ileal (about 1%) and retro-ileal appendix (about 0.5%).

The uterus is normally anteverted as the long axis of the cervix is directed more anteriorly than the long axis of the vagina. A retroverted uterus is a common variant.

SIDE OR COMMUNICATION

Vessels on opposite side of body

A left sided superior vena cava and/or a left-sided inferior vena cava may be present. The vena cava develop from a complex bilateral set of venous channels (cardinal veins) which typically disappear on the left but may abnormally disappear on the right instead, or at least persist on the left.

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There may be no impaired function (at least overtly). However, a left superior vena cava drains into the coronary sinus prior to entering the right atrium, creating gross enlargement of this vein.

Fig.21.3 Left-sided inferior vena cava

There is also an association with some degree of visceral transposition (e.g. the heart being situated on the right side of the body) or vascular transposition (e.g. reversal of the azygos venous system with the azygos vein on the left, the hemiazygos and accessory hemiazygos veins on the right). These have clinical significance as unexpected findings on physical examination, imaging or at surgery.

Situs inversus

Rarely there may be a complete situs inversus (about 0.01%) where the thoracoabdominal viscera are in mirror image to normal. The heart may be situated on the right side of the body, termed dextrocardia (in an additional 0.01%).

Patients with Kartagener’s syndrome present with situs inversus, respiratory infections and male sterility. The common factor is a defect in motility of cilia on all ciliated cells.

Patent endothelial channels

Abnormal communications may occur from endothelial channels failing to close during development.

A probe-patent foramen ovale (about 25%) occurs without any functional impairment, as its overlying flap remains apposed during life. In contrast, an atrial septal defect is regarded as a congenital malformation, being an open pathway allowing shunting of blood.

Patent mesothelial channels

The testis descends into the scrotum with a prolongation of the peritoneal cavity (the processus vaginalis), which normally closes prior to birth. A patent processus vaginalis through the inguinal canal into the scrotum predisposes to an indirect inguinal hernia.

Variable patterns of communication between individual synovial tendon sheaths of the digits with the common synovial tendon sheath in the hand may occur (about 28%).

ORIGIN OR BRANCHING

Aberrant arteries

An aberrant (L. ‘straying’) artery arises from a different

artery. Aberrant arteries tend to arise from a neighbouring

artery, close to the normal artery of origin. An aberrant left vertebral artery arises from the arch of the aorta (about 5%) instead of the left subclavian artery. An aberrant superior thyroid artery arises from the common carotid artery rather than the external carotid.

Fig.21.4 Aberrant left vertebral artery

An aberrant cystic artery (to the gallbladder) arises from neighbouring arteries and not from the right hepatic (its usual origin). Anomalous origins include from the left hepatic artery (about 6%), the common hepatic artery (about 2%) or the gastroduodenal artery (about 2%). An aberrant dorsal pancreatic artery arises from the coeliac trunk instead of the splenic artery.

Arteries that normally arise from a common trunk may instead arise independently. An aberrant right hepatic artery can arise from the superior mesenteric artery (about 12%) instead of the common hepatic artery. A branch of the thyrocervical trunk may arise directly from the subclavian artery. Aberrant circumflex femoral arteries can arise directly from the femoral artery instead of the beginning of the profunda femoris artery. Aberrant perforating branches can arise from the femoral artery rather than the profunda femoris artery. An aberrant posterior circumflex humeral artery can arise from the subscapular artery (about 20%) instead of the axillary artery and an aberrant profunda brachii artery can arise from the posterior circumflex humeral artery (about 7%) instead of the brachial artery.

Aberrant arteries may also occur in addition to, rather than instead of, the normal artery. Such aberrant arteries are termed aberrant accessory arteries.


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A left hepatic artery can arise from the left gastric artery (about 22%) instead of, or as well as, from the normal origin (the common hepatic artery). Half of these aberrant accessory hepatic arteries arise from the left gastric artery in addition to a normal left hepatic artery.

Origin on abnormal arterial trunk

Whenever multiple branches lie close to each other, they may arise from a common stem rather than independently. This particularly applies to the arch of the aorta. The left common carotid artery may arise in common with the brachiocephalic trunk (about 22%). The left subclavian and left common carotid arteries may arise by a common trunk (about 1%). The lingual and facial arteries may arise from a common trunk (about 20%) rather than independently from the external carotid. Both anterior and posterior circumflex humeral arteries may arise from a common trunk (about 20%) rather than independently from the axillary artery. The profunda brachii artery may arise from a common trunk with the posterior circumflex humeral artery (about 14%).

Abnormal arterial branching

Vessels develop from networks that have the potential for change, where preferred channels remain while others regress (providing scope for variation).

Fig.21.5 Scope for variation during arterial development

The pattern of preferred channels produces alternative pathways (e.g. where the same set of arteries may supply a particular territory but via different routes). These variations tend to be reciprocal (i.e. if one branch is

smaller, another is larger to compensate). Branches may also arise at a more proximal or more distal site along an artery. Ultimately every individual has a unique branching pattern for each vessel, as there is no requirement at this level for symmetry. Like fingerprints, even the branches of the central artery of the retina (as seen in examination of the optic fundus) are peculiar to each individual.

Branching patterns often vary, particularly for arteries that anastomose with other arteries. Any alternative path may become the preferred channel. The inferior epigastric and obturator arteries normally anastomose via their pubic branches. An aberrant obturator artery arises from the epigastric artery (about 30%) where a large pubic branch replaces the normal origin from the internal iliac artery or is additional to a normal obturator artery (i.e. an aberrant accessory obturator artery).

Fig.21.6 A normal and an aberrant obturator artery

A few ‘classic' patterns (described in textbooks as the standard) are themselves atypical or present in less than 50% of cases. It is contentious as to which pattern is anomalous. For example, a complete superficial palmar arch in the hand (linked to the superficial palmar branch of the radial artery) occurs in fewer than 30% of cases. A classic thyrocervical trunk is present in fewer than 50% of cases. A classic branching pattern of the axillary artery occurs in only about 10% of cases. Even a classic circle of Willis (at the base of the brain) is present in just over 50% of cases.

Abnormal nerve branching

Branching patterns of nerves are also variable (although less so than vessels), particularly those arising from a nerve plexus and associated components (including roots and branches).

Although axons in a nerve plexus do not directly communicate with each other (in contrast to vascular anastomoses that intercommunicate via lumens) they are wrapped in common connective tissue sheaths. Axons therefore tend to be bundled in a variety of ways, leading to variation in the origin of a nerve from a plexus or branches arising from a particular nerve (depending on the epineurium they finally get wrapped in).

The musculocutaneous nerve and the median nerve both arise from anterior divisions of the brachial plexus and can contain branches from each other (incidence: about 20%). The brachial plexus may arise from more cranial spinal nerve roots (pre-fixed) or more caudal (post-fixed). These may be associated with vertebral column anomalies (e.g. cranial and caudal shift), particularly those affecting the number of vertebrae.

COURSE OR RELATIONSHIP

Abnormal course of an artery

An artery may take an abnormal course, even though it has a normal site of origin.

The vertebral artery may enter the foramen transversarium of the fifth cervical vertebra (about 5%) or less commonly the seventh, fourth or third rather than its normal course into the sixth.

Abnormal course of aberrant artery

A retro-oesophageal right subclavian artery (about 1%) arises aberrantly from the arch of the aorta (to the left of the left subclavian artery) instead of from the brachiocephalic trunk (which is absent). It takes an abnormal path behind the oesophagus and may compress it, causing dysphagia (G. ‘bad’ + ‘eat’; i.e. difficulty in swallowing). The special case of dysphagia due to compression by an anomaly is termed dysphagia ‘lusoria’

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(L. ‘a sport of nature’). The anomaly itself has no functional impairment of blood flow, but may have clinical impact on an adjacent structure (in this case, the oesophagus).

Fig.21.7 Retro-oesophageal right subclavian artery

10% of aberrant obturator arteries pass medial to the femoral canal rather than lateral to it and are regarded as ‘endangered’ aberrant obturator arteries (about 3% overall).

Because it lies along the free margin of the lacunar ligament, the artery is potentially endangered in femoral hernia surgery (where the lacunar ligament is cut).

Abnormal relations of an artery

The inferior thyroid artery has a variable relationship to branches of the recurrent laryngeal nerve adjacent to the thyroid gland. The artery (in about equal proportions) may run superficial to, deep to or through the nerve branches (which are therefore endangered in thyroid surgery).

The right hepatic artery passes in front of the common hepatic duct (about 24%) and behind the portal vein (about 9%).

The cystic artery passes in front of the common hepatic duct (about 24%). It is endangered in gall bladder surgery.

Abnormal course of a nerve

In coursing from the axilla to the anterior compartment of the arm the musculocutaneous nerve may pass superficial to coracobrachialis rather than through it.

A ‘non-recurrent’ right recurrent laryngeal nerve occurs in association with an aberrant right subclavian artery. Instead of looping under the right subclavian artery it passes from the right vagus nerve directly to the larynx and is endangered in thyroid surgery.

DIVISION OR DEPTH

Abnormally high division of artery

During development, the axillary artery divides into a pair of arteries (brachial artery and superficial brachial artery). The latter normally disappears and the brachial artery divides into two branches: radial and ulnar, at the level of the cubital fossa.

Fig.21.8 Superficial ulnar artery from a high division

A high division of the axillary artery (about 4%) or of the brachial artery (about 6%) can occur. The high division may either be due to persistence of the superficial brachial artery (the brachial artery dividing subsequently into radial and ulnar arteries) or division directly into radial and ulnar arteries.

The roots of the median nerve normally pass in front of the axillary artery, however with a high division of the axillary artery one branch passes in front of them. The median nerve normally passes in front of the brachial artery in the arm. If the brachial artery disappears, instead of the superficial brachial artery, the median nerve remains deeply located (about 12%). Where there is a pair of brachial arteries in the arm (about 10%) the median nerve passes between them. The popliteal artery normally divides at the distal border of popliteus however it may divide at its proximal border (about 2%).

Abnormally superficial artery

The ulnar artery normally passes deep to the common origin of the forearm flexor muscles however; it may pass superficial to them (about 3%) particularly if associated with a high division of the brachial artery. A superficial ulnar artery can be endangered by inadvertent injection (as it tends to be mistaken for a superficial vein). Intra-arterial injection of certain drugs may produce intense vasoconstriction causing death of forearm and hand musculature. Injection of anaesthetic agents should not be given in the cubital fossa for this reason.

Abnormally deep artery

The maxillary artery normally passes superficial to the lateral pterygoid muscle but may pass deep to it (about 30%).

Abnormally high division of a nerve

The sciatic nerve normally passes below piriformis, however with a high division (about 12%) the common peroneal portion of the nerve pierces piriformis or may even pass above it (about 0.5%).


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ENDING OR DISTRIBUTION

Abnormal duct termination

The cystic duct may terminate lower than normal on the common hepatic duct or drain abnormally into the right hepatic duct at a higher level.

The bile duct and main pancreatic duct may terminate separately on the duodenal papilla (about 5%). In about 9% of cases, the accessory pancreatic duct does not open into the duodenum (about 50%) but into the main pancreatic duct.

Abnormal arterial termination

The arterial circulus vasculosus (‘circle of Willis’) at the base of the brain is created by the terminations of the two internal carotid and two vertebral arteries (via the basilar artery) that form the anterior, middle and posterior cerebral arteries. This anastomosis is typically completed by an anterior and a pair of posterior communicating branches (between the anterior cerebral arteries and between the middle and posterior cerebral arteries, respectively). The normal pattern occurs in just over 50%, with the most common variant being absent or small posterior communicating arteries (about 20%). There are also variations in the relative sizes and distributions of the cerebral arteries, which can also be asymmetrical.

Fig.21.9 Asymmetrical distribution of cerebral arteries

Generally blood from the two arterial systems: internal carotid and vertebro-basilar) does not mix, but the anastomosis provided by the communicating arteries enables the potential for alternative paths in occlusion affecting one or more of them. The clinical features of a stroke from such occlusion are therefore influenced by the anatomical arrangement of the circle of Willis.

Abnormal lymph trunk or vein ending

Terminations of the lymph trunks (jugular, subclavian, bronchomediastinal) may enter veins independently of the thoracic duct or right lymphatic duct. The inferior mesenteric vein typically terminates in the splenic vein but may terminate in the superior mesenteric vein (about 10%) or at the junction of the splenic and superior mesenteric veins (about 30%). Tributaries of the great saphenous vein

may terminate in the femoral vein. The external jugular vein may terminate in the cephalic vein (and pass superficial to the clavicle prior to this).

Arterial dominance

The pattern of vascular distribution is compensatory. If one territory is larger (from arterial dominance) a neighbouring territory tends to be smaller.

The heart is supplied by the left and right coronary arteries. Typically, the right coronary artery provides the posterior interventricular branch also known as the posterior descending artery (PDA). It supplies territory beyond the posterior interventricular groove. This is termed ‘right dominance’.

Fig.21.10 Normal coronary arteries (right dominance)

The left coronary artery may provide the posterior interventricular branch/’PDA’ (about 10%), termed ‘left dominance’. When both coronary arteries contribute equally to the PDA, it is termed ‘balanced’ (about 5%).

Fig.21.11 Angiogram of dominant left coronary artery

Other variations even include a single coronary artery, which provides all branches. The sino-atrial nodal branch may arise from the left coronary artery (about 45%) and the atrio-ventricular nodal branch from the left coronary artery (about 10%).

The area of myocardial infarction from a coronary occlusion in a particular individual is dependant on the pattern of distribution and degree of coronary artery dominance. This is of particular significance regarding supply of the conducting system and the associated risk of a life-threatening arrhythmia.

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CONGENITAL MALFORMATIONS

ACQUIRED DISORDERS

CONGENITAL MALFORMATIONS

Congenital (L. ‘with’ + ‘born’’) malformations, also

termed birth defects, are pathological structural changes that arise before birth. However, certain abnormalities from defective prenatal growth and development may not be diagnosed until later. Major malformations are recognised in 2- 3% of live newborn babies and an additional 2- 3% during infancy. Single minor malformations occur in about 15%.

Functional impairment

There is a fine line between the anatomical variation of anomalies and the pathological changes of congenital

malformations.

In contrast to anatomical variation (with abnormal structure or position but no functional impairment) pathological changes have impaired function, even if not immediately evident.

The abnormalities most difficult to classify are those

present at birth without functional impairment, but with a diminished functional reserve. A structural abnormality (e.g. a patent foramen ovale or horseshoe kidney) that remains symptomless throughout life as an incidental finding (e.g. on imaging or at post-mortem) is considered to be an anatomical anomaly. In contrast, a structural abnormality (e.g. an atrial septal defect or polycystic kidney) that is symptomless at birth but later manifests functional impairment is considered to be a congenital malformation.

Spontaneous abortions

About 50% of conceptions do not result in a live birth but spontaneously abort early (and if prior to implantation, undetected). At least half have severe chromosomal abnormalities.

Malformations occur when organ systems are forming (between the third to eighth weeks) and most major malformations spontaneously abort.

About 20% of perinatal deaths are also due to

congenital malformations. Genetic mutations and chromosomal abnormalities are solely responsible for about 15% of malformations. Environmental causes are solely responsible for about 10% and include physical agents (e.g. radiation), chemical agents (e.g. drugs) or organisms (e.g. certain viruses). The remainder (about 75%) are from multifactorial or unknown causes.

Major and multiple minor malformations

Multiple minor malformations generally signify an underlying major malformation.

This is important in the routine examination of a

newborn child.

About 25% of newborns with a major malformation have

other major malformations and generally do not survive for long.

There are associations between certain birth defects. The specific malformations produced depend on the critical stage of development reached by organs at the time of influence from the causative factors.

The most common major malformations involve the: - central nervous system (about 30%) - cardiovascular system (about 30%) - digestive system (about 10%) - urogenital system (about 10%) - musculoskeletal system (about 10%) A malformation syndrome (G. 'running together') is a

cluster of certain birth defects, particularly due to chromosomal abnormalities.

Down's syndrome (e.g. from an extra chromosome

21) typically includes cardiac, craniofacial and many other defects, both major and minor, along with mental retardation.

Defective closure or migration

Defective closure of the inter-ventricular septum or inter-atrial septum of the heart results in ventricular septal defects (incidence: about 0.4% of live births) and atrial septal defects (incidence: about 0.2%), respectively. Ventricular septal defects may be isolated or part of Fallot's tetralogy.

Fig.22.1 Defective closure of interatrial septum

Fig. 22.2 Tetralogy of Fallot


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Defective closure of the distal part of the urethra results in hypospadias (about 0.3%).

A patent ductus arteriosus (about 0.2%) may persist, rather than closing at birth.

Communication between the pleural and peritoneal cavities may persist causing a congenital diaphragmatic hernia (about 0.005%) with a large part of the stomach lying in the chest. However, a minor deficiency at the left vertebrocostal trigone of the diaphragm is not uncommon, with the left kidney lying in contact with pleura.

The umbilicus may rarely fail to close resulting in an omphalocoele with large herniation of gut, although a minor congenital umbilical hernia (about 15%) may be present for a short time after birth.

Failure of a lip or the palate to unite may result in hare lip and/or cleft palate (about 0.1%).

A congenital cerebral aneurysm results from deficiency of the media of arteries at a branch point in the circle of Willis (about 1%).

Spina bifida cystica (about 0.1%) is a serious defect involving exposure of the coverings of the spinal cord (meningocoele) or even the spinal cord/cauda equina in addition (meningo-myelocoele).

Fig.22.3 Spina bifida

However, spina bifida occulta (about 2%) is minor affecting the neural arch of one or more vertebrae and often not detected.

Fig.22.4 Tracheo-oesophageal fistula

An abnormal passage may remain from defective closure of the trachea from the oesophagus as a serious tracheo-oesophageal fistula (about 0.05%). An auricular pit,

sinus, fistula or cyst (incidence: about 1%) is not uncommon, although a branchial fistula or cyst is rare.

An undescended testis (about 0.3%) results from failure of a testis to migrate into the scrotum. Although a testis has not quite reached the scrotum in 3% of full term births and in 30% of premature births, it does so soon after. An ectopic testis is one that has migrated to a site other than the scrotum. Malrotation of the gut may occur.

Failure of ganglia to migrate from the neural crest to the wall of the large intestine results in congenital megacolon (Hirschsprung’s disease).

Transposition of great vessels occurs with the aorta and pulmonary trunk in the heart.

Defective opening or formation

Defective opening or canalisation may occur with tubes or tubules.

Oesophageal atresia, intestinal atresia and biliary atresia result from failure of the lumen to canalise . Imperforate hymen and imperforate anus (about 0.02%) result from defective opening of the cloacal membrane.

Polycystic kidneys (about 0.2%) occur when tubules of the nephron fail to open into those derived from the ureteric bud. Multiple cysts occur in polycystic liver and in cystic fibrosis of the pancreas.

Fig.22.5 Polycystic kidney

Aqueductal stenosis in the brain may cause congenital hydrocephalus, with accumulation of cerebrospinal fluid and enlargement of the cranium. Stenosis of the pulmonary or aortic valves in the heart may occur and in coarctation of aorta (about 0.1%) a constriction is located near the ductus arteriosus.

Certain structures may fail to form. These include digits and even limbs. An absent or

rudimentary brain is known as anencephaly (about 0.1%) and in bilateral renal agenesis (about 0.03%) both kidneys are absent. Congenital absence of lymph vessels in a lower limb (Milroy's disease) results in lymphoedema. An absent uterus (about 0.02%) and even vagina may occur.

Certain structures may form defectively. Chest deformities (about 0.1%) may result in funnel

chest (depressed sternum) or pigeon chest (protruding sternum). Congenital dislocation of hip may be due to defective formation of the acetabulum of the hipbone. It requires recognition (and splinting) prior to weight bearing to prevent long-term effects. Talipes (club foot) results in a deformity. In polydactyly there is an extra digit or digits and in syndactyly two or more digits are fused.

Failure of development of a centre of ossification in the body of a vertebra results in hemivertebra with associated scoliosis of the spine.

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Fig.22.6 Scoliosis due to a hemivertebra

Failure of development of epiphyses of long bones produces an achondroplastic dwarf. Microcephaly of the head and micrognathia of the jaw may occur. Certain genetic disorders (e.g. adrenogenital syndrome from congenital adrenal hyperplasia) and chromosomal disorders (e.g. Down's syndrome, Turner's syndrome and Klinefelter’s syndrome) also produce defective formation of organs as secondary effects.

ACQUIRED DISORDERS

There is a fine line between normal variation and pathological variation regarding acquired disorders. The normal variations most difficult to classify are those due to aging. The normal changes associated with senescence merge with certain degenerative disorders. There is even a fine line between congenital malformations and acquired disorders. The most difficult to classify are those birth defects that do not manifest themselves until later in life, and those acquired conditions that have a primarily genetic basis.

Traumatic disorders

Trauma is the disruption of tissues by physical injury.

The nature of the traumatic condition is dependent on the type of anatomical structure involved: skin or mucous membrane ulceration (loss of epithelial continuity), soft tissue laceration (tearing) or contusion (crushing), muscle or tendon strain, ligament sprain, bone fracture or joint dislocation. For certain of these (e.g... muscle strains or

ligament sprains) the degree of disruption may be classified as microscopic (first degree), partial (second degree) or complete (third degree).

Fig.22.7 Spinal injury with associated cord damage

Inflammatory disorders

Inflammation is the response of living tissues to injury. The cause of the injury may be physical, chemical, organismal or autoimmune. The inflammatory response

is produced by vascular and connective tissues, directed towards protection. However, it may result in some collateral damage (and in autoimmune conditions the body is ‘tricked’ into attacking a particular normal host tissue).

The degree of the inflammatory response depends on the anatomical site and local blood supply, as well as on general health and immunity.

Fig.22.8 Acute lobar pneumonia

The two types of inflammation are acute and chronic. Acute inflammation is characterised by the cardinal

signs of redness (‘rubor’), swelling (tumor’), heat (‘calor’) and pain (‘dolor’). This response is due to increased blood flow from vasodilatation (‘hyperaemia’), leakage of fluids and plasma proteins from increased vascular permeability (‘exudation’) and passage of certain white blood cells from the blood to surrounding tissues through the vessel wall (‘emigration’).

The possible sequelae of acute inflammation are restoration to normal (resolution), passage along anatomical pathways (spread), abscess formation (suppuration), scar formation (fibrosis) or procedure to chronic inflammation.

Spread may be direct (e.g. along fascial planes), lymphatic (via local lymph vessels to regional lymph nodes (containing collections of defence cells that filter lymph prior to its return into the venous system) or blood (via local veins prior to circulating around the body).

An abscess (L. 'to go away') is a localised collection of

pus (dead tissue, defence cells and micro-organisms) due to massive emigration of certain white blood cells. A scar is

due to the production of collagen fibres from connective tissue cells that proliferate after damage associated with acute inflammation. Chronic inflammation may follow acute inflammation if the causative agent remains (e.g. persistence of a foreign body) or may occur from the outset with an organism of low virulence. There is infiltration of certain defence cells together with the proliferation of connective tissue.

Degenerative disorders

Degeneration is the effect on living cells by injury. The

cells primarily affected are those embedded in the vascular and connective tissues of an organ. These specific cells (parenchymal cells) are most sensitive to direct harm from the injury while the vascular and connective tissues react to the injury (via an inflammatory response). The cause of the injury may be direct (e.g. by a physical, chemical or organism agent) as well as indirect (from associated inflammation). Parenchymal cells are also sensitive to injury from lack of oxygen. Effects on cells range from injury of the cytoplasm (cell damage) that is reversible

(e.g. hydropic swelling, fatty change) through to injury of the nucleus, resulting in cell death (necrosis) that is

irreversible. The sequel of necrosis is generally repair, involving surviving parenchymatous cells (via regeneration) and the connective tissues (via fibrosis).

Certain highly specialised cells (e.g. nerve and muscle) have lost the capacity to divide (and thus replace necrotic cells). Other possible sequelae are deposition of calcium in damaged tissue (calcification), dissolving dead tissue (lysis) with cyst formation and infection of a necrotic part (gangrene).


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Fig.22.9 Cirrhosis of the liver

Where continuous damage occurs, some necrosis and repair may even take place simultaneously (although associated with a degree of disruption of normal architecture) in the organ (e.g. cirrhosis of the liver from chronic alcoholism).

Apoptosis (G. 'dropping off') is programmed cell death,

particularly associated with aging. Other degenerative conditions involve extracellular

infiltrations into surrounding tissues including protein

deposition (e.g. amyloid) around blood vessels of certain organs and pigmentation from products of red blood cell breakdown (e.g. haemosiderin, bilirubin) or from inhalation of particles (e.g. carbon, silicone) into the lungs.

Neoplastic & growth disorders

A neoplasm (G. ‘new’ + ‘moulding’) is an abnormal

mass of tissue capable of progressive growth. A neoplasm may be either benign or malignant. A malignant neoplasm is often called cancer (L. 'crab') tending to spread by local invasion (direct spread) and/or dissemination via lymphatics, blood vessels or even across a body cavity. Malignant neoplasms include those in the host tissue (a primary) or those spread to distant sites (secondaries).

Fig.22.10 Carcinoma of the colon

Secondary neoplasms are also termed metastases (G. ‘beyond’ + ‘standings’). A carcinoma (G. 'cancer' +

'swelling') is a malignant neoplasm derived from epithelial cells (whether of a surface lining or of a gland), while a sarcoma (G. 'flesh' + 'swelling') is derived from connective

tissue or muscle cells. Other disorders of growth include atrophy (a decrease in size from wasting), hypertrophy

(an increase in cell size), hyperplasia (an increase in cell number), dysplasia (abnormal cellular development) and metaplasia (transformation of mature cells into an

abnormal form).

Circulatory disorders

Circulatory disorders include an accumulation of blood within the vessels of an organ (congestion), an accumulation of extravascular fluid (oedema), bleeding from vessels (haemorrhage) and an inadequate perfusion of tissues throughout the body (shock)

A haematoma (G. ‘blood’ + ‘swelling’) is a localised

extravascular collection of blood. A localised dilatation of an artery due to a weakness in its wall is termed an aneurysm (G. ‘widening’).

A mass formed from blood elements within the vascular system during life is termed a thrombus (G. ‘clot’) while a

substance that is carried in the blood stream and lodges in a vessel is termed an embolus (G. ‘plug’).

Arterial occlusion may produce a restriction of blood flow, termed ischaemia (G. ‘keep back’ + ‘blood’), through

to tissue death from interruption of supply, termed infarction (L. ‘stuffing’).

Fig.22.11 Cerebrovascular accident from hypertension

Mechanical disorders

Mechanical disorders, as distinct from traumatic (where there is tissue disruption from physical injury), involve a physical cause altering structure and/or impairing function. These include compression from the outside (e.g. by a surrounding structure) and collapse from the inside (e.g. of a lung). Other mechanical disorders include obstruction of

a hollow viscus. This may be associated with an abnormal dilatation (of the wall and lumen) proximal to the

obstruction. Hernia (L. ‘rupture’) is an abnormal protrusion of an

anatomical structure through an opening, defect or weakness. Prolapse (L. ‘falling’) is the dropping of an

organ from its normal position (e.g. due to weakness of its supports, coupled with gravity).

Fig.22.12 Hydronephrosis of kidney

The cause of a disorder may be the consequence of another. A mechanical disorder may be a sequel of a traumatic disorder (e.g. injury to one organ may produce compression of an adjacent organ) or of a neoplastic disorder (e.g. a carcinoma may obstruct a hollow viscus producing a dilatation proximal to it). In each of these cases the normal anatomy is altered.

Understanding of normal and abnormal anatomy is the basis for recognising clinical manifestations of disease processes.

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Section V

PRACTICAL PERSPECTIVES

Introduction: 'Anatomy involves exploration’'

Chapter 23: Surface and Functional Anatomy

Chapter 24: Radiographic Anatomy and Imaging

Chapter 25: Sectional Anatomy, CT and MRI

Chapter 26: Ultrasound Imaging

Chapter 27: Endoscopic Anatomy

Chapter 28: Clinical Procedures

Chapter 29: Postmortem Examination of Organs

Chapter 30: Cadaver Dissection

Introduction: ‘Anatomy involves exploration’

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Exploring a living body

Examining, investigating or treating a patient is a privilege and even if non-invasive, require informed consent.

Surface anatomy (including projections of underlying organs) together with functional anatomy (movements

actions and reflexes) forms the basis for conducting a physical examination.

Radiographic anatomy forms the basis for interpreting

the findings of imaging investigations. In plain radiography, an X-ray film (radiograph) is a

two dimensional representation of a three dimensional entity. The images comprise superimposed components, which correspond to the actual anatomical structures. In order to identify them and understand their relationships, each component of an image is analysed by following the path of the X-ray beam through the living body (from the source to the X-ray film). Different types of anatomical structures absorb X-rays to different degrees (which determine their ‘radiodensity’). On a radiograph, structures containing air which does not absorb X-rays (hence ‘radiolucent’) appear black, while structures such as compact bone which absorb X-rays (hence ‘radiodense’) appear white. Soft tissues are of intermediate radiodensity.

Hollow viscera are made of soft tissue density. Although certain hollow viscera contain a variable amount of gas, demonstration of the lumen and examination of the mucosa is made impossible if the viscus is collapsed (as the two soft tissue density walls do not make a contrast edge). Similarly, blood vessels are made of soft tissue walls (unless pathologically calcified) and contain blood (which is also of the soft tissue density). In contrast studies,

radiographic examination of certain viscera, cavities and vessels can be achieved by utilising a contrast material.

Sectional anatomy involves the appearance of the

body at a variety of levels and planes, particularly those of clinical importance. It forms the basis for interpreting CT, MR and Ultrasound images.

Computed Tomography (CT) is a technique displaying

a cross-sectional image of a living body using X-rays (by rotating the X-ray source and its detector around the long axis of the body). As with radiographs, the images are based on the differing radiodensities of different types of anatomical structures.

Magnetic Resonance Imaging (MRI) is a multi-planar

technique displaying sectional images that does not involve the transmission of X-rays. MRI is based on recording radio signals emitted from a living body placed within a strong magnetic field following transmission of radio frequency pulses into it or with rapid magnetic field changes.

Ultrasound (US) imaging techniques use specific

acoustic densities of different tissues to identify interfaces. The final image is a cross sectional image composed of many vertical lines, which together outline an image based on these acoustic interfaces. Different types of tissues are characterised by an ‘ultrasound scale’.

Endoscopic anatomy is the basis for interpreting

views of the body from within which also may be applied in new surgical techniques.

Endoscopy (G. ‘within’+’look’) is a procedure utilising a

long optical instrument (an endoscope) to illuminate and view the interior of a (living) body. The endoscope may be a rigid straight tube or a flexible fibre optic cord. There are two types of avenues for endoscopy. An endoscope may be passed along the lumen of a viscus (e.g... stomach, colon or bladder) via a normal opening on the exterior of the body (e.g... mouth, anus or urethra). Alternatively, a portal may be created by an incision to enable access into a body cavity (e.g... peritoneal cavity, pleural cavities), a joint cavity (e.g... shoulder joint, knee joint) or even a region (carpal tunnel, mediastinum). Endoscopy may also provide a route for surgical and/or imaging procedures.

Practical (including emergency) diagnostic and treatment procedures may be required of a first port-of-call

doctor. They are invasive and may involve manipulation of tissues (e.g. with the aid of surgical instruments) as well as piercing them. Ideally, procedures should be rehearsed on (dead) cadavers rather than performed for the first time on (live) patients.

In addition to knowledge of relevant surface markings, the anatomical basis of a procedure specifically requires awareness of the:

- anatomical factors in selecting an appropriate site - anatomical structures observed, palpated or pierced - anatomical hazards that may be encountered en route (i.e. structures endangered by the procedure). The associated clinical techniques and judgements are

beyond the scope of this book (with readers strongly advised to confirm that these comply with accepted current standards of practice).

Exploring a dead body

Viewing body parts, attending an autopsy or dissecting a human body are also privileges and require permission, usually within the context of a certified professional course. Respect for the deceased is important at all times.

An autopsy (G. ‘self + view’) or postmortem (L. ‘after death’) is performed as soon as possible to determine the cause of death. During an autopsy, the body is examined internally and externally as a prelude to microscopic examination and laboratory analysis. Organs can be examined with the naked eye; in situ, following excision and then in cut section.

Dissection (L. ‘apart’ + ‘cut’) provides a unique learning

experience into the structure of the body. A dead human body used for dissection is termed a cadaver (L. ‘fallen’).

Cadavers are preserved by the infusion of embalming fluid into the vascular system. Embalming fluid (typically including formaldehyde, phenol, ethanol and glycerol) permeates the entire body, disinfecting, fixing and moisturising tissues. In dissection, a regional approach is generally adopted. Each region is dissected layer-by-layer, from superficial to deep.

In authorised departments of anatomy, body parts and organs may be utilised as predissected wet specimens.

These can be stored in tanks or mounted in pots for further study. Plastinated specimens can be obtained from

special techniques that replace organic tissue with synthetic material. Individual bones or even the whole

skeleton (G. ‘dried up’ may be obtained when cartilage, periosteum and bone marrow has been removed.

In forensic osteology and odontology, skeletal and

dental remains as well as radiographs are examined to determine sex, age and possible causes of death.

Chapter 23: Surface and Functional Anatomy

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SURFACE REGIONS

SURFACE MARKINGS AND EXAMINATION

SURFACE MAPS OF SUPPLY TERRITORIES

FUNCTIONAL ANATOMY AND TESTING

SURFACE REGIONS

Fig.23.1 Surface of modules

Each body module includes some external surface.

The borders of these may be mapped on the skin by

imaginary lines representing the surface projections of their underlying bony and soft tissue boundaries.

Two in three (48 of 72) regions of the body include some external surface. Their borders may also be mapped on the skin by imaginary lines. They include more than half of the head and neck regions, some trunk regions, but all of the limb regions. The remaining (24) regions are deeply located without any direct skin covering.

Surface regions of head and neck

Fig.23.2 Surface regions of head

Fig.23.3 Surface regions of neck

Surface regions of trunk

Fig.23.4 Surface region on back of trunk


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Fig.23.5 Surface regions on front of trunk

Fig.23.6 Surface regions on perineum

Surface regions of upper limb

Fig.23.7 Surface regions of upper limb

1. - pectoral region 2. - axilla 3. - anterior compartment of arm 4. - cubital fossa 5. - anterior compartment of forearm 6. - carpal tunnel 7. - palm of hand 8. - palmar aspect of digits 9. - scapular region 10- deltoid region 11. - posterior compartment of arm 12. - posterior compartment of forearm 13. - anatomical snuffbox 14- dorsum of hand 15. - dorsal aspect of digits

Surface regions of lower limb

Fig.23.8 Surface regions of lower limb

1. - femoral triangle 2. - subsartorial canal 3. - anterior compartment of thigh 4. - medial compartment of thigh 5. - anterior compartment of leg 6. - lateral compartment of leg 7. - dorsum of foot 8. - dorsal aspect of digits 9. - gluteal region 10. - posterior compartment of thigh 11. - popliteal fossa 12. - posterior compartment of leg 13. - tarsal tunnel 14. - sole of foot 15. - plantar aspect of toes

23. Surface and Functional Anatomy

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SURFACE MARKINGS AND EXAMINATION

Skin features and body build

Fig.23.9

Body surface area

Skin (including its specialisations) covers the entire external surface of the body. The surface area of an average adult male is approximately two square meters.



This is calculated to determine the amount of fluid replacement required.

According to the ‘rule of 9’s’: the trunk = 4x9%

lower limbs = 4x9%, upper limbs = 2x9%, head and neck = 1x9% Total = 99% (+ genitals the remaining 1%).

Fig.7.7 'Rule of nines’ for adult body surface area

Bony landmarks

Fig.23.10


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Fig.23.11

Soft tissue landmarks

Fig.23.12

Surface projections of viscera

Fig.23.13

Fig.23.14

Normal variation with posture

When standing, due to gravity, all abdominal viscera descend, particularly those that are more mobile.

The viscera that are most mobile are those suspended by a mesentery.

The stomach and transverse colon are especially mobile, each having two mesenteries (which may be of considerable length). In certain individuals the stomach or transverse colon may even descend to the pelvis.


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Fig.23.15 Postural variation in position of organs

The surface markings and vertebral levels for organs based on anatomical descriptions of a recumbent cadaver may be vastly different to those in a living person standing upright.

The curvatures of the spine and the arches of the foot are affected by lying, sitting or standing. Other effects of posture include distension and pooling of blood in veins.

Normal variation with respiration

Fig.23.16 Movement of organs during breathing

During inspiration the lungs expand and viscera directly below the diaphragm, particularly the liver (and gall bladder), spleen and kidneys, are pushed downwards as it descends.

Physical examination of abdominal organs includes attempting to palpate them on full inspiration.

Sites where motor nerves are superficial

Fig.23.17

Sites where arteries are palpable


Fig.23.18 Sites for palpation of arteries against bone

The usual site for clinical examination of an arterial pulse is where the radial artery lies on the distal end of the radius just deep to skin of the wrist.


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Fig.23.19 Palpating the radial artery against bone

Other sites where arteries may be pressed against bone include the common carotid artery on the carotid tubercle (of the 6

th cervical vertebra) and the facial artery

on the body of the mandible. Although the femoral artery is not directly related to bone, it is large and is close to skin just below the mid-inguinal point (at the base of the femoral triangle) where its pulsation may also be palpated.


Systolic and diastolic blood pressure can both be measured clinically (utilising a sphygmomanometer and cuff) by auscultation (with a stethoscope). The cuff is wrapped around the arm to overlie and (when pumped up) compress the brachial artery. This site is selected because it is at the approximate level of the heart (thus without additional hydrostatic pressure). The diaphragm of the stethoscope is placed over the brachial artery near its termination. Tapping sounds are produced when flow becomes intermittent (between systolic and diastolic blood pressures) as pressure in the cuff is gradually released.


Pulse rate and rhythm may be detected clinically by palpation of any accessible artery. The radial artery at the wrist is usually chosen because at this site it is easily felt between skin and bone (the distal end of the radius). Pulse volume and character may be detected clinically by palpation of the common carotid artery in the neck against the carotid tubercle (on the transverse process of C6). Palpation should not be performed near the carotid sinus (at the level of C3/4) where compression of baroreceptors may cause reflex bradycardia and subsequent hypotension).

Sites where veins are accessible

Fig.23.20

Sites where lymph node groups are palpable

Fig.23.22


Enlargement of this left supraclavicular lymph node may signal lymph spread of cancer from a structure within the territory drained by the thoracic duct. It may even be the first (although late) sign of cancer in a thoracic organ (e.g. lung) or abdominal organ (e.g. stomach or testis), since the thoracic and abdominal lymph nodes are all deeply located and none are readily palpable.

Fig.12.17 The final sentinel lymph node

Palpation of both left and right supraclavicular (groups of cervical) lymph nodes should be performed in the routine examination of the thorax. Palpation of the left supraclavicular lymph nodes should be performed in the routine examination of the abdomen (and is mandatory if there is suspicion of cancer in an abdominal organ).

Examination of major lymph node groups

The cervical, axillary and inguinal lymph nodes are readily palpable in a physical examination.


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SURFACE MAPS OF SUPPLY TERRITORIES

Cutaneous nerve supply

Fig.23.24


Clinical testing for diminished cutaneous sensation (due to a specific lesion involving either a spinal cord segment or a peripheral nerve) is best performed across axial lines. It is recommended to commence from an area of normal sensation and proceed across the axial line to the suspected area of sensory loss.

Segmental nerve supply

Fig.23.25

Arterial supply

Fig.23.26

Lymph drainage

Fig.23.27

FUNCTIONAL ANATOMY AND TESTING

Ligament integrity


Extensive ligament damage produces great impairment of function and increased potential for instability.

Fig.5.21 Stress test for cruciate ligaments of knee joint

Ligament integrity may be tested clinically by stressing

the ligament (putting it on stretch) and comparing the observable movement between the injured and uninjured sides. With a ligament sprain, pain tends to be exacerbated by stressing the ligament.

Reflex muscle spasm

Abnormal or excessive joint movement is an important diagnostic feature in an acute ligament injury, particularly a grade III injury. This may be masked initially by the other stabilising structures at a joint, particularly muscles (due to protective reflex muscle spasm).

Nerve fibre rupture

Stressing a ligament to elicit pain is also a diagnostic feature in an acute ligament injury (particularly for grade I or grade II sprains). This may be masked in grade III injuries as sensory nerve fibres (including pain fibres) within the ligament are also likely to be severed.

Range of joint movement

Passive and active movements

Movements are either passive or active. A movement at

a joint is passive when it is not directly due to contraction of its associated muscles (e.g... purely via gravity).

An external agent may also be utilised to assist a passive movement throughout its full range of motion (‘passive assistance’). This enables a clinical assessment

of joint mobility (the potential range for each movement at a joint) that may be otherwise masked by muscle weakness or paralysis.

Muscle function


Muscle function may be tested using active range or resisted contraction. A movement at a joint is active when

it is directly due to contraction of its associated muscles. Active movements may also be assisted (active assistance) or resisted (active resistance) by an external

agent. In clinical assessment of muscle function, the active range of movement (associated with muscle contraction) is compared to the passive range (allowed by joint mobility),


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to determine which structures may limit movement (or produce pain).

Muscle strength is gauged by the degree of active resistance required to prevent movement.

Assessing muscle tone and wasting


Skeletal muscle tone (G. ‘tension’) is measured as

resistance to stretch. Muscle tone is under reflex control. It is dependent on a nerve supply (both motor and sensory) and is modulated by the recruitment of more or fewer motor units.

Skeletal muscle tone may be either increased or decreased by certain lesions of the nervous system. Assessment of skeletal muscle tone involves resistance to stretch of a major muscle group ideally through its full range of movement (with increasing velocity). This is an important step in a neurological examination.


Muscle is a very highly specialised tissue. Even though mature muscle cells have lost the capacity to replicate they respond to changes in demand. Muscle fibres undergo progressive enlargement, termed hypertrophy (G. ‘over-nourishment’) with increased demand. Muscle fibres progressively waste away with inactivity (‘disuse atrophy’)

and particularly after loss of their motor nerve supply (‘denervation atrophy’).

Being structural changes, muscle hypertrophy and atrophy are not evident immediately but only after a variable period of time. Assessment of skeletal muscle wasting involves comparing both sides of the body and, where possible, measurement of circumference. This is also an important step in a neurological examination.

Testing reflexes

Fig.23.

Somatic and visceral reflexes

There are two major types of reflexes: somatic and visceral.

With somatic reflexes the effectors are skeletal muscles, while with visceral reflexes the effectors are smooth muscle, cardiac muscle or glands. Somatic reflexes may be subdivided into superficial and deep according to the afferent nerve fibre type. Superficial somatic (cutaneous) reflexes (e.g. withdrawal reflexes) arise from skin.

A special group of superficial reflexes (e.g. cough and swallow reflexes) arise from mucous membranes, although they involve skeletal muscle effectors. Deep somatic (proprioceptive) reflexes (e.g. stretch reflexes and tendon jerks) arise from skeletal muscles and joints. Visceral reflexes include pupillary, lacrimal, salivary, baroreceptor and chemoreceptor reflexes.


The visual pathway travels from the front to the back of the brain (hence the importance of visual field examination for identifying the site of a lesion within the brain).

Assessing posture and gait

Line of gravity and stable joints

In an adult standing upright, the line of gravity passes

between the mastoid processes of the skull, balancing the head. It continues through the S-shaped vertebral column behind the centres of the cervical and lumbar spine and in front of the centres of the thoracic and sacral spine. It then passes behind the centre of the hip joints and in front of the centre of each of the knee and ankle joints.

While standing (with hips and knees extended and ankles dorsi-flexed) the weight bearing joints are in the position of maximal stability. Articular surfaces are apposed and associated ligaments taut (to conserve muscular effort). Minimal skeletal muscle tone is therefore required to maintain upright posture, other than to correct for body sway.

Fig.23. Line of gravity in erect posture


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BIPEDAL LOCOMOTION

In contrast to standing where muscular effort is conserved, bipedal locomotion enlists the actions of

many muscles. Walking on level ground involves cycles (between heel-

strike of the same foot) of swing (limb not in contact with the ground) phase and stance (weight bearing) phase. Muscles not only act to accelerate the swinging lower limb (from the beginning of swing phase to mid-swing), but also to decelerate it (from mid-swing to the end of swing phase).

Fig.23. Phases of the walking cycle

The line of gravity moves forwards in the direction of motion. At one phase of the cycle (mid-swing and mid-stance) it passes through both limbs. At all other phases it passes between the limbs.

Roles of the gluteal muscles

The large gluteus maximus muscle is located

posteriorly (creating the unique form of the human buttock) producing powerful hip extension in running and jumping.

Fig.23. Stabilisation of the pelvis during locomotion

Gluteus medius and minimus muscles prevent

excessive tilting of the pelvis (supporting the trunk above it) towards the unsupported side during locomotion.

Chapter 24: Radiographic Anatomy and Imaging

192

PLAIN RADIOGRAPH PRODUCTION

RADIODENSITIES OF TISSUES

RADIOGRAPHIC VIEWS

PROPERTIES OF PLAIN RADIOGRAPHS

BONES ON RADIOGRAPHS

JOINTS ON RADIOGRAPHS

OTHER STRUCTURES ON RADIOGRAPHS

CONTRAST RADIOGRAPH PRODUCTION

CONTRAST STUDIES OF VISCERA

CONTRAST STUDIES OF CLOSED CAVITIES

CONTRAST STUDIES OF VESSELS

DIGITAL SUBTRACTION ANGIOGRAPHY

PLAIN RADIOGRAPH PRODUCTION

The production of a plain radiograph involves beaming X-rays through an object onto a recording medium.

Fig. 24.1 Context for a plain radiograph

X-rays

X-rays are electromagnetic waves of radiation of a very short wavelength (only about 1/10,000 the wavelength of visible light).

In the electromagnetic spectrum, the shorter the wavelength of radiation, the greater the energy of radiation.

Radiation at the low end of the spectrum (e.g. visible

light) does not penetrate through human tissues. Radiation at the high end of the spectrum (e.g. cosmic rays) is not absorbed in human tissues, and passes through unchanged.

Energy of X-rays is optimal when some is absorbed by, and some passes through human tissues (with greater amounts absorbed by more electron dense tissue than by less electron dense tissue). The non-uniform beam that emerges from the patient carries within it information about the location and size of structures of different electron density in the patient (i.e. a tissue density map).

X-ray source

X-rays are produced in an X-ray tube, which is the

central component of every X-ray machine. It consists of an air-evacuated glass cylinder in which a tungsten filament (cathode) and anode are located. A high potential (voltage) difference (up to several hundred kV) is applied between the cathode (which is also heated) and the anode. Electrons are ejected from the cathode and are attracted to the anode.

Fig. 24.2 An X-ray tube

When these highly accelerated electrons collide with the anode, their kinetic energy is transformed to heat and radiation, including X-rays. The X-ray beam exits through a window (usually rectangular) bounded by lead collimators.

Recording media

On specially designed receptor materials (X-ray film and image intensifier screens), X-rays produce a short, tiny burst of light for every X-ray photon that is absorbed. This flash of light is recorded as a single dot on the X-ray film, or as a single impulse in a digital image. An X-ray image comprises millions of such dots.

The X-ray film is still the most commonly used

recording medium in radiography. Once exposed, the X-ray film is called a radiograph (or an X-ray image).

Unexposed film consists of a plastic sheet covered with an emulsion sensitive to the visible light (photo-sensitivity) and X-rays (radio-sensitivity). The exposure to the X-ray radiation, followed by the interaction with a developer, results in chemical changes characterised by deposition of the metallic silver in the emulsion, which produces blackness on the film.

The intensity of blackness on a radiograph is directly proportional to the intensity of radiation which reaches the film.

24. Radiographic Anatomy and Imaging

193

Digital radiography is progressively replacing film

based radiography. Digital X-ray receptors are large solid-state plates which convert the photon energy directly to electrical signals that can be read out electronically. The advantage of digital radiography is that the image is a dataset and not dependent on a physical carrier. A digital radiograph can be manipulated, copied and sent like any other digital image.

Image intensifier tubes are still in use. These are

highly evacuated tubes which convert incoming X-ray photons into electrons accelerated towards a photocathode converting them into light photons. The light photons are in turn recorded with a (digital) video camera that generates the final image. Image intensifier tubes are part of X-ray equipment used for real-time procedural imaging.

All CT scanners already collect their information in digital form, as do nearly all procedural X-ray machines (angiography and fluoroscopy machines, and operating theatre mobile image intensifiers).

Steps in radiograph production

Production of a radiograph includes: 1 - proper patient positioning 2 - protecting the patient from unnecessary radiation 3 - correct placing of film, X-ray source and the patient 4 - selecting optimal settings on the X-ray machine (voltage, tube current, exposure time, focal spot) 5 - exposure of the film or digital receptor 6 - development, fixing, washing and drying the film The film or digital image is interpreted and reported.

RADIODENSITIES OF TISSUES

Effects of X-rays on tissues

In living tissues, X-rays can either cause no effect (pass through unchanged), or become absorbed or deflected. When an X-ray (more specifically an ‘X-ray photon’ to differentiate it from colloquial uses of the word ‘X-ray’) is absorbed or deflected, all or some of its energy (respectively) is absorbed by tissue electrons, which in turn are ‘knocked out’ of their usual energy levels (‘orbitals’ or ‘shells’). This ‘knocking out’ can ionise atoms and molecules (ionisation: loss of an electron by an atom, to acquire an overall electrical charge). Most of the time, ionisation reverses almost immediately, without any effects.

However, it potentially has biochemical consequences via ionisation of living molecules. DNA in particular may be affected. X-rays can have both cancer-killing and cancer-promoting effects.

The gonads of both the patient and staff should be shielded from X-ray exposure by an appropriate covering (e.g. a lead apron). An embryo is potentially vulnerable to radiation, particularly during organ development and it is important to be aware of the possibility of an unsuspected pregnancy. For women of reproductive age, pelvic or abdominal radiography should be performed within two weeks of the onset of menstruation.

Attenuation of X-ray beam

X-rays interact with different tissues of the body. As an X-ray beam penetrates through the body it progressively loses X-ray photons (i.e. the beam becomes less intense). X-rays which are stopped (absorbed) or deflected (scattered) by the tissue they pass through are excluded from the beam. This reduction in intensity of the X-ray beam is termed attenuation.

Tissue radiodensities

Capacity of a tissue to absorb or scatter X-rays depends on its electron density, because X-ray photons are absorbed or deflected primarily by electrons. The closest easily measurable physical parameter to electron density is physical density (i.e. mass per unit volume, usually expressed as grams/cm

3), which is not only related to the

physical state of the tissue but also to the atomic number of the elements which form it. When describing degrees of X-ray attenuation, density of living tissue is termed ‘tissue radiodensity’.

The greater the tissue radiodensity the greater the attenuation of X-rays.

This results in fewer X-rays reaching and interacting with the X-ray film (or other recording medium).

Radiodensity spectrum

On the basis of their (plain film) radiodensities, tissues can be classified into four groups from the least to the most dense:

- air - fat - soft tissues - bone

Fig. 24.3 Tissue radiodensities on a plain film

The air density includes gases (which are normally present in some hollow viscera) as well as air in air sinuses.

Soft tissues include all body fluids, muscles, water, cartilage, liquid bowel contents and parenchymal organs.

Bone density includes teeth. A fifth non-anatomical density, often seen in

radiographs, is that of metal, which is much denser than bone (e.g. total hip prostheses, fracture fixation plates, prosthetic heart valves, etc). Other commonly used prosthetic and medical device materials include plastics and silicones, which have densities close to that of soft tissue (but often have a radiodense stripe or marker to make identification easier).


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Degrees of lucency or opacity

Images of different tissues appear on X-rays as different shades of grey, from almost black (least dense) to almost white (most dense).

When referring to the appearance of different densities on the X-ray film, the terms lucency and opacity used. Dark structures on the film are referred as radiolucent (e.g. air) while light or white structures (e.g. bones) are called radio opaque.

Fig. 24.4 How various densities appear on a film

Radiological interfaces

An interface is created when different anatomical structures lie in contact with each other.

A radiological interface is created when tissues of different radiodensity lie adjacent to each other.

Depending on the positioning relative to the path of the x-rays, these radiological interfaces may or may not be visible on a radiograph.

Lines on a radiograph

Lines (or edges) may be seen on a radiograph when radiological interfaces are parallel to the path of the X-rays.

Fig. 24.5 How a line is formed on an image

A radiological interface viewed ‘end-on’ is seen as a distinct line on the radiograph while a radiological interface ‘face-on’ to the x-ray beam is never seen. A line seen on a radiograph implies two absolute conditions have been met. The first condition is that two tissues of sufficiently different radiodensity lie next to each other (i.e. form an interface). The second condition is that the interface is parallel to the X-ray beam.

The difference in density of the two structures has to be sufficient to produce a noticeable difference in X-ray attenuation. A relatively radiodense but small structure will produce a visible edge (e.g. a sesamoid bone) when surrounded by less radiodense tissue (e.g. muscle).Even tissue which has little difference in radiodensity to its surroundings can still produce an edge if the radiological interface is long and straight enough.

In the chest, a thin structure such as a blood vessel or a bronchial wall, will produce a visible edge against air-filled lung parenchyma for as long as it runs parallel with the X-ray beam. This is called the ‘end-on’ effect. The same

blood vessel or bronchial wall may be invisible if it runs at an angle to the X-ray beam.

RADIOGRAPHIC VIEWS

Images of the same structure from different angles are termed radiographic views (or projections).

Radiographs are two-dimensional images of three-dimensional structures positioned between the source of X-rays (the X-ray tube) and the film. Not only is the anatomy ‘collapsed’ into a flat image on the film but structures are superimposed on each other without indication of their order.

Each view provides an image of an object from a different angle.

An object is usually radiographed in at least two projections at right angles to each other.

This enables the viewer to construct a 3-dimensional mental image from the complementary pair of flat (2-dimensional) radiographs. It also enables superimposed (overlying) structures to be identified as separate entities.

Types of radiographic views

The name of a projection (and its abbreviation) is derived either from the direction of the X-ray beam or from the position of the object relative to the recording medium. The front-to-back view is known as anteroposterior (A-P) and the back-to-front is posteroanterior (P-A). These

describe the direction of an X-ray beam, with the X-ray film being always close to their exit from the body.

Fig. 24.6 Views from beam via back and front

Side-on projections are known as lateral views.


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An A-P view and a lateral view are standard

radiographic views. They are sufficient for many radiological examinations, being at right angles to each other.

Sometimes, a particularly important structure can be visualised optimally only if an oblique view is obtained as

well. In those instances the oblique view becomes a standard view in addition to the A-P view and a lateral view.

Fig. 24.7 Standard views of lumbar spine

In addition to these, a number of oblique projections can also be used enabling certain features of a body part to be visualised.

The X-ray tube is always perpendicular to the middle of the film. The optimal degree of required obliquity varies for different structures.

Looking at a radiograph

The image, whether an AP or PA view, is looked at as if facing the patient. The patient’s right is on the observer’s left and vice versa. ‘R’ and ‘L’ are marked on a radiograph to indicate the respective side.

For lateral and oblique projections, abbreviations indicate which aspect of the body is adjacent to the film. Right lateral (‘R lateral’) view indicates that the right aspect of the imaged body part is placed against the film. Right anterior oblique (‘RAO’) view indicates the positioning of the right antero-lateral aspect of the body against the film.

PROPERTIES OF PLAIN RADIOGRAPHS

The quality of an image on a radiograph depends on the ability to record closely placed objects (particularly if they are of similar densities) as separate entities.

Film penetration and sharpness

How black (or white) the overall film is and how sharp (or unsharp) the edges on it are will determine the ability to interpret the image.

The degree of penetration of the film will limit the

viewer’s ability to tell different tissues from each other. A film that is overpenetrated is too black; a film that is underpenetrated is too white. A correctly exposed film (correctly penetrated) has a full range of white, grey, and black shades. A film that is underpenetrated will lose most tissues other than the blackest (e.g. air in lungs). A film that is overpenetrated will lose all tissues other than the whitest (e.g. bones). While an image that is too black can be partly compensated by bright light translumination, an image that is too white cannot be manipulated further.

With the introduction of digital radiography the problem of incorrect exposure leading to under or overpenetration of the film will become largely overcome because of very wide optical latitude of the digital receptor, so that incorrect exposure can be compensated by window and level manipulation of the resulting data set.

Sharpness is a descriptive term that conveys the

success with which thin interfaces are depicted as thin on the image. A ‘sharp’ edge retains its pencil-thin quality (e.g. bone cortex interface with muscle). An ‘unsharp’ edge is where an interface appears smeared or blurred on the image. Unsharpness is due to the inevitable blurring of thin interfaces and edges that occur on all films. When severe, unsharpness will limit the diagnostic interpretation of the film.

Geometric unsharpness is the result of the X-ray tube

source being a finite size and not a true point source. This produces half-shadows (‘penumbras’) which lead to blurring of otherwise sharp edges.

Fig. 24.8 Geometric and motion unsharpness

Because of natural magnification of the image with increasing object-film distance, geometric unsharpness is worst for structures furthest from the film (and, by corollary, closest to the X-ray tube). For the same reasons, the greater the source-object distance compared to the object-film distance, the less is the geometric unsharpness. In part, this is why chest X-rays are taken with a large source-object distance.

Motion unsharpness is the result of the edge moving

while the film is being exposed. Motion unsharpness produces the X-ray equivalent of photographic blur when a fast moving object is photographed with a long exposure. Therefore, radiographs of moving objects (such as the heart and pulmonary vessels) are taken with as short an exposure as possible. For the same reason, a patient may be asked to keep still, not to breathe, or not to swallow during certain exposures.

Image resolution and noise

Capacity of an imaging system to register very small, closely positioned objects as two separate objects and to present them as distinct images is known as the resolving power or spatial resolution of the system. The greater the

resolution, the smaller are the objects that can be identified as separate.

Image noise describes the point-to-point variation in

image optical density, where a uniform image is expected. It is the visual equivalent of background noise produced by an audio system in place of expected total silence. Image noise limits the contrast resolution of an imaging system

(i.e. the ability to distinguish radiographic density differences between two objects, particularly if the objects are small). In radiographs or scans taken with progressively lower exposure, image noise makes up a progressively greater proportion of the total imaging signal. Therefore,


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reducing X-ray exposure parameters (to reduce patient X-ray dose) will eventually produce a non-diagnostic image.

Magnification and distortion

X-rays being divergent, will magnify the image of any structure in their path in the same way as a point source of light will magnify shadows. The closer an object is to the film, the less will be its magnification. The further an

object is away from the film (and by corollary the closer it is to the X-ray source), the greater will be its magnification.

The X-ray beam cone has a central ray, and peripheral rays symmetrically distributed on either side. Because the peripheral rays have a longer path than the central rays, structures imaged by the peripheral rays will be magnified more than structures in the path of the central ray.

The combination of progressive magnification and mis-mapping of structures that lie progressively further away from the central ray produces distortion.

Structures of most interest should be placed centrally within the X-ray beam.

When the central ray is not perpendicular to the film, distortion is particularly severe for any structures not located immediately against the film. This is particularly significant for a mobile film cassette.

The X-ray film should be placed perpendicular to the centre of the X-ray beam.

Fig. 24.9 Magnification

Positioning of the patient

Being three-dimensional, the body will naturally have some of its parts closer to the film than others.

The organ or body part of most interest is positioned as close as possible to the recording medium to minimise magnification and loss of sharpness.

Fig. 24.10 Positioning chosen part near film

When the anterior aspect of the body (or a part of the body) needs to be least magnified and distorted on a radiograph, a P-A view is chosen.

For example in a routine chest X-ray, the heart and lung hila (both nearer to the anterior thoracic wall) are of more interest than the spine. The patient is positioned with the front of the chest leaning against the X-ray film (while the X-ray source is located behind the middle of the patient’s back).

Superimposition and summation

An overlap (partial or total) of images of structures positioned in the path of the X-ray beam is termed superimposition. The order of these structures is not

evident from the X-ray image alone, but can be correctly determined by combining the skill of ‘thinking in layers’ when looking a radiograph (enhanced by experience) with good knowledge of anatomy. This combination of skill with knowledge is particularly required to interpret images of complex structures (e.g. skull or vertebral column).

Fig. 24.11 Summation from superimposed structures

The combined optical density (as seen on the final image) of several superimposed structures is the sum of the optical densities produced by each structure individually, because the X-ray beam is attenuated by each structure independently and in turn. The increase in optical density on the film produced by several overlapping structures is termed summation. For example, the

vertebral column and mediastinal soft tissue structures are superimposed on a frontal chest X-ray, and where superimposed they produce a whiter area (i.e. summation takes place).

In order to avoid interpretation difficulties resulting from the superimposition and summation phenomena, multiple projections are routinely used with the object often radiographed in at least two projections commonly at right angles to each other.

BONES ON RADIOGRAPHS

Bones, being very radiodense, are easily identifiable on a plain X-ray film because of their contrast with surrounding structures. Each type of bone has a particular radiographic appearance.

Long bones on images

The differences between compact and cancellous bone are particularly visible on radiographs of long bones.


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Compact bone (densely packed bone tissue infiltrated with calcium) appears more opaque than cancellous bone (containing many little compartments).

Compact bone is particularly thick in the shaft of a long bone where it forms weight-bearing walls surrounding the medullary cavity.

Compartments within cancellous bone and the entire medullary cavity are filled with radiolucent bone marrow.

Fig. 24.12 Long bones on an image (elbow)

Parts of the shaft circumference are parallel and other parts are perpendicular to the X-ray beam. Four interfaces are visible, two with soft tissues surrounding the shaft and two with the medullary cavity filled with soft tissue (bone marrow).

The ends of a long bone are predominantly cancellous bone organised into groups of parallel struts (representing weight bearing paths) known as trabeculae. Compact

bone is reduced to a thin superficial covering (‘subchondral bone’) that usually supports the articular

cartilage.

Short and irregular bones on images

In short and irregular bones, the thin compact layer present on the surface of bones is clearly seen only when the X-rays are parallel with these surfaces.

Fig. 24.13 Short bones on an image (wrist)

Underlying cancellous bone is organised into a system of weight bearing trabeculae.

Irregular bones often have a complex radiographic appearance, with their parts and surfaces superimposed on

each other. Hence, a careful layer-by-layer approach is necessary.

Fig. 24.14 Irregular bones on an image (spine)

Flat and pneumatic bones on images

In flat bones, the cancellous bone (containing bone marrow) is usually reduced to a thin layer between two compact layers. Its interface with cortical bone is only visible when parallel with the X-ray beam.

Flat bones of the cranium have a typical appearance. Two parallel densities (representing internal and external compact bony tables) sandwich a ‘reticular’ opacity representing diploe.

Fig. 24.15 Flat and pneumatic bones (skull)

Certain skull bones have air-filled cavities (the paranasal sinuses), appearing as paired, clearly demarcated lucent areas (air radiodensity) of variable size.

Assessing bony integrity

Fig. 24.16 Bony integrity (hip)


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Margins of every normal bone on a radiograph (regardless of size, shape and complexity) should appear as sharp, clear and continuous lines demarcating the edge of the bone from the surrounding tissues.

Trabecular patterns within cancellous bone should form clear and continuous parallel lines where present.

Optimal bony alignment (and immobilisation to maintain it) is crucial in fracture management.

Ossification centres

In the developing skeleton, primary centres of

ossification (except for the short bones of the wrist and foot) appear well before birth. Further development and growth of certain bones occur until the end of adolescence.

Secondary centres of ossification (epiphyses) appear

at different ages in cartilaginous parts of developing skeletal elements. They appear on radiographs as small and irregular (often nodular) radiopacities representing calcified cartilage in the centre of cartilaginous bone ends. These islands of calcification quickly become ossified, expand in size and develop denser margins and trabecular cores. As they grow they replace the hyaline cartilage of the epiphysis (with hyaline cartilage remaining only on articular surfaces).

Fig. 24.17 Epiphyses of knee (age 3)

Epiphysial plates

Fig. 24.18 Growth plates of knee (age 10)

A radiolucent ‘gap’ seen on the radiograph of a growing bone between the primary and secondary ossification centres (or between epiphysis and metaphysis in long bones), represents the epiphysial (growth) plate. The

radiolucency is due to soft tissue epiphysial cartilage that is end-on to the X-ray beam.

Zones of calcified cartilage form thin but distinguishable white (radiopaque) lines clearly demarcating the epiphysial plate from surrounding bone tissue.

Epiphysial lines

Growth of bones finishes with closure of the epiphysial (growth) plates. Calcified cartilage from the margins of the epiphysial plate extends into its middle converting it to bone. With time, this area becomes thinner and merges with nearby trabeculae. However, a fine line of compact bone, termed the epiphysial line, usually persists

throughout life.

Fig. 24.19 Epiphysial line on tibia (adult)

Accessory and sesamoid bones on images

An accessory (supernumerary) bone appears when an ossification centre fails to fuse with the main bony mass. Accessory bones are relatively common, and on radiographs should not be mistaken for a bone fragment produced by a fracture.

Fig. 24.20 Sesamoids and accessory bone (foot)

Sesamoid bones occur in certain tendons, where they pass across bony convexities. In addition to the patella (the largest sesamoid bone), they are found most frequently in


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the foot and hand. On radiographs they appear as discreet round or oval bones with clearly defined margins. They should not be mistaken for abnormal calcification that may occur in tendons, for loose bodies in a joint cavity or for bone fragments.

JOINTS ON RADIOGRAPHS

Bony articular surfaces may be directly observed on a

plain X-ray film of a joint. Other joint components of soft tissue radiodensity are indistinguishable from each other. However, articular fat pads (fat radiodensity) can be seen.

Radiological joint space

The radiolucent gap between opposing bony articular surfaces visible on plain radiographs is termed the radiological joint space. In a normal synovial joint, this

‘space’ is occupied by articular cartilage covering each bony articular surface as well as the thin layer of synovial fluid between them. The space is visible because the apposed cartilage layers are end on to the X-ray beam.

Fig. 24.21 Plain film of hip joint

In certain joints, the radiological joint space contains intra-articular fibrocartilage (e.g. intervertebral discs, menisci of the knee) or ligaments (e.g. cruciate ligaments of the knee).

Fig. 24.22 Plain film of lumbar spine

Being of soft tissue density, all these structures blend into a uniform opacity filling the space between articular surfaces and cannot be distinguished from each other.

Some joints contain fat pads that push their synovial lining against non-apposed articular cartilage. These can be seen with care, and are important in the assessment of a joint.

Fig. 24.23 Plain film of knee joint

Assessing radiological joint space width

The width of the radiological joint space should be assessed. Changes (widening, narrowing or disappearance) are all signs of joint abnormality. Cartilage degeneration narrows the space. An abnormal accumulation of fluid inside a joint (e.g. due to bleeding) will widen the space.

Bony articular surfaces

Bony articular surfaces are clearly observable on plain radiographs.

The continuity of each surface (including the edges where it is continuous with the thin nonarticular compact bone) is carefully examined. The line(s) representing an articular surface should be sharp and smooth (without pits or bumps).

The bone adjacent to an articular surface should not display excessive opacity or lucency and its trabecular pattern should be regular (oriented according to forces transmitted through the supported articular surface).

Assessing joint congruence and alignment

Joint congruence is assessed by following the

opposing articular surfaces. These should be parallel and equidistant in all projections.

Joint alignment is of course dependent on joint position

at the time of the radiograph. The conventional way to measure alignment is by measuring the angle between the long axes of the two articulating bones. For complete assessment of alignment, more than one view is required.

Obtaining optimal joint congruence and alignment is crucial in management of dislocations. Laxity of the joint capsule, ligaments or surrounding tendons can also result in joint malalignment that is also seen radiographically.

Joint soft tissues

Although not visible separately on plain radiographs, individual soft tissue components of synovial joints should


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be considered. Soft tissue structures around a joint that typically present as uniform soft tissue density are the joint capsule, surrounding tendons, bursae and muscles, and also any overlapping neurovascular bundles. Only joint fat pads will be visible as fat density structures (if large enough, and if the interface with other soft tissue structures is parallel to the beam).

Assessing soft tissue calcification

Calcification of articular soft tissues can occur with degeneration, following injury and in certain diseases. This may become visible on a plain radiograph and should be distinguished from calcification in tissues not associated with the joint.

OTHER STRUCTURES ON RADIOGRAPHS

Certain structures other than bones and joints may be distinguished on a plain radiograph.

Anatomical soft tissues include many different types (e.g. muscle, cartilage, fascia, tendon, glands and fat). In addition, body fluids (e.g. water, blood bile, urine, cerebrospinal fluid) can not be distinguished from radiographic soft tissue on plain film.

Fat-soft tissue interfaces

Only fat has sufficient radiographic contrast compared to all other types of soft tissues (and body fluids) to form visible interfaces on a plain film.

Kidneys and lateral borders of psoas muscles are surrounded with extraperitoneal fat on the posterior abdominal wall. Therefore, they can be easily outlined as well defined radiopacities on the background of the more radiolucent adipose tissue.

Fig. 24.24 Abdominal fat-soft tissue interfaces

The breast is composed of glandular tissue supported by ligamentous suspensory elements (of soft tissue density) and clumps of fat dispersed through it. In mammography, the fibro-glandular tissue of the breast

can be distinguished and visualised on the breast fat background (although this varies with age and hormonal status).

Fig. 24.25 Mammogram of a dense breast

Fig. 24.26 Mammogram of a fatty breast

Air-soft tissue interfaces

When an organ or a tissue of soft tissue density is adjacent to air or gas, the difference in radiodensity will form a clean and sharp edge provided the interface is parallel to the x-ray beam.

This is best seen on a plain chest X-ray where blood vessels in the lung can be recognised and clearly outlined by air contained within the lung parenchyma.

Fig. 24.27 Air-soft tissue interfaces (chest)

Intralumenal air or gas

In viscera which contain air or gas the interface between the air filled lumen and mucosa becomes visible. On a plain chest radiograph air is seen in the trachea and principal bronchi and as well as in the stomach. On a plain radiograph of the abdomen air is seen in the stomach and gas (produced by microbial fermentation as well as swallowed air) is seen in the intestine.

Significance of extralumenal air or gas

Air of gas seen outside the lumen of a viscus may indicate perforation. If this occurs to an intraperitoneal viscus (e.g. stomach, large intestine) gas enters the peritoneal cavity and may accumulate under the diaphragm.

CONTRAST RADIOGRAPH PRODUCTION

The production of a contrast radiograph involves

beaming X-rays through an object following the administration of contrast material (contrast medium) to

change the radiodensity of certain structures.


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Fig. 24.28 Context for a contrast radiograph

The passage of contrast material is often followed in real time during the duration of contrast examination to observe the patterns of its distribution.

Contrast enhancement

Since body fluids (and non-gaseous contents of hollow organs) cannot be distinguished from their surrounding walls on a plain radiograph, administration of contrast material is required to see them. This is achieved by filling body cavities or lumina of viscera and of vessels with materials of different radiodensity. The resulting change in radiographic density between the lumen and walls of the imaged organs (or body parts) allows their visualisation.

Contrast material can be introduced into just about any body cavity or potential space.

Intravenously administered contrast material, (different to that for non-vascular administration), is concentrated in various parenchymal body organs depending on its biochemical properties, and so may opacify these organs, making them more visible against adjacent soft tissue. The most common example is contrast opacification of the kidneys, making the renal parenchyma and collecting systems visible in turn (‘intravenous pyelogram’ or ‘intravenous urogram’).

Positive and negative contrast media

Any compound introduced into the body in order to change the radiodensity of soft tissues is called contrast material (or contrast medium).

Certain contrast media are denser than the organ or tissue imaged (‘positive’ contrast media) like iodinated

injectable compounds for intravascular administration or barium sulphate suspensions for oral or rectal administration.

Other contrast media are less dense (‘negative’

contrast media) like air, oxygen and carbon dioxide for gastrointestinal or body cavity administration.

Avenues of contrast administration

Contrast material can be introduced into the body through normal body orifices. Contrast material can also be injected through needles or catheters into various body cavities, spaces or structures (e.g. joint cavity, subarachnoid space, blood vessels).

Contrast material may even be injected into loose connective tissues (to subsequently highlight lymph vessels and nodes) as it is gradually included in the lymph flow.

Fluoroscopy systems

Often, the flow and distribution of the contrast material has to be demonstrated in real time, so the design of the X-ray machine reflects those needs (capacity for continuous observation, quick successive image capture and video recording).

A fluoroscopy system consists of an image intensifier (which transforms X-rays into a flux of electrons in an evacuated tube, and then into visible light) and a camera or a solid state light deflector which converts this light to still or video images.

CONTRAST STUDIES OF VISCERA

The lumen of a viscus can be outlined using contrast material introduced either directly or indirectly. The direct

approach involves filling a lumen with contrast media introduced via normal body openings (e.g. swallowed, or utilising an endoscope). The indirect approach involves

ingestion or injection of a contrast medium that gets concentrated and excreted into the lumen of a different organ than what it entered.

The viscera most commonly examined by contrast studies are of the digestive and urogenital systems.

Certain hollow viscera may also be outlined indirectly after excretion of contrast media through them.

Upper GI studies

Several different studies examine the upper digestive tract, each tailored to one specific part. Fluoroscopy is used to demonstrate the pharynx (pharyngogram), oesophagus (barium swallow), stomach (barium meal) and small intestine (small bowel series) with only liquid barium as contrast (for ‘single contrast’ study), or liquid barium and

gas-producing crystals or liquid or even injected air (for a ‘double contrast’ study).

Fig. 24.29 Barium meal

Lower GI series

The large intestine is examined (in single or double contrast) using a barium enema. Liquid barium is run into

the colon via a rectal tube (under fluoroscopic vision). To obtain double contrast, extra barium is then evacuated and air insufflated into the colon to distend it. The patient turns


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or stands to best demonstrate successive parts of the large intestine in turn. Double contrast barium enema optimally details the large intestinal mucosal lining. Appropriate patient preparation in order to achieve a clean colon is an absolute prerequisite for this examination.

Fig. 24.30 Barium enema

Oral cholecystography and ERCP

Oral cholecystography is a contrast examination of

the gall bladder following the oral administration of a special iodine medium, which is resorbed in the intestine and delivered to the liver via the portal system. In the liver it is metabolised and excreted into the biliary tract and finally concentrated in the gall bladder, where the material increases radiodensity of the lumen making it visible. Oral cholecystography has now been superseded by intravenous CT cholecystography (CT-IVC), where the contrast material is injected intravenously and images acquired on a CT scanner.

Fig. 24.31 Oral cholecystogram

Endoscopic Retrograde Cholangiopancreatography (ERCP) is a contrast radiographic examination in which the

contrast material is injected directly (and retrogradely) into the biliary and pancreatic ducts by use of a fiberoptic endoscope. Although, this examination offers excellent demonstration of ducts, it is performed only when less

invasive radiological approaches (e.g... ultrasound, contrast studies, CT and MRI) are not conclusive.

Urography and cystography

Intravenous urography (intravenous pyelography), retrograde pyelography and cystography are contrast examinations of the urinary tract.

Intravenous urography involves intravenous

administration of a contrast medium followed by obtaining a precisely timed sequence of radiographs. Several radiographs taken within the first minute from the bolus injection show the renal parenchyma (nephrogram phase), while the radiographs at about 5 minute intervals post-injection demonstrate calyces, renal pelvis, ureters and the urinary bladder. Compression of the ureters where they cross the pelvic brim (by a tight band) is often applied after the first 5 minutes in order to distend the ureters and renal pelvis. Following release of compression, the contrast accumulates in the urinary bladder. A post-voiding radiograph is obtained to check bladder emptying.

Retrograde pyelography is a contrast radiographic

examination in which the contrast material is injected directly into the renal pelvis via a ureteral catheter passed retrogradely (introduced with the aid of a cystoscope). The calyces and renal pelvis are well displayed with this examination.

Fig. 24.32 Retrograde pyelogram

Cystography is an examination of the urinary bladder

and it is often performed with urethrography in males to assess the functional anatomy of the lower urinary tract. Following catheterisation, the bladder is filled with contrast material and examined using fluoroscopy. The second part of the examination involves removal of the catheter followed by fluoroscopic examination of micturition.

Hysterosalpingography

Hysterosalpingography is a contrast radiographic

examination of the uterus and uterine (Fallopian) tubes. A vacuum cup device (with a small cannula) or a specially designed catheter is attached to the vaginal opening of the cervix and contrast material is directly introduced into the uterine cavity. The contrast medium then fills the tubes and spills into the peritoneal cavity.


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Fig. 24.33 Hysterosalpingogram

This examination can also have a therapeutic effect on female infertility by opening the uterine tubes previously occluded by adhesions.

CONTRAST STUDIES OF CLOSED CAVITIES

Certain closed cavities (spinal and cranial cavities, joint cavities and the abdominal cavity) can also be injected with contrast media in order to visualise surrounding or associated structures. However, these invasive contrast studies have generally been superseded by modern non-invasive imaging techniques (e.g. CT or MRI).

Myelography

Myelography is performed by injecting the contrast

media (usually opaque myelographic contrast material, rarely gas) into the spinal subarachnoid space. The contrast material is introduced either via a lumbar puncture or via a cervical lateral puncture under direct fluoroscopic vision. The patient is carefully turned and tilted to distribute the contrast through the CSF in a way that will show the suspected abnormality, and may then proceed onto CT scanning after a variable time delay.

Fig. 24.34 Lumbar myelogram

Contrast arthrography

Contrast arthrography utilises a gaseous medium

(pneumoarthrography), positive contrast medium (opaque

arthrography) or combination of two (double-contrast arthrography) to examine synovial joint cavities (joint space, also outlining any contained soft tissue structures such as menisci, ligaments, cartilage and bursae). Single contrast arthrography is commonly performed under local anaesthesia. The contrast is injected into the synovial cavity, the joint is manipulated to achieve optimal distribution of media and then examined with radiography, CT or MR.

Peritoneography

Peritoneography is used occasionally to look at

location and distribution of peritoneal compartments, and to diagnose difficult peritoneal hernias. Contrast material is injected into the peritoneal cavity under fluoroscopic control.

CONTRAST STUDIES OF VESSELS

Angiography is a specialised radiological examination

utilising contrast media (organic iodine solutions) to directly visualise vessels and indirectly visualise organs by opacifying their capillaries.

Angiography may be performed to investigate primary vascular diseases (e.g. aneurysms), bleeding, trauma and neoplastic diseases.

Arteriography

Arteriography refers to the contrast examination of

arteries in general. Specialised arteriography includes angiocardiography, aortography, cerebral and coronary angiography. These are concerned with contrast examination of the heart chambers, thoracic and abdominal aorta, intracranial and coronary arteries, respectively.

Contrast medium is injected via a catheter inserted into a peripheral artery (e.g. femoral or brachial) and passed retrogradely to the origin of the desired artery, or even into cardiac chambers. Arterial blood flow directs the distribution of the contrast medium from the catheter tip.

Angiography of certain solid organs (e.g. kidney, liver) includes three phases: arterial, capillary and venous. The

capillary phase enables visualisation of the organ parenchyma. Associated veins are also commonly sufficiently opacified in the last stage of arteriography (after the contrast medium passes through the capillary system of an organ whose arteries are opacified).

Fig. 24.35 Arteriogram (abdominal aorta)


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Interventional radiology

Arteriography is the prelude to radiologic intervention. Therapeutic applications such as angioplasty, embolisation, thrombolysis and selective chemotherapy may be performed following diagnostic arteriography. The subspecialty of radiology that utilises radiological techniques for treatment is called interventional radiology.

Venography

Venography is the contrast examination of veins

(peripheral and central). A venous catheter is inserted (and positioned) into a

peripheral vein. The injected contrast medium is carried in the direction of venous blood flow towards the heart, opacifying lumina of the veins along this path.

Fig. 24.36 Venogram (inferior vena cava)

Lymphography

Lymphography includes the contrast examination of

lymph vessels (lymphangiography) and of lymph nodes (lymphadenography).

Fig. 24.37 Lymphogram

Both the media and the application approach are unique. Oily iodine contrast media are utilised because of their longer retention by the lymphatic system.

A blue dye is injected into the tissues of the dorsum of the foot or hand (depending on which part of the lymphatic system needs to be viewed). The dye is taken-up by local lymphatic vessels which are outlined enabling subsequent cannulation and injection with a radiodense medium.

The first set of radiographs, to demonstrate lymph vessels, is taken within one hour of the injection. A second set, to demonstrate lymph nodes, is taken about 24 hours later (allowing time for the contrast to accumulate in them).

Lymphography has been taken over by cross sectional imaging, in particular CT.

DIGITAL SUBTRACTION ANGIOGRAPHY

Digital Subtraction Angiography (DSA) involves

‘removing’ unwanted parts of an image by the use of digital manipulation. It is widely used in angiography in order to subtract bones, gas-filled organs and soft tissues from the image, so that contrast filled blood vessels are not obscured by them.

Masking unwanted structures

Before contrast is given to opacify blood vessels, an initial image (termed a mask) is taken in exactly the same position as the subsequent images. As contrast is being injected, several images in rapid succession are taken. The mask film is subtracted from the run films, hence the terms 'subtracted image' for the final product and 'subtraction

angiography' for the technique. However, any movement by the patient will lead to differences between the mask and the run images not caused by vascular opacification. Such confusing artefacts on the subtracted images may render them useless for diagnosis.

Fig. 24.38 Digital subtraction aortogram

Chapter 25: Sectional Anatomy, CT and MRI

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SECTIONAL ANATOMY

CT IMAGE PRODUCTION

TISSUE PROPERTIES IN CT

PROPERTIES OF CT IMAGES

ADDITIONAL CT TECHNIQUES

MR IMAGE PRODUCTION

TISSUE PROPERTIES IN MRI

PROPERTIES OF MR IMAGES

SPECIAL MR IMAGING

SECTIONAL ANATOMY

Images of sections may be correlated with those from CT and MR and are presented with the same left-right orientation as plain film radiography. An image is viewed as if facing the patient; the patient’s right is on the viewer’s left and the patient’s left is on the viewer’s right. Historically, for axial images, this has been called ‘the view from the feet’. The same convention applies to coronal slices (right on the left and left on the right). However, no convention exists with sagittal images.

Axial sections of the body

Sections in axial planes are oriented transversely, being perpendicular to the long axis of the body (or specific body part). When the body is standing erect (e.g. in the anatomical position) axial planes are horizontal.

Fig. 25.1 Axial section (mid-thoracic level)

Sagittal sections of the body

Sections in sagittal planes pass between the front of the body and the back, parallel to the sagittal suture of the skull. They are oriented longitudinally, being parallel to the long axis of the body (and are vertical when standing erect).

The midline of the body is in the mid-sagittal (or median) plane, as it is directly along the sagittal suture.

Fig. 25.2 Sagittal section (left parasagittal)

Coronal sections of the body

Fig. 25.3 Coronal section

Sections in coronal planes pass between the right and left sides of the body, parallel to the coronal suture of the skull. They are also oriented longitudinally but are perpendicular to sagittal planes.

CT IMAGE PRODUCTION

Computed tomography (CT) is a radiologic technique

which utilises X-rays to produce cross-sectional images of the patient.


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Fig. 25.4 Context for CT images

The formation of CT images is a multi-step process with the equipment design reflecting the order in which this takes place.

CT image acquisition

The two major components of any CT scanner are the gantry (shaped like a donut) and the patient bed, which slides through the middle of the gantry. The gantry contains a mobile X-ray tube which continuously rotates around the gantry opening, and an array of many small digital X-ray detectors lying opposite the X-ray tube.

The X-ray tube produces a thin, fan shaped X-ray beam. The width of the beam (also the width of the slice) is controlled partly by physical tube collimators, and partly by electronic detector masking. The beam traverses the patient and is ‘read’ by the detectors on the other side of the gantry opening.

Fig. 25.5 Components for CT image production

The X-ray tube continuously rotates around the patient (at the speed of 1 revolution per second, or faster), and images from many angular projections are collected during any one revolution. These images are analogous to multiple overlapping photographs in panoramic photography, but are only slice-thick.

The patient bed (with the patient lying as still as possible) is moved through the gantry, presenting successive parts of the patient to the X-ray beam, so that contiguous slices are acquired.

CT image reconstruction

The resulting huge volume of data from the image acquisition process is passed to the CT computer for image construction.

Each reconstructed slice consists of ‘voxels’ (3 dimensional brick-shaped pixels). The size of the digital X-ray detectors and the number of angular projections determine the smallest possible size of the voxel within each axial slice (dimensions in the X and Y directions, i.e. ‘the short sides of the brick’). The size of the voxel in the Z direction (craniocaudal direction, ‘the long side of the brick’) is the width of the X-ray beam.

Fig. 25.6 A CT voxel

Each CT voxel has a value, traditionally displayed as a shade of grey. The value in each voxel is a measure of tissue radiodensity in the corresponding voxel in the patient. CT can measure radiodensity with exquisite accuracy, and can detect extremely subtle radiodensity differences in adjacent voxels.

TISSUE PROPERTIES IN CT

As in plain radiography, when a fan-shaped X-ray beam penetrates the body during Computed Tomography (CT) examination, it interacts with different tissues.

Depending on the tissue radiodensity, some X-rays are absorbed, some are scattered.

Tissue radiodensities in CT

While plain radiographs have four groups of radiodensities, CT images provide a greater range of shades that allow differentiation between many tissue types.

Fig. 25.7 Tissue radiodensities in CT imaging

Traditionally, tissue radiodensity as measured by CT is expressed in Hounsfield Units (HU) or CT units.

On the HU scale, the attenuation value for water is zero. Positive values are for structures with higher attenuation than water (soft tissues and bones) while

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negative values are for structures with lower attenuation than water (fat and air).

The attenuation values for tissues of different radiodensities expressed in Hounsfield Units are:

air -1000 HU, fat -100 to -60 HU, water 0 HU soft tissues 40 to +60 HU, bone +1000 HU or higher

Fig. 25.8 Attenuation values (in HU) and grey scale

A CT image is simply a tissue density map (expressed in HU) in that particular slice. Although CT images can be displayed in any colour scale of the user’s choice, by tradition they are displayed in the same way as radiographs: radiolucent tissues are black, and increasingly radiodense tissues are progressively white.

PROPERTIES OF CT IMAGES

The images displayed on a monitor following CT scanning consist of a matrix of picture elements (pixels). Each individual pixel on the screen represents the HU value of the corresponding tissue voxel in the patient.

Body slices in CT images

Radiographs display the entire body part or an organ that is imaged, whereas CT images display slices of body parts or organs.

The recording medium in radiography is traditionally the

X-ray film whereas in CT, signals from detectors are transformed into digital images. However, these may subsequently be recorded on film.

CT images can be presented in all the ways than any other digital images can. They can be viewed on a computer monitor, television screen, be printed to reflective paper or transparent film. Conventionally, to allow side-by-side hanging of radiographs and CT scans on the same light box, hardcopy CT is printed to transparent film.

Pixels from voxels in CT

A CT image is displayed for interpretation as a flat, two dimensional picture composed of small picture elements (pixels). The slice it represents is composed of small brick-shaped volumes of tissue termed voxels. The voxel depth is not evident from the flat display, but can be read from the information provided with the image. On single-slice CT

scanners, The third dimension ('Z' axis) of a voxel is usually the thickness of the X-ray fan beam (i.e. the collimation). On multi-slice CT scanners voxel depth and slice thickness is determined by physical and electronic collimation and also by reconstruction parameters.

Windowing in CT

The limited capacity of the human eye to differentiate between different shades of grey limits the total number of shades of grey that can be displayed on a CT monitor or on CT film at any one time. The number of shades of grey that can be usefully displayed is far smaller than the number of different Hounsfield Unit values that are measured by a CT scanner. In order to fit the large dynamic range of the measured Hounsfield Units into the narrow dynamic range of the human eye, only select portions of the dynamic range are displayed, and may be ‘stretched’ or ‘compressed’ into only a few steps of grey. The process of displaying the HU range of interest is called ‘windowing’.

Window level refers to the mid-value of the HU range to be displayed with the limited shades of grey, while window width describes the extent of this range. In general, to display a particular tissue optimally, the window level should be comparable to that tissue’s usual HU number, and the window width can then vary depending on how many other structures need to be included.

A ‘narrow window’ shows great detail in the structure

of interest, but everything above the window will be presented as uniform white, and everything below as uniform grey. A ‘wide window’ displays many different

tissues as grey, but there is little differentiation between them.

Optimal window level and width vary for each tissue. For optimal display of mediastinal soft tissues, the window level is around 40 HU, and the width is 400 HU. For lungs, however, the window level is around -700 HU and the width is 1000 HU. These values are usually displayed on the image.

Fig. 25.9 Windows in CT (of thorax)

CT resolution

In-slice CT spatial resolution is limited by the size of

individual X-ray detectors, and the number of angular


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projections that are collected. CT spatial resolution in the cranio-caudal (Z axis) is limited by slice collimation (whether physical or electronic). CT spatial resolution, although impressive, is much coarser than of plain film.

CT contrast resolution (the ability to tell different

tissue radiodensities apart) is extremely high, and well above that of plain film. Similar to plain radiography, it is limited by image noise.

ADDITIONAL CT TECHNIQUES

Rapid progress in digital technology and engineering, as well as routine utilisation of contrast media, led to the refinement of routine CT techniques and development of targeted CT techniques.

Use of oral and intravenous contrast media

In abdominal CT examinations, the use of orally or rectally administered contrast materials enables contrast-filled lumina of hollow organs to be more confidently distinguished from solid masses or cysts.

Intravenously administered contrast material opacifies blood vessels, making identification easier, and also opacifies vascular parenchymal organs, allowing better detection of abnormal areas within them (e.g. tumour masses). Intravenous contrast is used routinely for imaging all areas of the body where vessels need to be distinguished from non-vessels (e.g. in the neck, identifying vessels and lymph nodes).

In children and thin patients with little intra abdominal fat, the anatomical borders between soft tissue density structures are often difficult to find, because these are usually outlined by fat. Oral and intravenous contrast administration is particularly helpful in these cases.

Fig. 25.10 Contrast media in CT (abdomen)

Thin section CT

High-resolution computed tomography (HRCT)

refers to thin-section CT. In HRCT the beam collimation is the thinnest possible, often as thin as 0.5 to 1.0mm. This means that voxel depth (Z axis) is the thinnest possible, and partial voluming artefact that degrades in-slice resolution with thicker slices is minimised. However, it is impossible to scan an entire body with 1.0mm slices (this will produce at least 1000 images to look at the torso alone), and of necessity such thin slices are taken with a spacing, often 10mm. Hence, this is a sampling study that is most frequently used to examine lung parenchyma in fine detail for diffuse lung disease where only representative tissue slices are sufficient.

Fig. 25.11 High resolution CT (of lungs)

Multislice CT

With the development of CT X-ray tubes, detector technology and electronic collimation, detector arrays are evolving to allow acquisition of multiple slices with each tube rotation. Four and 16 slice CT scanners may become superceded by new 64 slice machines.

Multislice CT dramatically decreases the total

examination time, because fewer tube revolutions are required to cover the same cranio-caudal distance. This is of particular use with patients who have difficulty holding still (e.g. short of breath, or children) and in trauma cases.

Helical CT

Helical CT imaging involves constant advancement of a

patient through the gantry with a continuously revolving X-ray tube. This is equivalent to helical motion of an X-ray tube around the patient. Helical CT images are of a continuous volume of tissue, rather than a single slice at a time, hence the term volume scanning. It eliminates gaps

between slices in a conventional CT and allows multi-planar and 3-D reconstructions (e.g... of the skull).

Because CT slices are continuously acquired, the total data set is volumetric, and the reconstructed axial slices can be reformatted into coronal, sagittal or oblique slices with little information loss.

Almost all new CT scanners are helical and helical CT is rapidly replacing conventional CT.

MR IMAGE PRODUCTION

Magnetic Resonance Imaging (MRI) produces high

quality cross-sectional images of the patient in any plane (horizontal, sagittal, coronal and/or oblique).

MRI (unlike radiography and CT) avoids using ionising radiation.

It is based on recording radiofrequency signals emitted from within the body (rather than on the transmission of X-rays through the body).

Fig. 25.12 Context for MR images


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Magnetization of body

In MR imaging, the body is placed into a strong, permanent magnetic field. Superimposed on this is a much weaker, rapidly variable magnetic field, produced either by transmitting specific radio frequency (RF) electromagnetic waves into the body, or by rapidly switching magnetic coils.

Fig. 25.13 Emission of RF signals from body

The variable magnetic field leads to radiofrequency electromagnetic waves being emitted from the body. These are recorded by receiver coils, the location of their source is decoded, and spatial maps of several magnetic properties of the body are slowly built up with successive pulse-receive cycles.

RF application and detection

The formation of an MR image is not an instant event, but occurs after many repetitive cycles. These pulse-receive cycles consist of sending RF wave pulses into the body, followed by detecting RF signals that are emitted from the tissues. The number of repetitions depends on the field of view (i.e. how large a volume is being imaged), the matrix size (i.e. how many voxels this volume is broken up into) and how much signal is wanted above system noise (desired signal to noise ratio). The larger the volume, the finer the matrix, and the better the signal to noise, the longer it will take for a particular imaging sequence to run. Most imaging sequences last between one and 10 minutes, with most common being around 3 to 5 minutes long. To be sufficiently thorough, most examinations consist of at least several imaging sequences.

Magnet and coils

The main part of an MRI machine is a very large magnet with a long and narrow tunnel or bore in which the patient is placed during the MR examination. This is the most obviously visible part of the MR machine.

The transmitter RF coils (and coils to create magnetic field gradients) are no less important, but hidden in the machine housing.

The receiver coils (used for reading the emitted RF waves) are placed as close as possible to the area being imaged, and so are quite obvious. The ‘birdcage’ placed in front of a patient’s face when imaging the brain is the head receiver coil. The coil for imaging the spine is usually built into the patient table. Coils for imaging joints and other body parts are usually placed around, or applied directly on, the body part to be imaged. To obtain the best possible image, the body part is placed in the centre of the main magnet bore.

MR control processing and storage

The control console of an MR scanner works analogously to the control console of a CT scanner. It is a high powered computer, running specialised software to drive the magnet and to rapidly reconstruct the read out signal into anatomical images.

Once reconstructed, the images are handled the same way as any other cross sectional computerised images, and can be printed to paper or film, read on the screen (soft copy reading), recorded on a CD or a backup tape.

Contraindications to MR Imaging

Implanted electronic devices and potentially mobile ferromagnetic material are contraindications to MRI.

Any implanted electronic device can either be disabled by the magnetic field of an MR scanner or generate current loops which can cause burns. In particular, pacemakers usually stop functioning in an MR scanner, potentially leading to asystole and death. Pacemakers, neurostimulators and bionic ear implants are absolutely contraindicated in MR scanning. No electronic implant should even enter the MR scanning room.

Metallic implants can be ferromagnetic (i.e. move in a magnetic field) or not. Generally, firmly implanted joint prostheses and other orthopaedic hardware will degrade the MR image to useless in their vicinity, but can enter the magnet. However, freely mobile metallic implants must be considered ferromagnetic till proven otherwise. In particular, cerebral aneurysmal clips or coils can not enter the MR scanning room until cleared as MR compatible. Metallic intraocular foreign bodies can cause severe damage if they move in the magnetic field. These can be detected with a plain X-ray.

Ferromagnetic objects (e.g. oxygen cylinders, hand tools) may be drawn into the magnet and become unintended projectiles.

In MR examination, the patient is placed on the moveable table, which slides inside a narrow tunnel barely large enough to contain an average human body. Patients with claustrophobia may not cope with the procedure and obese patients may not even fit in the tunnel.

TISSUE PROPERTIES IN MRI

When placed within a strong magnetic field and exposed to electromagnetic radiation of specific frequency, tissues emit the absorbed energy in the form of RF signals. Properties of these signals reflect certain physical and chemical properties of the tissue. The collected RF signals carry interpretable information about the spatial location of various tissues within the body.

Magnetic properties

Just like with CT, MR imaging creates volume maps of several physical properties of the tissues being imaged. CT creates a map of only one such property: tissue radiodensity (which in turn reflects electron density in the tissue).

MR imaging can image three fundamental tissue properties: proton density, and two others, called T1 and T2 constants.

All tissues have the properties of T1 and T2 constants, which relate to how quickly magnetization returns to steady state after being altered by the RF pulses sent in by the transmitter coils. T1 weighted sequences show fatty tissues as bright and fluids as dark, while T2 weighted sequences


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show water and fluid as bright. Modern T2W sequences ('fast spin echo T2W') also show fat as bright.

Fig. 25.14 MR images (axial) weighted differently

PROPERTIES OF MR IMAGES

The image occurring on a video monitor following MR imaging consists of a matrix of picture elements (pixels). Each individual pixel on the screen represents the degree of signal intensity for the corresponding voxel in the body translated into a different shade of grey scale.

Pixels from voxels in MR

The slab of tissue imaged with MR is broken up into voxels the same way as in CT. The resulting voxel map of proton density, T1 weighting or T2 weighting in the imaged volume is presented as a series of 2-D slices made up of 2-D pixels. Conventionally, these PD, T1W or T2W maps are displayed as shades of grey (although any other colour scheme could be used).

Tissue magnetic properties displayed with MR images (proton density, T1W and T2W) reflect the different chemical composition of different tissues. Compact bone and air appear black on all sequences (no signal emitted).

Appearances on T1 weighted images

On T1 weighted images tissues with a high fat content appear bright.

These include: - adipose tissue - yellow bone marrow - white matter in the CNS In contrast, tissues with a high water content and fluids

(e.g. CSF) appear dark. Muscle is of intermediate brightness, but this depends

on the amount of fat infiltration and fluid content of the muscle.

Appearances on T2 weighted images

On T2 weighted images tissues with high water content appear bright.

These include Cerebrospinal Fluid (CSF), mucus, urine and bile, or in case of a pathological process, oedematous mucosa and non-clotted blood.

Fig. 25.15 MR images (mid-sagittal) weighted differently

Distinguishing soft tissues on MR images

The most important advantage of MR over other imaging modalities is the ability to distinguish types of soft tissues from each other.

MR imaging distinguishes white matter from grey matter

in the CNS. The ability to distinguish tissues on MR images applies

not only to normal tissues, but also to pathological changes in them. Because many disease processes lead to local oedema (e.g... inflammation), the increased water content of oedema becomes visible as an area of increased signal on T2 weighted images.

Modern T2 weighted sequences also image fat and very fatty tissues as bright. This leads to occasional confusion between fat and water if only the T2W sequences are studied. In general, to begin deciding on the composition of a particular tissue, both T1 and T2 weighted images need to be reviewed.

A quick way to recognise a sectional image as being MR rather than CT, is to look at the subcutaneous fat layer. Fat on MR appears bright (T1W) or less bright (T2W) but on CT fat typically appears black (subject to windowing).

SPECIAL MR IMAGING

By using contrast media that have magnetic properties, the MR images of blood vessels and vascular organs can be enhanced. Alternatively, manipulation of MRI data can display only the structures that move quickly. This enables imaging of blood vessels without an application of any contrast media.

Gadolinium based material

Gadolinium is a toxic rare metal. However, when firmly bound to organic chelates, it produces a chemical compound with distinct magnetic properties. Following an intravenous injection, gadolinium based contrast materials circulate within the vascular system, and enter the extracellular tissues but do not cross the intact blood-brain barrier. Gadolinium based contrast material is excreted via the kidneys and accumulates in the urine. On T1 weighted images gadolinium contrast agents lead to tissue brightening (‘enhancement’) in a comparable way to how


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iodinated contrast material behaves in CT. There is no effect on T2 weighted images.

Proton movements

Dedicated MR sequences allow selective demonstration of moving protons, and in particular of flowing blood. In these dedicated sequences, moving blood can be either bright (‘white blood sequences’) or have no signal (‘black blood sequences’).

The vast majority of MR angiography (MRA) and MR venography (MRV) is based on white-blood sequences, sometimes following contrast material administration.

This imaging approach enables 3-D reconstruction of vascular tree images.

Heavily T2 weighted sequence

On heavily T2 weighted sequences fluid has very bright signal. A number of MR applications which image normal fluid spaces within the body utilise this phenomenon. One of these is MR Cholangiopancreatography (MRCP)

which produces a map of the biliary tree and pancreatic ducts.

Other sequences have been used to image ureters, fluid filled loops of intestine, and even dilated lymphatic vessels.

Fig. 25.16 MRCP

Chapter 26: Ultrasound Imaging

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ULTRASOUND IMAGE PRODUCTION

TISSUE PROPERTIES IN US IMAGING

PROPERTIES OF US IMAGES

DOPPLER ULTRASOUND IMAGES

ULTRASOUND IMAGE PRODUCTION

Ultrasound imaging or ultrasonography is an imaging

technique which produces cross-sectional multiplanar images of the patient’s body by using ultra high frequency sound or ultrasound (US).

Ultrasound allows real time cross-sectional imaging without any ionizing radiation.

Ultrasound reflection

Ultrasound is a high frequency vibration transmitted as

a series of longitudinal waves through the body. When it travels through the tissues, the vibration interacts with them. The most important form of interaction for creation of US images is reflection, which occurs when the US waves cross boundaries of tissues which are of different acoustic densities. The reflected US wave is picked up by the transducer, and the location and distance of the boundary is calculated. These boundaries (interfaces) form the ultrasound image are built up by an ultrasound machine.

Fig. 26.1 Context for US images

Ultrasound formation and detection

Both formation and detection of ultrasound is done in a transducer. The transducer contains a piezoelectric

material (crystal) which has an intrinsic ability to change shape when voltage is applied to it, and rapid oscillation of the piezoelectric crystal shape produces ultrasound waves.

The most convenient material used in US transducers is ceramic because it has large charged atoms which are loosely held in a complex crystal structure. When placed in an electric field, these atoms move and change the shape of crystal. This in turn produces high frequency sound or ultrasound.

The piezoelectric effect is symmetrical, so the crystal shape change will produce a small electric signal when the crystal is struck by the reflected ultrasound wave. Thus, the transmitter is also used as the receiver.

The transducer produces pulses of high frequency sound which are sent to the body.

After each pulse of ultrasound is emitted, there is a ‘listening’ period, when the transducer detects US waves returning from the body. Small electric pulses coming from the transducer are amplified, decoded and placed into their correct location in a cross sectional image.

TISSUE PROPERTIES IN US IMAGING

An ultrasound wave propagates through different tissues at different speeds depending on their composition.

Reflection, absorption and scatter

Ultrasound is a high frequency vibration transmitted as a series of longitudinal waves. When they travel through the patient’s body, these waves can be reflected, absorbed, or scattered, and a combination of these three interactions of sound with tissues forms its ultrasound appearance. Reflection produces weak US waves which travel back to

the transducer. Absorption and scatter (like in radiography) lead to

the reduction of the US beam energy. This depends on the tissue composition and thickness as well as on the transducer frequency. These factors affect the ability of tissue interfaces to reflect the sound in order to be interpreted as a signal in the US machine.

Acoustic impedance and echogenicity

Acoustic impedance is a property of material

determined by its density coupled with the velocity of ultrasound (and may be regarded as the 'elastic resistance' to US wave propagation).

An acoustic interface exists at the junction of two tissues of different acoustic impedance.

At each acoustic interface, an incident ultrasound wave is partly refracted (changes direction) and partly reflected (leading to image production). The amount of sound reflected depends on the difference in tissue acoustic impedance between the two tissues.

The larger the difference in density of adjacent tissues, the larger the reflection, resulting in a brighter signal from their acoustic interface.

Tissues can be classified into different groups based on their tendency to uniformly reflect sound. In ultrasound this tendency to reflect ultrasound is referred to as echogenicity.

PROPERTIES OF US IMAGES

Ultrasound images can be printed to paper or film, or displayed in real time on a monitor. By convention, US data is displayed as shades of grey, while flowing blood is colour-coded.

Ultrasound tissue scale

On the basis of their echogenicity the appearance of tissues can be classified (and is conventionally presented on a grey scale) as follows:

Fluid (water, urine, cyst fluid, bile, non-clotted blood) has very few reflections and appears black. Fluid with internal debris and particles (intestinal content) has an irregular reflecting (patchy white) appearance. Debris and other floating reflectors move gently in real time.

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Solid soft tissue has grey appearance from internal reflections, as well as its own granularity (‘echotexture’), in

the following order: (whiter) pancreas > liver, spleen > kidneys (darker).

Fat is very reflective, and looks bright white on ultrasound.

Gas absorbs sound and does not transmit it, and has the appearance of ‘dirty shadowing’ on ultrasound.

In contrast, bone (or calcification) does not transmit sound, but does give a sharp reflection, producing ‘clean shadowing’.

Fig. 26.2 US image of gall bladder (bile-filled)

Real time sector scans

All modern ultrasound machines produce real-time images, which change constantly with tissue movement. The frame refresh rate is sufficiently high to demonstrate normal cardiac, diaphragmatic, tendinous and other movements as continuous and smooth.

Ultrasound probes are the detachable scan heads that

carry the transducer on their end, and are applied directly to the patient (through an acoustic coupling gel).

Fig. 26.3 Echocardiograms

Most ultrasound probes are either curved or linear. Curved probes produce a wedge-shaped image but with a

concave apex that corresponds to the probe face. Linear probes produce a square or rectangular image. Some probes are designed to have a small footprint (for example, to image the heart by scanning between ribs). These sector scanners or vector arrays produce a triangular image with a point apex. Intra-cavity probes (for endorectal and endovaginal ultrasound) are elongated and thin, with usually a curved semicircular transducer on their tip.

The operator can decide at any time to obtain a series of characteristic ‘frozen’ images for detailed studies of anatomy or pathology appearing on certain slices.

Fig.26.4. Routine US at 18 weeks of pregnancy

Ultrasound examination during pregnancy may be obtained by moving the transducer across the abdomen. It is performed to assess the foetus as well as the position of the placenta.

DOPPLER ULTRASOUND IMAGES

Doppler ultrasound detects and analyses rapidly moving objects. Specifically, this applies to flowing blood.

The Doppler effect

When an emergency vehicle with a siren is travelling towards a listener, the frequency of sound (the pitch) is higher than when it is travelling away from the listener. This phenomenon is an example of the Doppler effect. The

Doppler effect is produced when the effective frequency of a longitudinal sound wave is changed because its source is also moving (in the same direction or in the opposite direction to the sound wave).

Colour Doppler

Colour Doppler imaging detects moving blood and its

direction but provides no information about its spectral properties. The velocity of motion commonly presented as an intensity map. By convention, flow towards the transducer is coloured red, while flow away from the transducer is coloured blue (although any colour scale could be chosen).


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Fig. 26.5 Colour Doppler images (right kidney)

Colour Doppler information can be superimposed on real-time grey scale structural imaging to provide a composite image ('duplex scanning').

Pulsed Doppler

In pulsed Doppler imaging, a continuous narrow beam

of US is used to 'listen' along one chosen direction in a small target volume. The reflected US beam carries information about the flow velocity within the target volume as a function of time as well as spectral information about all its velocities (i.e. the spread of different velocities and their relative prevalence within the target volume). The sound equivalent of pulsed Doppler US is the familiar arterial 'whoosh'. The visual image is a very accurate representation of the flow waveform and is used for vascular physiological analysis (e.g... to derive peak systolic velocity, diastolic velocity etc.).

Chapter 27: Endoscopic Anatomy

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LOOKING WITHIN HOLLOW VISCERA LOOKING WITHIN BODY CAVITIES LOOKING WITHIN JOINT CAVITIES

LOOKING WITHIN HOLLOW VISCERA

The mucosal features of most tubular viscera can be examined endoscopically. Such (intralumenal) endoscopy is typically performed via a normal opening on the exterior of the body. As well as looking at normal anatomy and for abnormalities (e.g. tumours, ulcers), function (e.g. of sphincters) can be assessed, biopsies of suspicious areas taken and treatment performed (e.g. removal of polyps, stones or foreign bodies).

Bronchoscopy

The respiratory tract to the segmental bronchi can be viewed through a bronchoscope passed via the nose. Vocal cord function is also assessed.

Fig.27.1 Tract viewed at bronchoscopy

1. Inlet of larynx and vocal folds 2. Trachea 3. Tracheal bifurcation 4. Right main bronchus 5. Right upper lobe bronchus 6. A segmental bronchus

Fig.27.2 Internal views of respiratory tract

Gastroscopy

The upper gastrointestinal tract to the duodenum can be viewed through a gastroscope passed via the mouth. Sphincteric function is also assessed.

An additional diagnostic and therapeutic procedure such as ERCP can be performed where a tube is passed into the bile duct. This enables investigation of the biliary tract and removal of stones.

Fig.27.3 Tract viewed at gastroscopy

1. Upper end of oesophagus with folds (from surrounding cricopharyngeal sphincter)

2. Lower end of oesophagus (with folds from surrounding functional sphincter) and line of epithelial transition to gastric type

3. Body of stomach (with rugae) 4. Pyloric antrum and canal with orifice (and

surrounding pyloric sphincter) 5. Smooth part of first part of duodenum (‘duodenal

cap’) 6. Second part of duodenum with circular folds and

duodenal papilla (for bile duct and main pancreatic duct)

Fig.27.4 Internal views of upper digestive tract


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Colonoscopy

The lower gastrointestinal tract to the caecum and terminal ileum can be viewed through an endoscope passed via the anus. Prior to the procedure, the large intestine is cleared by oral ingestion of a special bowel preparation. During the procedure, gas can be introduced to distend the lumen. Normal mucosal features are visualised, abnormalities detected, biopsies of suspicious areas taken and polyps removed.

Fig.27.5 Tract viewed at colonoscopy

1. Rectum (with superior, middle and inferior rectal valves) lined by a characteristically thin mucosa

2. Sigmoid colon 3. Splenic flexure of colon (with shadow of spleen

seen through the wall) 4. Transverse colon 5. Ascending colon and caecum (with ileocaecal

valve) 6. Caecal pole (with mucosal folds and orifice of

appendix))

Fig.27.6 Internal views of lower digestive tract

Cystoscopy

The lower urinary tract including the bladder (and ureteric orifices) can be viewed through a cystoscope passed via the external urethral meatus. The urethra in a male is much longer than that in a female. Particular care is taken to ensure the cystoscope negotiates the change in direction of the urethra at the bulb of the penis (to smoothly enter the narrow membranous urethra). This is achieved by manoeuvring the penis.

Fig.27.7 Tract viewed at cystoscopy (male and female)

1. Penile urethra 2. Bulbar fossa in urethra (and opening to

membranous urethra) 3. Prostatic urethra (with urethral crest and seminal

colliculus) 4. Internal urethral meatus (and uvula of bladder

projecting into lumen) 5. Trigone of bladder 6. Right ureteric orifice

Fig.27.8 Internal views of lower urinary tract

27. Endoscopic Anatomy

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LOOKING WITHIN BODY CAVITIES

The interior of major body cavities and external features of contained organs can be examined endoscopically. A portal of entry is created by a small incision through the body wall allowing passage of the endoscope (which is then manoeuvred around the cavity).

Thoracoscopy

The pleural cavity, lung and mediastinal structures can be viewed through the relevant side of the thoracic wall.

At the beginning of the procedure, the lung is partially collapsed by entry of air into the pleural cavity. This potential space is normally occupied by only a thin film of fluid (between parietal and visceral layers of the pleura). As it collapses, the lung separates from the thoracic wall. The lung is re-inflated after the thoracoscope has been removed.

Fig.27.9 Body cavities viewed at thoracoscopy

1. Right sympathetic trunk descending on the posterior thoracic wall (crossing intercostal veins)

2. Right lung (middle and lower lobes) and anterior costodiaphragmatic recess

3. Right dome of diaphragm and posterior costodiaphragmatic recess

4. Upper lobe of left lung and lateral thoracic wall 5. Left phrenic nerve on pericardium over left ventricle

Fig.27.10 Internal views of pleural cavities

Laparoscopy

The peritoneal cavity and abdominopelvic organs can be viewed through a laparoscope introduced via a small incision through the anterior abdominal wall, typically at the umbilicus.

The peritoneal cavity is a potential space normally occupied by only a thin film of fluid (between parietal and visceral layers of the peritoneum). The viscera separate from the anterior abdominal wall when gas is introduced to distend the space.

Fig.27.11 Body cavity viewed at laparoscopy

1. Left lobe of liver (lifted, showing left subhepatic space), stomach (with greater omentum) and spleen, diaphragm and left subphrenic space

2. Diaphragm and right subphrenic space, right lobe of liver and fundus of gall bladder

3. Anterior abdominal wall and small intestine 4. Uterovesical pouch of peritoneal cavity (between

uterus and bladder), uterine tubes and ovaries (with follicles)

5. Rectouterine pouch of peritoneal cavity (between rectum and upper part of vagina) and fundus of uterus

Fig.27.12 Internal views of peritoneal cavity (in female)


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LOOKING WITHIN JOINT CAVITIES

Joint cavities can be examined through an arthroscope introduced via a portal of entry created by a small incision into the joint capsule. Normal intra-articular structures can be identified, abnormalities detected and therapeutic procedures performed (e.g. trimming a torn meniscus).

Fig.27.13 Major joint cavities viewed at arthroscopy

Arthroscopy of upper limb joints

1. Articulation between head of right humerus and glenoid cavity of scapula (with labrum)

2. Attachment of long head of biceps tendon (to supraglenoid tubercle)

3. Anterior capsule of shoulder joint (with defect to subscapular bursa)

4. Inferior capsule of shoulder joint 5. Right elbow joint (with annular ligament around

head of radius) 6. Left wrist joint (with articulation between distal end

of radius and proximal row of carpal bones)

Fig.27.14 Internal views of major upper limb joints

Arthroscopy of lower limb joints

7. Right hip joint (with articulation between head of femur and acetabulum of hip bone)

8. Patella and suprapatellar bursa of right knee joint 9. Medial femoral and tibial condyles with medial

meniscus 10. Anterior cruciate ligament in intercondylar notch 11. Lateral femoral and tibial condyles with lateral

meniscus 12. Left ankle joint (with articulation between dome of

talus and distal end of tibia)

Fig.27.15 Internal views of major lower limb joints

Chapter 28: Clinical Procedures

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INCISIONS

WOUND CLOSURE

SYNOVIAL CAVITY PUNCTURE

BODY CAVITY PUNCTURE

INJECTIONS

NERVE BLOCKS

ARTERIAL PUNCTURE

VENEPUNCTURE

INTRAVENOUS CANNULATION

INCISIONS

An incision is a surgical cut through skin. Incisions are

made to remove skin lesions or to provide access to deeper anatomical structures. For surgical exploration of body cavities, incisions are also made through the other layers of the body wall.

Skin characteristics of the region (including relaxed skin tension lines) should be assessed with functional

and cosmetic implications considered.

Relationship to skin tension lines

Skin incisions made parallel to lines of tension heal with a minimal scar, while those crossing lines of tension tend to produce a wider scar.

Fig 28.1 Relaxed skin tension lines

This may be even more critical for patients with a

tendency to form a nodular mass of scar tissue (termed a keloid) along a skin wound.

Incisions should ideally be placed along prominent skin creases (particularly in the trunk, neck and face) to disguise the scar.

Fig.28.2 Incisions and skin creases

Incisions in special areas

Special care should be taken for incisions in certain areas, particularly on the face, to avoid disfigurement. Special care should also be taken near cutaneous orifices (e.g... at the vermilion border of the lips).

Sites where incisions should be avoided

Incisions crossing joint lines should be avoided due to subsequent restriction of movement even from normal scar contraction.

Similarly, incisions crossing facial expression lines

(including eyebrows) should be avoided as normal scar contraction may result in an altered appearance. In addition, with hairlines, scar tissue does not form hair follicles.

Incisions crossing mucocutaneous junction lines should be avoided where possible.

Pressure areas (e.g... behind elbow or heel, over patella or on the sole) should also be avoided because of wound tension coupled with restricted blood supply when bearing pressure.

Bleeding from incisions and healing

Incisions (particularly if deep) pass through vascular structures (dermis, subcutaneous tissue and muscle) and will bleed (particularly where hydrostatic pressure is increased). Certain regions tend to possess a richer arterial supply than others and are more likely to bleed profusely. They will also heal rapidly where there is a low venous pressure (e.g... scalp, face and neck) as the arteriovenous


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pressure difference enables effective circulation at the tissue level.

Fig.28.3 Elevation to reduce bleeding

Those with high venous pressure (e.g. legs and feet) heal slower due to a more sluggish microcirculation. Areas with a poor arterial supply (e.g. skin over the tibia) do not tend to heal well. This is accentuated in patients with impaired arterial flow (e.g. from arterial occlusion) or impaired venous flow (e.g. from varicose veins).

Pain and need for prior anaesthesia

Many structures are supplied by sensory nerves with significant numbers of pain receptors and some (e.g. skin) produce extremely severe pain while being cut. It is most important to provide adequate anaesthesia (whether general, nerve block or local) for any incision.

The dermis of skin has a particularly rich supply of superficial somatic pain fibres.

Deep fascia, aponeurosis and muscle are supplied with deep somatic pain fibres and serous membranes (parietal layer) are particularly richly supplied.

If local anaesthesia is adopted, each of the pain sensitive layers needs to be adequately injected. If a nerve block is adopted, infiltration around the appropriate nerve (or nerves) needs to be achieved (noting that adjacent sensory nerve supply territories may overlap).

Layers traversed by incisions

Layers traversed from superficial to deep may include skin, subcutaneous tissue, deep fascia and muscle.

Incisions to open a joint cavity may additionally include, ligament, fibrous capsule and synovial membrane.

Incisions to open a body cavity typically involve traversing body wall layers in the following sequence:

- skin - subcutaneous tissue - aponeurosis and/or muscle layers - parietal layer of serous membrane In abdominal incisions, where possible, muscles are

split parallel to the direction of fibres (rather than incised) to minimise damage (with pain and bleeding) and avoid nerve injury (e.g. to the ilioinguinal nerve). This also enables more rapid healing and less chance of the postoperative complication of a protrusion through the area of weakness from the incision (an incisional hernia).

Fig.28.4 Incision via layers (anterior abdominal wall)

Structures endangered by incisions

Incisions should be planned with an awareness of underlying structures (particularly nerves and vessels) and special care must be taken to avoid damaging them.

Fig.28.5 Sites where vital nerves are superficial

Nerves that are superficial and vulnerable at specific sites include the:

- facial nerve branches (on face) - accessory nerve (in posterior triangle of neck) - ulnar nerve (behind elbow and at wrist) - median nerve (at wrist) - recurrent thenar branch of median nerve (in palm) - common fibular nerve (behind knee) All of the above nerves contain motor fibres, with

significant functional impairment to muscle action resulting if inadvertently severed.

28. Clinical Procedures

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Sensory nerves that are superficial and vulnerable include the:

- great auricular nerve (in neck) - palmar cutaneous branch of median nerve (at wrist) - saphenous nerve (in leg) Incisions in the thoracic wall endanger intercostal

neurovascular bundles. Anterior abdominal wall incisions endanger epigastric vessels (superficial, inferior and superior).

Lateral abdominal incisions may divide nerves to the rectus abdominis muscle as they approach it from laterally. They may also endanger branches of the circumflex iliac vessels (superficial and deep).

Fig.28.6 Vessels and nerves endangered on abdomen

Internal organs (e.g... abdominal viscera) may even be endangered if an incision is made too deeply.

WOUND CLOSURE

A wound is an injury involving a break in the skin, produced by either an incision or a laceration (traumatic

tear).

Prevention of dead space

Wounds should be closed layer by layer to prevent dead space and maximise wound strength

Closing wounds in layers prevents dead space that otherwise tends to accumulate blood and is prone to subsequent infection. Closing in layers also maximises wound strength (although fat does not hold sutures well) and minimises risk of disruption.

Muscles also tend to pull apart with contraction of their fibres and the most critical layer (e.g. an aponeurosis or a fascial wall of a compartment) should be closed without creation of any weakness or gap.

Adherence of skin to deeper structures is avoided by closing in layers, otherwise the scar tends to retract and move with underlying muscle contraction (e.g. a scar from thyroid surgery moving on swallowing).

Fig.28.7 Closure in layers (anterior abdominal wall)

Wound layers sutured

Interrupted sutures or a continuous suture may be

used for a particular layer where appropriate. Layers are closed in the reverse order to those traversed by an incision and may include muscle, deep fascia, subcutaneous tissue and skin. Subcutaneous tissue is generally closed with the skin as fat does not hold sutures well.

Closing wounds from incisions into a body cavity typically involve the following layers of the body wall:

- parietal layer of serous membrane - aponeurosis and/or muscle layers - subcutaneous tissue with skin In abdominal incisions, where more than one layer of

muscles are split parallel to the direction of fibres (rather than incised) and the fibre direction for these layers cross each other, they may spring back together (and overlap sufficiently) without need for suturing.

Sutures and other options for skin

For skin interrupted sutures may be used for most regions. They also tend to stop bleeding even in highly vascular areas (e.g. scalp, face and neck). These areas tend to bleed more readily but heal more rapidly (with sutures removed in about 5 days). Less scarring is obtained by using a larger number of fine sutures rather than fewer, heavier sutures more widely spread.

A subcuticular suture heals with minimal scarring and

may be used for cosmetically important sites particularly in females and children. However, it directly apposes only the epidermis and dermis, with more sutures probably being required in a deeper layer to compensate.

Glue or clips may be helpful in certain circumstances. External adherent strips may be used to close

superficial wounds (except across joints, where movement disrupts them) and are of particular value in children.

Wound edge alignment and suture tension

Wound edges require precise alignment with anatomical landmarks (e.g... borders of lips or eyelids) to


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prevent disfigurement. Edges heal better when they are slighted everted. Sutures should be tied away from the wound edge.

Fig.28.8 Eversion and alignment of wound edges

Excess tension (e.g... sutures tied too tight) impairs blood supply to the wound (especially its edges), causes pain and delays healing. Impaired blood supply may even result in the wound breaking down.

Fig.28.9 Suture tension and tissue blood supply

Although many areas can tolerate some tension to bring wound edges together, in certain regions (e.g. lower leg, foot, palm and fingers) only minimal tension is permissible. Areas already under considerable tension (e.g. over the tibia) require particular care. Excessive tension may require mobilization of a skin flap or even skin grafting.

Fig.28.10 Postoperative elevation to minimise swelling

Post-operative swelling will further increase wound tension. Swelling can be minimized by elevation (which reduces hydrostatic pressure). Tension from muscle action can be minimized by immobilization from a splint. Firm dressings may reduce wound tension and swelling, but if too tight will impair blood supply.

Fig. 28.11 Optimal compression from dressings

Sutures should be left for a greater time in areas under tension with a poor blood supply (or other factors delaying healing). If there is doubt, they can be removed in stages rather than all at the one time (and risk wound disruption). However, if sutures appear too tight (and causing inadequate blood supply) they should be removed early.

Lacerations and their management

Lacerations are wounds produced by trauma rather

than by controlled surgical incisions. In addition to skin, fascial and muscle layers, the laceration should be systematically inspected for injury to each of the following types of structures where applicable:

- vessels - nerves - tendons - bones and/or joints - internal organs Damage to deeper vital structures may be concealed.

Fig.28.12 Layers of a laceration and associated injuries

Bleeding from wounds, even in highly vascular areas (e.g... scalp) is usually controlled by pressure applied to each side of the wound. Small spurting arteries can be clamped (with artery forceps) for a couple of minutes and larger spurters ligated, if necessary. Bleeding will tend to stop from compression by the sutures when the wound is closed.

The wound should be explored by searching for any foreign bodies. The site, number and depth are assessed (including by x-ray where appropriate) before attempting removal. Use of a tourniquet may be considered to provide a bloodless field.


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The wound is debrided by removing any foreign or

dead tissue (particularly fat or deeply located tissue). Dead tissue is more extensive with contusions and crush injuries.

Contaminated wounds are likely to become infected particularly if time has elapsed. Infection results in wound breakdown. After inspecting, exploring and debriding, it may be best to delay wound closure for 5 days (delayed primary closure).

SYNOVIAL CAVITY PUNCTURE

A synovial cavity includes a closed sac with a

potential space enclosed within the serous membrane. The sac contains a small amount of fluid for lubrication. Fluid, blood or pus may accumulate in it as a result of trauma or disease. A synovial cavity may be drained by aspiration via a needle inserted through the layers overlying the joint or associated bursa.

Sites for a joint puncture

In a knee joint puncture, the needle may be inserted from either side near the superior border (base) of the patella, into the gap between the patella and the femur or into the suprapatellar bursa (which is continuous with the synovial cavity above the patella). Alternatively a lower approach may be adopted from either side of the ligamentum patellae near the inferior border (apex) of the patella.

Fig.28.13 Drainage of knee joint synovial cavity

Layers pierced in a joint puncture

For a synovial cavity puncture the following layers

are pierced in sequence by the needle: - skin - subcutaneous tissue - deep fascia (with or without underlying muscle) - fibrous capsule (with or without overlying ligament) - synovial membrane

Prior to the puncture, it is important to anaesthetize skin and synovial membrane as these layers are the most richly innervated by pain fibres.

Hazards of a joint puncture

In a joint cavity puncture, if the needle is pushed too firmly, its point may damage hyaline cartilage covering a bony articular surface within the joint.

For the lower approach in a knee joint puncture other intra-articular structures (the menisci and cruciate ligaments) are also endangered.

BODY CAVITY PUNCTURE

A body cavity includes a closed sac with a potential

space located between the parietal and visceral layers of a serous membrane. The sac contains a small amount of fluid for lubrication. Fluid, air, blood or pus may accumulate in it as a result of trauma or disease. A body cavity may be drained by aspiration via a tube or needle inserted through the layers of the overlying body wall.

Sites for a body cavity tap

Appropriate sites for access are determined by relationships to anatomical landmarks and potential hazards (e.g. vessels and nerves of the body wall and vital structures in the body cavity).

Prior to insertion of the needle, local anaesthetic is injected to infiltrate each layer of the body wall that is pain sensitive (particularly the overlying skin and underlying parietal layer of serous membrane).

The site is determined by clinical examination and may be confirmed by radiological imaging. The thickness of the body wall is estimated as a guide to maximal safe depth of needle insertion. Aspiration is attempted throughout needle advancement. If fluid is not encountered at the estimated depth, consideration should be given to the possibility of the needle tip being blocked by a plug of tissue (the needle may be flushed to unblock it, or rotated to bring the bevel away from adjacent tissue).

Fig.28.14 Drainage of pleural cavity


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Layers pierced in a body cavity tap

For a body cavity puncture the following layers are

pierced in sequence by the needle: - skin - subcutaneous tissue - muscle/fascial layers - parietal layer of serous membrane Drainage of a pleural effusion (accumulation of fluid in

the pleural cavity) may be performed via a needle inserted posteriorly through an appropriate intercostal space in the lower part of the thoracic wall.

Hazards of a body cavity tap

The structures endangered by a needle inserted into a body cavity are vessels and nerves of the body wall in addition to vital organs within the body cavity.

In a pleural tap, the needle is inserted near the lower margin of the intercostal space to avoid injury to the main intercostal neurovascular bundle that runs in a groove near the inferior border of a rib.

The following viscera are endangered: - lungs (by penetrating the visceral pleura) - liver on the right (by penetrating the diaphragm) - spleen on the left (by penetrating the diaphragm) - kidneys (by penetrating or passing below diaphragm) Assessment of lung status by physical examination is

mandatory after the procedure and a chest x-ray may also be required.

INJECTIONS

Intradermal (ID) injections are given directly into the dermis of the skin. Subcutaneous (SC) injections pass through the skin into superficial fascia. Intramuscular (IM)

injections penetrate into muscle.

Fig.28.15 Depth and optimal angle of injections

The more superficial the injection, the flatter the plane of entry, to prevent the needle from penetrating too deeply. For an ID injection, the needle is of narrower bore and the volume injected to raise a small bleb within the skin is much smaller. For a SC injection in a thin person, the skin and subcutaneous tissue may be pinched to ensure the correct layer is injected.

For an IM injection, the muscle should be relaxed (e.g. arm loosely by side). The skin may be stretched just prior to rapidly inserting the needle with a dart-like action, to minimise pain.

Preventing inadvertent IV injection

There is always the danger of inadvertently injecting into a blood vessel (especially veins which are numerous and variable) within subcutaneous tissue or within muscle. Aspiration, by drawing back the plunger of the syringe (and observing that blood does not appear), immediately before injecting, minimises this.

Aspirating before injecting avoids inadvertent intravenous injection

If blood is aspirated, the point of the needle should be moved. The tributaries of gluteal veins deep in the buttock are particularly numerous and large.

Fig.28.16 Aspiration before injecting

Sites for IM injections

IM injections are commonly given in the upper arm (deltoid muscle), buttock (gluteus maximus muscle) or lateral aspect of thigh (vastus lateralis muscle).

Layers pierced by an IM injection

For an IM injection the following layers are pierced in sequence by the needle:

- skin (epidermis and dermis) - subcutaneous tissue - deep fascia - muscle

Structures endangered by IM injections,

The structures endangered by an IM injection are primarily vessels and nerves coursing deep to the muscle. The arteries are accompanied by venae comitantes (a pair of intercommunicating veins that surround the artery). Inadvertent intravenous injection may occur if the needle tip enters them. IM injections into deltoid endanger the:

- axillary nerve and circumflex humeral vessels (deep to deltoid running transversely around the surgical neck of the humerus)

- cephalic vein (along the anterior border of the muscle within the subcutaneous tissue)

- radial nerve and profunda brachii vessels (behind the posterior border of deltoid deeply within triceps) IM injections into gluteus maximus endanger the:

- sciatic nerve (deep to the inner-medial quadrant of the buttock)

- superior and inferior gluteal vessels (deep to the gluteal muscles)


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An IM injection into gluteus maximus should be given in the upper-outer quadrant of the buttock (to avoid the sciatic nerve). IM injections into vastus lateralis endanger the descending branches of the lateral circumflex femoral vessels.

NERVE BLOCKS

A nerve block involves infiltrating local anaesthetic

around a nerve to interrupt conduction (temporarily).

Within a peripheral nerve, small fibres (mainly pain fibres) are most affected by local anaesthetic agents. Larger fibres are affected to a lesser degree (hence touch sensation may remain).

Although anaesthesia may be achieved by infiltration directly around certain structures (e.g. in a wound) much more local anaesthetic agent is required than with a nerve block. Directly injecting certain areas (e.g. palm or sole) may be painful and injection of fluid into tight, confined compartments may also raise pressure in the compartment, compromising vascular supply to its contents.

Fine needles reduce the rate of injection and the volume required. Dental syringes have cartridges and fine needles, allowing easier control of the volume injected. However, they preclude aspiration and should only be used for superficial injections. The point of the needle must be moved while injecting to avoid inadvertent intravenous injection of a large bolus.

Sites for nerve blocks

Appropriate sites for access to nerves are determined by relationships to anatomical landmarks and potential hazards (e.g. accompanying vessels and neighbouring vital organs). Landmarks directly observed or palpated, coupled with knowledge of anatomy and its variations, are vital for correct placement.

Fig.28.17 A digital nerve block

The area anaesthetized by a nerve block corresponds to the sensory distribution of the nerve (distal to the site of infiltration) minus the area of overlap from adjacent nerves.

For an intercostal nerve block, it is necessary to also

block the nerves above and below the level of injury (or region requiring anaesthesia) due to the extensive overlap in sensory supply of adjacent spinal nerves (and dermatomes).

For a digital nerve block, each of the 4 digital nerves

at the base of a finger (supplied by a palmar digital and a dorsal digital nerve on each side) may be blocked. An alternative approach is to infiltrate around the common palmar digital nerve in a web space. It is often necessary to block more than one common digital nerve because each provides digital nerves only to contiguous halves of adjacent fingers.

Use of vasoconstrictors with nerve blocks

Conditions that increase blood supply (inflammation, exercise) decrease the duration of action. Vasoconstrictor drugs (e.g. adrenaline) may be used to prolong the action of local anaesthetic agents (by slowing blood stream removal of drug) and reduce local bleeding. However, they must be used with caution and are forbidden for certain sites.

Adrenaline must never be injected into terminal parts (particularly digits or penis) because they are (collectively) supplied by end-arteries.

It may produce intense arterial spasm resulting in death of tissue distal to the injection site.

Fig.28.18 Vasoconstriction of end arteries from adrenaline

Hazards of nerve blocks

The structures endangered by a nerve block are accompanying vessels in addition to the nerves themselves:

- veins (with direct vessel damage and/or intravenous injection) - arteries (with direct vessel damage and/or arterial spasm) - nerve (with direct damage by intraneural injection) Damage indirectly due to compartment syndrome may

also occur with a nerve block (particularly if large volumes of agent are given).

Aspirating before injecting avoids inadvertent intravenous injection.

Ideally, nerve blocks should only be performed on patients who are awake. If a nerve is hit, reporting of paraesthesia enables the operator to reposition the needle prior to injection. If direct injection into a nerve occurs, reporting of paraesthesia or pain enables the operator to immediately stop injecting and reposition the needle to prevent further damage.


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ARTERIAL PUNCTURE

An arterial puncture may be performed to provide a

sample of arterial blood for analysis of blood gases (partial pressures of oxygen and carbon dioxide). An arterial cannulation may be performed for direct measurement of arterial blood pressure during surgery and for radiological procedures. Punctures are not performed on end-arteries.

Accessible and palpable sites of arteries

The radial artery is commonly chosen for an arterial

puncture and is readily palpable between skin and bone at the distal end of the radius on the front of the wrist (lateral to the tendon of flexor carpi radialis).

The brachial artery and the femoral artery are

alternatives for arterial puncture. They are readily palpable in the cubital fossa (medial to the tendon of biceps) and in the femoral triangle (below the mid-inguinal point), respectively.

Assessment of collateral circulation

Adequate collateral circulation should be tested for prior to an arterial procedure.

Fig.28.19 Compressing circulation to hand

Adequate collateral circulation to the hand is tested by compressing both the radial and ulnar arteries at the wrist until the skin of the palm has blanched. One artery is released with blushing in the entire palm indicating adequate collateral supply from that artery. The test can be repeated for the other artery to assess adequacy of collateral supply from it.

Fig.28.20 Releasing compression of ulnar artery

Layers pierced in an arterial puncture

The procedure is painful unless a local anaesthetic injection is given (particularly into the skin).

Tension may be put on the artery to immobilise it. This prevents the artery moving away from the needle, when it is inserted. The layers then pierced are skin, subcutaneous tissue and deep fascia. The artery is penetrated by the needle, until blood is seen pulsating into the syringe.

Fig. 28.21 Radial artery puncture

Compression of the artery immediately after removal of the needle or cannula is mandatory to minimise bleeding. Firm pressure is maintained (through a gauze swab) for at least 5 minutes.

Fig.28.22 Minimising bleeding after arterial puncture

Hazards of an arterial puncture

The needle tip may hit underlying periosteum and bone (with resultant pain). A neighbouring vein, nerve or tendon may also be damaged.

The artery itself is vulnerable to damage with bleeding and subsequent haematoma (which may be large and painful). Potential hazards are arterial spasm or thrombosis.


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VENEPUNCTURE

A venepuncture is used for obtaining a venous blood

sample or for giving an intravenous injection.

Sites for a venepuncture

The procedure is best performed on a distended vein and it is far more convenient to choose a superficial vein of the upper limb. However, in emergency situations, the femoral vein may be used, as more peripheral veins may be difficult to access (particularly with shocked, obese or very young patients).

Superficial veins are highly variable (and have numerous unnamed tributaries). Although any prominent vein in the upper limb may be used, the cubital fossa is the most satisfactory site for a venepuncture as large superficial veins meet in the roof of this region. A median cubital vein typically connects the cephalic vein to the basilic vein in the subcutaneous tissue. However, a median cephalic vein and a median basilic vein (rather than a single median cubital vein) may be present instead.

Fig.28. 23 Superficial venous patterns in cubital fossa

Being a flexor region, the skin is not as tough as on the extensor aspect of the limb and after the procedure compression of the vein can be easily assisted by elbow flexion.

Preventing inadvertent intra-arterial injection

Anatomical variations associated with the brachial artery or its branches may be potential hazards.

Fig.28.24 Inadvertent injection into an arterial variant

Intravenous injection of a general anaesthetic agent or vasoconstrictor should not be administered at the cubital

fossa because of the risk of inadvertent intra-arterial injection into an anomalous artery (e.g. a superficial ulnar artery, which occurs in about 3% of cases). This may result in intense vascular spasm compromising supply of the forearm and hand.

For other intravenous injections, a superficial vein situated laterally in the cubital fossa, such as the cephalic vein, may be chosen. The cephalic vein is located further away from the brachial artery than the median cubital vein (which is separated from the artery only by the thin bicipital aponeurosis).

Tourniquet to distend veins

Application of a tourniquet (e.g. around the arm)

distends the veins distal to the tourniquet. The tourniquet should be at a pressure less than diastolic arterial pressure to enable sufficient blood flow.

Veins are often best felt, not just seen. If a vein is not very prominent, gently tapping over it may help the vein to dilate. Veins tend to constrict in the cold and dilate with warmth (e.g. from a hot towel). They also tend to dilate when hydrostatic pressure is increased (e.g. utilising gravity, by placing the limb in a dependent position).

Technique of venepuncture

The needle should be directed along the chosen vein at a reasonably flat plane of entry (10 -15 degrees) through the skin, bevel upwards. It is inserted to at least 5 mm. into the interior of the vein.

Fig.28.25 Venepuncture in the cubital fossa

For obtaining a venous blood sample, aspiration should be performed gently; otherwise the vein (being at low pressure) will tend to collapse. Failure of the procedure may occur because the needle has missed the vein (the needle can be withdrawn slightly and the procedure reattempted) or penetrated right through the vein (the needle can be slowly withdrawn while gently aspirating, until the vein is re-entered).

After the sample is obtained, the tourniquet is released and the needle withdrawn. Compression of the vein (through a swab) immediately after removal of the needle will minimise bleeding and bruising. Gentle pressure is usually required for only a short time.


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Hazards of a venepuncture

The vein itself (being thin walled) may be nicked or even lacerated, resulting in bleeding and a subsequent haematoma.

A neighbouring artery or nerve may also be damaged. In the cubital fossa the brachial artery, or a variant of it, is endangered. The median nerve is medial to the artery and the lateral cutaneous nerve of the forearm runs near the lateral border of the cubital fossa (on the brachioradialis muscle).

INTRAVENOUS CANNULATION

Intravenous cannulation is performed to infuse fluid, transfuse blood and administer drugs. Typically a peripheral vein is chosen, but a central vein (e.g. internal jugular or subclavian) should be utilised to monitor central venous pressure, infuse certain drugs or supply parental nutrition.

Sites for peripheral IV cannulation

It is far better to choose a superficial vein of the upper limb (to avoid risk of thrombosis in the lower limb). The chosen vein should be as distal as possible along the limb (which preferably should be the non-dominant upper limb). Indwelling cannulae should not overlie a joint (to avoid kinking by flexion) unless the joint is splinted.

The veins are distended by application of a tourniquet proximal to the selected site for entry. If a vein is not very prominent, gently tapping over it may help the vein to dilate (which is also helped by warmth and dependency). Ideal sites for cannulation of veins are at an inverted 'V' junction point or where a vein pierces deep fascia.

Veins tend to be more fixed at these sites, which helps anchor them so that they do not move away from the tip of the needle.

Technique of peripheral IV cannulation

Fig.28.26 Intravenous cannulation on dorsum of hand

The needle (with surrounding cannula) should be directed along the chosen vein (located in the

subcutaneous tissue) at a reasonably flat plane of entry (10-15 degrees) through the skin, bevel upwards. It is inserted up the vein far enough to ensure that the cannula is also within the vein (when blood begins passing up the needle, it is pushed a bit further up the vein). The cannula is then slid forwards off the inside needle until it is as far as possible within the vein, prior to withdrawing the needle and releasing the tourniquet.

Hazards of peripheral IV cannulation

With a cannulation, the vein itself (being thin walled) may be nicked or even lacerated, resulting in bleeding and a subsequent haematoma.

Infusion fluid may leak into the surrounding tissues. An additional hazard is thrombosis. This is more likely

with immobilisation, hence splints are not ideal.

Chapter 29: Postmortem Examination of Organs

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POSTMORTEM EFFECTS ON TISSUES

ORGANS IN-SITU AT AUTOPSY

EXCISED VISCERA AT AUTOPSY

POSTMORTEM EFFECTS ON TISSUES

The colour, texture and shape of organs at postmortem more closely represent their living state than in embalmed cadavers (due to the effects of embalming fluid). However, postmortem tissues are more friable and may be obscured by blood. They are also not disinfected and tend to deteriorate (particularly at room temperature).

During autopsy, organs are first examined in-situ. Viscera are also examined following excision and in cut section (demonstrating their external and their internal appearance, respectively).

ORGANS IN-SITU AT AUTOPSY

Skin and abdominal wall incised

After the body is placed in the supine position, a line of incision to expose contents of the thoracic and abdominal cavities is planned.

The skin and muscles covering the anterior thoracic wall and those of the anterior abdominal wall are incised and reflected. This reveals the sternum and rib cage (with intercostal muscles) in the thorax and opens the peritoneal cavity below the costal margin.

Within the abdomen, the liver and stomach are exposed, together with the fatty membrane (greater omentum) covering most of the intestines.

Fig.29.1 Thoracic wall and abdominal cavity exposed

Transverse colon lifted

The greater omentum is excised from the transverse colon to uncover the small intestine and ascending colon.

The transverse colon is lifted, revealing its mesentery and that of the small intestine (the mesentery).

Fig.29.2 Intestines with mesenteries revealed

Anterior thoracic wall removed

The ribs are cut laterally and the front of the rib cage removed, by cutting it from the diaphragm. This opens the pleural cavities and reveals the lungs. The pericardium (located between the pleural sacs) is also exposed.

Fig.29.3 Intestines with mesenteries removed

The transverse colon and most of the small intestine (jejunum and ileum) are excised along with their mesenteries. This reveals the duodenum and descending colon.

Lungs excised and pericardium opened

The lungs are excised. The pericardial sac is opened anteriorly and reflected downwards, exposing the pericardial cavity and serous pericardium (its parietal layer, which lines the interior of the fibrous pericardium). The surface of the heart is also revealed. It is partly covered by epicardial fat (particularly over the right atrium).

Within the pericardial sac, the pulmonary trunk and ascending aorta can be seen arising from the ventricles of the heart. The auricles (ear-shaped appendages) of the atria can also be seen.


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Fig.29.4 Heart within pericardial sac

Heart excised

The heart is removed by cutting all the great vessels at the reflections of the serous pericardium.

Fig.29.5 Parietal layer of serous pericardium

The parietal layer of the serous pericardium is reflected onto the heart (to become the visceral layer) via two connecting roots for the great vessels. One set of reflections is for the arteries (pulmonary trunk and ascending aorta) while the other set is for the veins (superior vena cava, inferior vena cava and the pulmonary veins). The right pulmonary veins are demonstrated while the left pulmonary veins have been removed.

Pericardium removed and diaphragm lifted

Fig.29.6 Mediastinum and abdominal viscera revealed

The pericardium is removed, revealing other structures located in the mediastinum. The mediastinum is the region of the thoracic cavity between the pleural sacs. The first two parts of the aorta (ascending and arch) are visible, with the third part (descending aorta) obscured by the oesophagus. Pulmonary veins are also seen passing horizontally.

The diaphragm is lifted, revealing the superior surface of the liver and the upper part of the stomach. The ascending colon has also been displaced upwards, to reveal the caecum and appendix.

Liver, stomach and spleen excised

Fig.29.7 Retroperitoneal unpaired viscera

All remaining contents of the thoracic cavity are removed, revealing the thoracic vertebral column. The liver (with the gall bladder), stomach and spleen are excised, revealing the duodenum and pancreas. The ascending colon, descending colon, duodenum and pancreas are retroperitoneal as their (dorsal) mesentery became absorbed during development.

Retroperitoneal unpaired viscera excised

Fig.29.8 Paired viscera on posterior abdominal wall

The diaphragm, ascending colon (with caecum and appendix), duodenum and pancreas are all excised. The paired viscera (kidneys, suprarenal glands and ureters) on the posterior abdominal wall are exposed, together with the inferior vena cava.

The left renal vein is seen passing across the abdominal aorta before draining into the inferior vena cava. The descending colon is displaced laterally and is seen crossing the pelvic brim to become the sigmoid (pelvic) colon. The peritoneal cavity of the pelvis is continuous with that of the abdomen.

29. Postmortem Examination of Organs

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EXCISED VISCERA AT AUTOPSY

Hollow viscera excised and in cut section

Fig.29.9 Excised hollow viscera

1. Oesophagus 2. Trachea and main bronchi 3. Heart 4. Stomach 5. Gallbladder 6. Duodenum 7. Jejunum and ileum 8. Caecum (with appendix) and abdominal colon 9. Bladder (with ureters) and deferent ducts 10. Vagina and uterus (with uterine tubes) 11. Sigmoid colon, rectum and anal canal

Fig.29.10 Excised hollow viscera in cut section

Solid viscera excised and in cut section

Fig.29.11 Excised solid viscera

1. Thyroid gland 2. Lungs 3. Liver 4. Spleen 5. Pancreas 6. Suprarenal glands 7. Kidneys 8. Ovaries 9. Testes (with epididymes) 10. Prostate gland (with seminal vesicles)

Fig.29.12 Excised solid viscera in cut section

Chapter 30: Cadaver Dissection

232

DISSECTION PREPARATION

SURGICAL INSTRUMENTS

SKIN INCISION

SKIN REFLECTION

SUBCUTANEOUS FAT REMOVAL

DEEP FASCIA INCISION AND REFLECTION

MUSCLE & FASCIAL PLANE SEPARATION

NEUROVASCULAR BUNDLE DISSECTION

EXPOSURE OF DEEP STRUCTURES

DISSECTION PREPARATION

Prior to dissection, (as in surgery on a living patient) the cadaver is carefully placed in position on the table for optimal access to the appropriate region. The body is uncovered and relevant anatomical landmarks identified.

Dissection involves exposing anatomical structures by separating them from surrounding tissue, which is then removed. Connective tissue (fascia) keeps some structures together and keeps others apart, but also tends to obscure them from view (loose connective tissue contains a variable amount of fat).

Fig.30.1 Cadaver supine on a dissecting table

Dissection utilises surgical instruments to manipulate tissues and is performed layer-by-layer (from superficial to deep) commencing with a skin incision.

Fixation by embalming fluid

Embalming fluid fixes tissues of the cadaver so that connective tissue becomes firmer and fat is solidified, in contrast to living or unembalmed tissue. Embalming provides the advantage of a bloodless field and less friable tissues (in addition to the benefits of disinfection and moisturisation). However, organs appear drained of colour. Viscera are particularly affected and hollow viscera tend to deflate.

Dissecting safely

As in surgery, all needles and blades are classified as sharps and must be handled with care. They should only be discarded in specially designed and labelled containers for safe disposal. Particular care is taken when changing scalpel blades (attaching a blade to the scalpel handle or detaching a blade from it). Artery forceps (instead of fingers) are used for grasping the blade, to minimise risk of injury.

Dissecting gloves are used to protect the hands. Adequate lighting is not only essential for visualising the structures encountered, but also for visualising instruments and any other potential hazards (e.g. sharp cut ends of bones).

Maintaining the cadaver

The cadaver can be maintained in good condition by covering when not in use, to prevent parts from drying up and hardening. Once dissected, spraying the region with a special wetting and disinfecting solution and wrapping it in plastic also helps to keep it moist.

SURGICAL INSTRUMENTS

Fig.30.2 Surgical instruments used for dissection

Scalpels

Scalpels are used to make incisions and to clean visible surfaces (e.g. muscles) by piecemeal removal of fat. They have a handle and a blade, each of which come in various sizes. The larger blades are more useful for incisions and removal of skin or of deep fascia, while the smaller blades are more useful for very fine dissection (e.g. clearing connective tissue from a small vessel).

Scalpel blades become blunt with use and need to be changed regularly (with great care). The scalpel handle is notched obliquely on one side; which abuts the oblique end of the blade and the two may only be correctly fitted on that side.

30. Cadaver Dissection

233

The scalpel should be held like a pen, with the index finger used to guide the blade for incisions. The little finger can rest on the cadaver to steady the hand for fine work.

Fig.30.3 Holding a scalpel correctly

Forceps

There are a variety of forceps. Plain (non-toothed) forceps are used for grasping tissues other than skin and may be used in fine dissection to separate tissues.

Fig.30.4 Holding tissue forceps correctly

Toothed forceps are used for grasping cadaver skin, (that has been fixed by embalming fluid). However, toothed forceps should not be used on living skin to avoid localised trauma.

Other special types of forceps are artery forceps, used for clamping vessels (and for changing scalpel blades). They have a ratchet, enabling the joints to be locked at various tensions. Bone forceps are strong and heavy, used for fragmenting and removing bone.

Plain and toothed tissue forceps should be held like a pen, but controlled between the thumb and middle finger for optimum ease and accuracy.

Scissors

A pair of scissors can be activated by inserting the distal phalanx of the thumb and of the ring finger into each loop. The scissors may then be guided by the index finger, which is positioned over the joint for fine control and steadied by the middle finger.

Fig.30.5 Holding tissue scissors correctly

Large scissors with round-tipped blades can be used to separate tissues by inserting their blades closed, then gradually opening them. Curved blades enable these scissors to be controlled from an angle (not in the plane of dissection) so that they do not obscure the field of view.

Small, pointed scissors should be used for very fine dissection or trimming surfaces and edges of soft tissue.

Retractors and probes

During deep dissection, retractors are used to expose the field of view by maintaining separation of overlying structures.

A ‘cat’s paw’ retractor is usually held by an assistant. Alternatively, a self-retaining retractor has two ‘paws’ that can be locked apart to maintain separation of structures, without need for an assistant.

A blunt probe is used to explore avenues obscured from view. A duct can be explored by inserting the probe through its orifice.

SKIN INCISION

A fresh, sharp blade is applied to the scalpel handle to begin dissection. Larger scalpels are generally chosen for skin incisions on an embalmed cadaver, due to the force required to penetrate the skin (which might snap a small scalpel blade).

An incision is made with the scalpel blade held perpendicular to the plane of the skin, contacting it along the blade’s edge (rather than just with the point).

Fig.30.6 Incising the skin (on leg)

The epidermis and dermis (approximately 2mm deep) are pierced. In ‘thick’ skin, on the palm of the hand or sole of the foot, the epidermis is greatly thickened and much tougher. These areas require a more forceful incision, so special care must be taken to avoid injury.

Once the blade enters the (fatty) subcutaneous tissue, a reduced resistance is felt.

SKIN REFLECTION

Skin may be reflected after two incisions meeting at a right angle have been made.

The skin at the junction of the incisions is gripped firmly (using toothed forceps) and traction applied to it, while the blade of the scalpel progressively frees dermis from underlying subcutaneous tissue. This is done using small strokes, rotating the blade so it cuts parallel to the plane of the subcutaneous tissue. Reflection along this plane avoids damage to structures in the subcutaneous tissue.


234

Skin flaps are reflected, rather than removed, so that they can be replaced following dissection to keep underlying tissue moist.

Fig.30.7 Reflecting skin from underlying tissues

SUBCUTANEOUS FAT REMOVAL

After the subcutaneous tissue is entered, dissection is continued within this plane to preserve the structures coursing in it. Plain forceps and a finer scalpel are used to free fatty tissue from them.

Fig.30.8 Detaching subcutaneous tissue

Cutaneous nerves and superficial veins receive their fine branches (or tributaries) from the skin. The main nerves and veins are preserved and may be followed until they pierce the deep fascia. The remaining fat is reflected and removed.

DEEP FASCIA INCISION AND REFLECTION

A larger scalpel is required to incise the thin but tough deep fascia.

Fig.30.9 Incising and reflecting deep fascia

Deep fascia is removed from the field of dissection and, in this case, incised along its attachment to bone (where it becomes continuous with the periosteum). In general, deep fascia is easily removed from underlying muscles except where it merges with intermuscular septa.

At some sites, muscle may be attached to deep fascia by its associated connective tissue (surrounding epimysium, aponeurotic expansions or tendinous prolongations).

MUSCLE & FASCIAL PLANE SEPARATION

Fig.30.10 Blunt dissection along a fascial plane

30. Cadaver Dissection

235

Individual muscles may be separated from each other by blunt dissection. This is the parting of structures without using a scalpel blade. It is best performed along fascial planes, particularly mobile fascial planes, where tissues are only loosely connected to one another.

Several instruments can be used for blunt dissection, including plain forceps, large round-tipped scissors (with blades closed), or even the tip of the finger.

NEUROVASCULAR BUNDLE DISSECTION

Fig.30.11 Opening scissor blades to separate structures

Neurovascular bundles tend to course along mobile fascial planes, in parallel with them. Branches tend to pass along fixed fascial planes (e.g. intermuscular septa). The fascial sheath of a neurovascular bundle is thinner around veins, enabling them to expand. This may be removed (usually using a fine scalpel and forceps) and structures within the bundle separated by inserting closed scissors, then gradually opening the blades.

Deep arteries in the periphery are surrounded by venae comitantes (pairs of veins with numerous intercommunicating branches). These in turn also communicate with superficial veins by communicating veins, which pierce the deep fascia.

EXPOSURE OF DEEP STRUCTURES

Major nerves and vessels tend to run deeply between muscles. A retractor may be required to help expose these structures.

Veins are much more numerous and variable than arteries and many smaller veins (together with their tributaries) may need to be removed for adequate exposure of an accompanying artery or nerve.

Fig.30.12 Exposing and displaying a deep structure

Locating nerves and vessels at dissection

During dissection, a nerve or vessel can be located by backtracking after finding the neurovascular hilum of their target organ. Even if nerves and vessels take separate paths or one has an anomalous course, they eventually converge near their destination.

Appendix 1: List of Principles

236

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The developmental history of an individual reflects the evolutionary history of its species.

The potentials (and limitations) of cells, tissues and organs are determined by the germ layers from which they are derived.

Only mesoderm derived structures are vascular

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When describing the relationship between one structure and another, the body is considered to be in the anatomical position.

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Branchial arch derivatives retain their nerve supply despite migration.

The nerve supply to a muscle is retained even if the muscle migrates during development.

Each limb develops with a principal bone proximally, a pair of long bones distal to it, then short bones and five digits.

The most distinctive human characteristic is the habitual adoption of upright stance and locomotion based solely on the two lower (hind) limbs.

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Bony trabeculae are oriented along lines of stress (both compressive and tensile).

Articular surfaces are the only external surfaces of a bone not surrounded by periosteum.

Bony elevations are produced at sites of traction

Hyaline cartilage is avascular and aneural

Unlike cartilage, bone requires a blood supply, as the calcified matrix does not allow diffusion.

Almost all secondary centres appear after birth (females generally at an earlier age than males).

Growth in length occurs at the metaphysial surface of an epiphysial plate.

Epiphyseal fusion occurs after puberty (females generally at an earlier age than males).

The earlier an epiphysis appears the later it fuses.

Epiphyses for larger long bones tend to appear before (and fuse after) those for smaller long bones.

Damage to an epiphysial plate will impair subsequent growth.

Adults tend to have stronger bones than ligaments, while children have the reverse.

Healing, including of fractures is more rapid in children than in adults.

Weight bearing bones heal slower than non-weight bearing bones.

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The shape of the articular surfaces determines the particular movements permitted.

Bony articular surfaces do not come in direct contact with each other unless the overlying articular cartilage has worn away.

Synovial membrane lines the internal surface of the capsule and all non-articular structures on the interior of a synovial joint.

Ligaments, within a joint or between two joints acting as a functional unit, are positioned along the axis of movement.

Collateral ligaments are important contributors to stability by preventing unwanted side-to-side movement.

Children are more likely to fracture a bone before tearing a ligament.

The weakest points of a ligament are at or near their attachments, rather than between them.

A ligament that is arranged in discrete parts rather than a continuous band allows more joint mobility but is weaker and therefore more vulnerable.

Discs or menisci create compartments, allowing different movements to occur simultaneously on each side of the partition.

Bursae tend to be more numerous at joints with greater mobility.

The contribution to joint stability from bones is dependent on the congruence of their articular surfaces.

Muscles are the most important stabilising factor for mobile joints, providing the first line of defence against dislocation.

Nerves supplying muscles that produce movements at a joint also typically supply the joint.

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Tendinous attachments to bone, in contrast to those of fleshy muscle fibres, produce bony markings.

A large tendon attaching to a developing bone is likely to be associated with a traction epiphysis (to allow for growth of the bone at the site of attachment).

In contrast to a ligament, a muscle tends to rupture at other sites in addition to its attachments.

Muscles crossing more than one joint are particularly prone to injury from over-stretching.

Fleshy muscle fibres tend to be replaced by tendons at sites of pressure or friction.

Deep fascia is not found as a continuous sheet around parts of the body that expand significantly.

Deep fascia is not found over the subcutaneous surface of a bone

Muscles with a common action are generally located in the same fascial compartment.

Where nerves and vessels have a common course they tend to be enclosed within a common fascial sheath (as a neurovascular bundle).

The active range of movement at a joint is proportional to the length of muscle belly.

Strength is proportional to the cross-sectional area of the muscle.

Muscles crossing more than one joint can generate extra force but are also prone to overstretch.

Prime movers tend to be located superficially and fixators deep.


237

Skeletal muscles with a common action often share a common nerve supply and occupy a common compartment.

A muscle located on the border between two compartments may receive a dual nerve supply (and have dual prime mover actions).

The nerve supply to a muscle reflects its developmental origin (nerves remain ‘faithful’ to their muscles).

The segmental pattern of nerve supply in the trunk is in a simple cranial to caudal sequence.

An individual limb muscle typically receives its supply from two consecutive spinal cord segments.

Proximal flexor muscle groups are supplied from more cranial (pairs of) segments than those for distal flexor muscles.

The most caudal segment distributed via the limb plexus supplies the most distal muscle group for the upper limb and for the lower limb (intrinsic muscles of palm and of sole, respectively).

Where there is a major source artery (and principal vein) it enters as part of the neurovascular bundle at the hilum, on the deep surface of the muscle.

The majority of anastomoses in the body are via skeletal muscles.

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The dermis on extensor surfaces tends to be thicker and tougher increasing protection from injury.

Connective tissue in living skin is oriented along the relaxed skin tension lines.


Territories supplied by peripheral nerves derived from consecutive spinal segments overlap extensively (and their branches intermingle).

Overlap for pain and temperature is more extensive than that for touch.

Nerve branches do not cross the midline of the body.

Adjacent dermatomes that are consecutive overlap extensively.

The middle segment of a limb plexus is distributed to the most distal skin.

Adjacent dermatomes that are not consecutive do not overlap.

Cutaneous nerve branches do not cross axial lines.

Pain from a deep source is referred to the same neurosome.


Pain from an unpaired viscus is referred to the midline.

Pain from a paired viscus is referred to the same side.

Vessels, being derived from mesoderm, develop only in mesoderm-derived tissues.

Continuous arteries supply continuous organs.

Arteries travel with connective tissue via fascial planes particularly associated with muscles.

Vessels do not cross mobile planes.

Vessels cross planes at sites (of least mobility) where connective tissue is anchored.

Arteries course from fixed (concave) areas to mobile (convex) areas.

Veins converge on fixed areas from mobile areas.

The vast majority of muscles are part of more than one angiosome.

Lymph capillaries are not present in epithelia (including epidermis) but are abundant directly under an epithelial surface.

Lymph vessels tend to accompany veins.

Lymph normally passes through at least one set of lymph nodes before reaching the venous system.

The skin of almost the entire body drains first to a superficial lymph node group before draining to a deep group.

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Normal constrictions of the lumen tend to occur at the beginning and end of a tubular viscus.

Structures directly related to an organ tend to produce grooves or impressions on it.

A duct opening into the lumen of a hollow viscus tends to narrow as it traverses the wall.

Endocrine glands have a very rich blood supply.

A paired viscus receives a unilateral neurovascular supply and refers pain to the same side.

Midline unpaired viscera receive nerve and vascular supply lines from both sides

Non-midline unpaired viscera have an arterial supply from unpaired branches of the aorta (arteries of the foregut, midgut and hindgut) and venous drainage into an unpaired system of veins (‘portal’ system).


Pain from an unpaired viscus is felt over the midline of the body as impulses are simultaneously received by the left and by the right side of the spinal cord.

Sphincters are often located near an external orifice (particularly on the perineum).

The direction of the orifice is at right angles to the direction of apposition of the walls of the tubular viscus (or duct) immediately proximal to it.

The epithelial lining of viscera is avascular (as is the epithelium of skin).

The underlying connective tissue of the lamina propria is highly vascular (as is the dermis of skin).

Arterial anastomoses, venous communications, watershed areas of lymph drainage and inter-nervous lines (of sensory nerve supply) occur at mucocutaneous junctions.

Visceral nerves supply smooth muscle sphincters, and somatic nerves supply skeletal muscle sphincters.

Transmucosal junctions tend to be located where territories of different developmental origin meet.

Inter-nervous lines for reflexes particularly occur where mucosa overlies skeletal muscle.

There tends to be no arterial anastomosis across vascular segments although there may be some venous communication.


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Visceral nerves supply smooth muscle and glands, while somatic nerves supply skeletal muscle.

The body wall and the (parietal) layer of serous membrane lining it are supplied by somatic nerves, while the gut and the (visceral) layer of serous membrane around it is supplied by visceral nerves.

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Although some peripheral nerves are purely motor or purely sensory, the vast majority are mixed.

In contrast to a receptor, an effector is not in direct continuity with a neuron.

The functional fibre type of a sensory nerve fibre corresponds to the type of organ associated with the receptor.

The functional fibre type of a motor nerve fibre corresponds to the type of effector.

Sympathetics primarily control smooth muscle tone of arterioles.

Most neural pathways in the CNS cross the midline.

Posterior nerve roots are purely sensory while anterior nerve roots are purely motor.

Each branchial arch is supplied by a mixed cranial nerve.

A ganglion, created by the collection of cell bodies of sensory neurons, is found on the posterior root of every spinal nerve.

Each posterior root ganglion resides in an intervertebral foramen, regardless of the length of the associated nerve root.

The sensory ganglia of cranial nerves are located in or near the associated foramina in the skull.

Each spinal nerve from T1-L2 is connected to the sympathetic trunk by a white rami communicans.

Every spinal nerve is connected to a sympathetic trunk by a grey ramus communicans.

Only anterior rami of spinal nerves take part in the formation of plexuses.

Peripheral nerves derived from anterior divisions of a plexus are distributed to flexor compartments while those derived from posterior divisions are distributed to extensor compartments.

A nerve which supplies a muscle producing movement at a joint also supplies sensation to the joint and skin overlying (the insertion of) the muscle.

The CNS receives blood supply from its periphery.

There are no lymph vessels in the CNS.

Large nerve fibres within a peripheral nerve are the most susceptible to pressure.

A neuron influences the vitality of its connections.

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The greatest drop in blood pressure occurs across arterioles.

Where arteries divide into terminal branches, the larger branch tends to be more directly in line with the main trunk, with the smaller at a greater angle.

The cardiovascular system is not only a closed system but also a double system with two distinct blood circulations.

Systemic arteries transport oxygenated blood.

Adjacent (branches of) arteries tend to anastomose with each other.

Skeletal muscles receive the most arterial branches and contain the majority of anastomoses.

Anastomoses occur around joints but are only significant within muscle bellies that cross the joint.

End organs are particularly vulnerable to having their arterial supply cut off.

End tissues within end organs are most vulnerable to having their arterial supply interrupted.

An embolus within an artery tends to lodge immediately distal to a branch point, where the main artery narrows.

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A portal system links two capillary beds at low pressure.

A valve is typically located at the termination of a vein.

The veins of the vena caval systems traversing body cavities of the trunk, together with the entire vertebral and azygos systems of veins, are valveless.

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Lymph capillaries are present only in tissues derived from mesoderm.

The termination of lymph ducts occurs where the venous pressure is about zero, whether upright or supine.

Lymph drains from superficial nodes to deep nodes.

After puberty, the thymus in particular (together with lymphoid tissue in general) involutes with age.

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Regional anatomy is concerned with the situational (extrinsic) properties of an organ – its position and relations.

The first step in a clinical diagnosis is to determine the (anatomical) site of a lesion.

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The branching patterns of vessels tend to be asymmetrical resembling the branching of a tree.

Flexor muscles with a richer nerve supply (for fine control of movements) tend to occupy compartments on the ventral aspect of the body and are covered by delicate skin with a correspondingly richer nerve supply (for fine sensory discrimination).

Course antigravity extensor muscles tend to occupy compartments on the dorsal aspect covered by hairier skin with tougher dermis.

Posterior rami of spinal nerves directly supply the dorsal aspect of the trunk (and also of the neck) with their associated extensor regions containing skin, joints and (deeply located) intrinsic muscles.

A limb plexus divides into anterior and posterior divisions, with their nerve fibres distributed (via associated peripheral nerves) to flexor regions and extensor regions, respectively.

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Compartments tend to be in layers.


239

While major vessels and nerves may course along them, few cross mobile fascial planes as they would overstretch or have their own mobility restricted.

Vessels tend to cross planes at sites of fusion, where connective tissue is anchored.

Vessels and nerves course from fixed to mobile areas.

Fluids (including blood and pus) tend to track along mobile fascial planes as they provide paths of least resistance.

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Within a neurovascular bundle, the vein and lymph vessels are located more peripherally.

The major limb arteries tend to run through flexor regions and are generally located on the flexor aspect of joints.

The nerve supply to a structure remains constant even if the structure has migrated.


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Anomalies found on physical examination or by imaging may be of clinical significance per se or when misdiagnosed as being pathological.

Encountering anomalies, particularly when not anticipated, can pose problems during invasive procedures or surgical operations.

It is vital for a clinician to distinguish typical from atypical, normal from abnormal, and health from disease.

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During the early embryonic phase, features appear from more primitive ancestors.

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The lymphoid organs are the first organs to involute.

The part of the skeleton that best distinguishes males from females is the bony pelvis.

The most mobile viscera are those suspended by a mesentery.

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Multiple branches arising close to each other can have a common stem.

Variations in venous patterns are extremely common as veins develop from numerous endothelial channels.

An arterial trunk arsing from a main artery and subsequently dividing can be absent, with its branches arising independently.

A large anastomosing branch of a neighbouring artery may replace an artery and take over its territory.

Abnormal fusion of vertebral elements tends to occur at transitional regions

Accessory bones are created by failure of a centre of ossification to fuse with the rest of the bone.

Anomalies of bony fusion and non-fusion may create a domino effect along the spine.

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During development migration may occasionally fall short of the normal site.

During development migration may occasionally overshoot the normal site or deviate to an abnormal site.

Abnormal communications may occur from endothelial channels failing to close during development.

Vessels develop from networks that have the potential for change, where preferred channels remain while others regress (providing scope for variation).

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In contrast to anatomical variation (with abnormal structure or position but no functional impairment) pathological changes have impaired function, even if not immediately evident.

Malformations occur when organ systems are forming (between the third to eighth weeks) and most major malformations spontaneously abort.

Multiple minor malformations generally signify an underlying major malformation.

Understanding of normal and abnormal anatomy is the basis for recognising clinical manifestations of disease processes.

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The viscera that are most mobile are those suspended by a mesentery.

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The intensity of blackness on a radiograph is directly proportional to the intensity of radiation which reaches the film.

The greater the tissue radiodensity, the greater the attenuation of X-rays.

A radiological interface is created when tissues of different radiodensity lie adjacent to each other.

Lines (or edges) may be seen on a radiograph when radiological interfaces are parallel to the path of the X-rays.

An object is usually radiographed in at least two projections at right angles to each other.

Structures of most interest should be placed centrally within the X-ray beam.

The X-ray film should be placed perpendicular to the centre of the X-ray beam.

The organ or body part of most interest is positioned as close as possible to the recording medium to minimise magnification and loss of sharpness.

Compact bone (densely packed bone tissue infiltrated with calcium) appears more opaque than cancellous bone (containing many little compartments).

Only fat has sufficient radiographic contrast compared to all other types of soft tissues (and body fluids) to form visible interfaces on a plain film.


240

When an organ or a tissue of soft tissue density is adjacent to air or gas, the difference in radiodensity will form a clean and sharp edge provided the interface is parallel to the x-ray beam.

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Radiographs display the entire body part or an organ that is imaged, whereas CT images display slices of body parts or organs.

MRI (unlike radiography and CT) avoids using ionising radiation.

Implanted electronic devices and potentially mobile ferromagnetic material are contraindications to MRI.

On T1 weighted images tissues with a high fat content appear bright.

On T2 weighted images tissues with high water content appear bright.

The most important advantage of MR over other imaging modalities is the ability to distinguish types of soft tissues from each other.

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Ultrasound allows real time cross-sectional imaging without any ionizing radiation.

An acoustic interface exists at the junction of two tissues of different acoustic impedance.

The larger the difference in density of adjacent tissues, the larger the reflection, resulting in a brighter signal from their acoustic interface.

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Skin incisions made parallel to lines of tension heal with a minimal scar, while those crossing lines of tension tend to produce a wider scar.

Incisions should ideally be placed along prominent skin creases (particularly in the trunk, neck and face) to disguise the scar.

Incisions crossing joint lines should be avoided due to subsequent restriction of movement even from normal scar contraction.

Incisions should be planned with an awareness of underlying structures (particularly nerves and vessels) and special care must be taken to avoid damaging them.

Wounds should be closed layer by layer to prevent dead space and maximise wound strength

Aspirating before injecting avoids inadvertent intravenous injection

Within a peripheral nerve, small fibres (mainly pain fibres) are most affected by local anaesthetic agents.

Larger fibres are affected to a lesser degree (hence touch sensation may remain).

The area anaesthetized by a nerve block corresponds to the sensory distribution of the nerve (distal to the site of infiltration) minus the area of overlap from adjacent nerves.

Adrenaline must never be injected into terminal parts (particularly digits or penis) because they are (collectively) supplied by end-arteries.

Ideal sites for cannulation of veins are at an inverted 'V' junction point or where a vein pierces deep fascia.

Appendix 2: List of Applications

241

SSSeeeccctttiiiooonnn 111::: TTThhheee HHHuuummmaaannn BBBooodddyyy

CCChhhaaapppttteeerrr 222::: HHHuuummmaaannn FFFooorrrmmm aaannnddd SSStttrrruuuccctttuuurrreee

Site of most stress on spine

Risk of choking and protective reflexes

SSSeeeccctttiiiooonnn 222::: BBBooodddyyy SSSyyysssttteeemmmsss aaannnddd SSStttrrruuuccctttuuurrreee

CCChhhaaapppttteeerrr 444::: SSSkkkeeellleeetttaaalll SSSyyysssttteeemmm aaannnddd BBBooonnneeesss

Marrow reversion after blood loss

Mistaking bones for fracture fragments

Mistaking epiphysial plates for fracture lines

Determination of skeletal age

Importance of imaging bones bilaterally

Epiphysial judgement

Epiphysial damage

Perichondrial stripping

Periosteal stripping

Interruption of blood supply to bone

Fractures

Fracture healing

CCChhhaaapppttteeerrr 555 AAArrrttt iiicccuuulllaaarrr SSSyyysssttteeemmm aaannnddd JJJoooiiinnntttsss

Joint degeneration

Osteophyte formation

Articular cartilage damage

Synovial effusion

Haemarthrosis

Septic Arthritis

Loose body

Grades of ligament injury

Tears and avulsion at ligament attachments

Ligament vulnerability


Masking of ligament tear by muscle spasm

Masking of pain by nerve fibre rupture

Laxity and loss of proprioception

Labrum or meniscal tears

Bursitis

Joint cavity communication

Pinched fat pad

Assessment of joint mobility

Joint dislocation and subluxation

Pain from degenerative arthritis

Sensory effects of ligamentous injury

Effects of injury on vascular joint tissues

Effects of capsular or ligamentous injury

Effects of articular cartilage injury

CCChhhaaapppttteeerrr 666::: MMMuuussscccuuulllaaarrr SSSyyysssttteeemmm aaannnddd MMMuuussscccllleeesss

Grades of muscle injury

Sites of muscle tears

Muscles prone to strain

Tenosynovitis

Infection of a synovial sheath

Effect of mesotendon injury


Active insufficiency

Passive insufficiency



Muscle injuries and healing

CCChhhaaapppttteeerrr 777::: IIInnnttteeeggguuummmeeennntttaaalll SSSyyysssttteeemmm aaannnddd SSSkkkiiinnn

Direction of skin incisions and scarring


Regeneration of skin after burns

Effect of nail bed damage

Subungual haematoma

Fingerprinting

Skin surgery

Area of anaesthesia in a nerve block


Shingles dermatomal distribution

Effects of lacerating dermal vessels

Planning grafts based on angiosomes

Lymphangitis

Lymph spread from watershed areas

CCChhhaaapppttteeerrr 888::: VVViiisssccceeerrraaalll SSSyyysssttteeemmmsss aaannnddd VVViiisssccceeerrraaa

Obstruction of a tubular viscus

Types of duct obstruction

Torsion of a viscus

Surgical removal of a segment

Strangulation of a viscus

CCChhhaaapppttteeerrr 999::: NNNeeerrrvvvooouuusss SSSyyysssttteeemmm aaannnddd NNNeeerrrvvveeesss

Features of a segmental nerve lesion


Pre-fixed and post-fixed plexus variants

Features of a peripheral nerve lesion

Reflex muscle spasm

Barrier to spread of brain tumours


242

Types of nerve injuries

Grades of nerve injury

Axonal degeneration

Axonal regeneration

Pain from meninges and dural sleeves

Neuralgia and phantom pain

CCChhhaaapppttteeerrr 111000::: AAArrrttteeerrr iiiaaalll SSSyyysssttteeemmm aaannnddd AAArrrttteeerrr iiieeesss



Arteriosclerosis

Atherosclerosis and arterial aneurysm

Haemorrhage

First aid management of haemorrhage

Effect of central retinal artery occlusion

Effect of sudden coronary artery occlusion

Vulnerability to vasoconstrictors

Danger of ligating a segmental artery

Types of arterial occlusion

Effects of anatomical end artery occlusion

Effects of functional end artery occlusion

Arterial occlusion to vital areas

Inadvertent ligation or injection

Thrombosis and embolism

Pulmonary embolus

Systemic arterial embolus

CCChhhaaapppttteeerrr 111111::: VVVeeennnooouuusss SSSyyysssttteeemmm aaannnddd VVVeeeiiinnnsss

Managing venous bleeding in surgery

Varicose veins and haemorrhoids

Venous valve incompetence

Deep vein thrombosis in the calf

Alternative routes of venous return

Venous spread of tumours and infections

Venous congestion and oedema

CCChhhaaapppttteeerrr 111222::: LLLyyymmmppphhhaaattt iiiccc SSSyyysssttteeemmm aaannnddd LLLyyymmmppphhh VVVeeesssssseeelllsss

Lymph vessel ligation

Effect of thoracic duct laceration

Lymphatic spread

First aid for venomous bites

Lymphoedema

Sentinel nodes in tumour spread


Accessory spleens and splenectomy effect

SSSeeeccctttiiiooonnn 333::: BBBooodddyyy RRReeegggiiiooonnnsss aaannnddd OOOrrrgggaaannn PPPooosssiiitttiiiooonnn

CCChhhaaapppttteeerrr 111555::: BBBooodddyyy CCCooommmpppaaarrrtttmmmeeennntttsss aaannnddd FFFaaasssccciiiaaalll PPPlllaaannneeesss

Compartment syndrome

Potential paths of tracking and direct spread

CCChhhaaapppttteeerrr 111666::: BBBooodddyyy WWWaaalll lllsss aaannnddd CCCaaavvviii ttt iiieeesss

Hernia

Drainage of accumulations in a body cavity

Prolapse

CCChhhaaapppttteeerrr 111777::: NNNeeeuuurrrooovvvaaassscccuuulllaaarrr PPPaaattthhhwwwaaayyysss

Detecting arterial pulsation

Predicting vascular endangerment

Predicting nerve endangerment

SSSeeeccctttiiiooonnn 444::: HHHuuummmaaannn DDDeeevvveeelllooopppmmmeeennnttt &&& VVVaaarrriiiaaatttiiiooonnn

CCChhhaaapppttteeerrr 111888::: GGGrrrooowwwttthhh aaannnddd DDDeeevvveeelllooopppmmmeeennnttt

Calculating fluid loss from burns in neonates

Forensic determination of age

CCChhhaaapppttteeerrr 111999::: NNNooorrrmmmaaalll VVVaaarrr iiiaaattt iiiooonnn

Forensic determination of sex

Obstetric assessment of pelvic dimensions

Vulnerability to fractures from a fall

Fat distribution and cardiovascular risk

Body Mass Index

Palpating abdominal organs on inspiration

SSSeeeccctttiiiooonnn 555::: PPPrrraaaccctttiiicccaaalll PPPeeerrrssspppeeeccctttiiivvveeesss

CCChhhaaapppttteeerrr 222333::: SSSuuurrrfffaaaccceee aaannnddd FFFuuunnncccttt iiiooonnnaaalll AAAnnnaaatttooommmyyy


Sites where arteries are palpable




Examination of major lymph node groups



Reflex muscle spasm

Nerve fibre rupture





CCChhhaaapppttteeerrr 222444::: RRRaaadddiiiooogggrrraaappphhhiiiccc AAAnnnaaatttooommmyyy aaannnddd IIImmmaaagggiiinnnggg

Steps in radiograph production

Assessing bony integrity

Assessing radiological joint space width


243

Assessing joint congruence and alignment

Assessing soft tissue calcification

Interventional radiology

CCChhhaaapppttteeerrr 222555::: SSSeeecccttt iiiooonnnaaalll AAAnnnaaatttooommmyyy,,, CCCTTT aaannnddd MMMRRRIII

Distinguishing soft tissues on MR images

CCChhhaaapppttteeerrr 222888::: CCClll iiinnniiicccaaalll PPPrrroooccceeeddduuurrreeesss

Sites where incisions should be avoided

Structures endangered by incisions

Lacerations and their management

Hazards of a joint puncture

Hazards of a body cavity tap

Preventing inadvertent IV injection

Structures endangered by IM injections,

Hazards of nerve blocks

Assessment of collateral circulation

Hazards of an arterial puncture

Preventing inadvertent intra-arterial injection

Hazards of a venepuncture

Hazards of peripheral IV cannulation

Appendix 3: List of Terms

244

SSSeeeccctttiiiooonnn 111 Ectoderm Mesoderm Endoderm

CCChhhaaapppttteeerrr 111

Anatomical Position Sagittal Coronal Transverse Anterior Posterior Superior Inferior Medial Lateral Proximal Distal Superficial Deep External Internal Ventral Dorsal Palmar Plantar Cranial Caudal Rostral Occipital Bilateral Midline Unilateral Ipsilateral Contralateral Flexion Extension Abduction Adduction Medial Internal Lateral External Lateral flexion Pronation Supination Plantar flexion Dorsiflexion Inversion Eversion Protraction Retraction Elevation Depression


Animal Coelomate Chordate Animal Coelom Gut tube Chordates Neural groove Notochord Pharyngeal pouches Branchial clefts Segmentation Polarity

Myomeres Metamerism Somites Sclerotome Dermamyotome Branchiomerism Branchial arches Branchial muscles Polarity Buccopharyngeal membrane Cloacal membrane Pre-axial border Post-axial border Dermatomes Myotomes Welcoming Position Vertebrate Skeleton Spinal cord Spinal nerves Quadrupeds Mammals Appendages Mammary glands Uterus Umbilical cord Placenta Pulmonary Systemic Forebrain Jawbone Teeth Ossicles Primates Brachiators Thermoregulation Hominid Homo sapiens Line of gravity Bipedal locomotion Gluteus Maximus Larynx Vocal cords Soft palate Nasopharynx Oesophagus Pharynx Oropharynx


Internal Genital Organs External Genital Organs

SSSeeeccctttiiiooonnn 222


Viscera Cells


Extracellular matrix Osteoblasts Osteoclasts Compressive Tensile Calcium Collagen Hyaline Fibro Elastic

Chondrocytes Diaphysis Accessory


Joint Cavity Suture Syndesmosis Gomphosis Synostosis Primary cartilaginous joints Secondary cartilaginous joints Symphyses Plane Uni Axial Hinge Pivot Bi-Axial Condylar Ellipsoid Saddle Multi-Axial Ball and Socket Simple Compound Complex Fibrocartilage Disc Menisci Articular Ovoid Sellar Pit Fossa Notch Fat Pad Labrum Discs Menisci Osteophytes Mobility Fusion Loose body Fibrous capsule Intracapsular Bursa Annular ligament Synovial cavity Synovial membrane Synovial cavity Synovial fluid Hyaluronic Acid Synovial effusion Haemarthrosis Haemophiliac Septic arthritis Loose body’ Locking Ligaments Elastic Ligaments’ Ligamenta Flava Intrinsic Ligaments Extrinsic Ligaments Cruciate Ligaments Collateral Ligaments Accessory Ligaments Grade I Grade II


245

Grade III Avulsed Stressing Laxity Proprioception Special structures Labrum Disc Menisci Labrum Loose body Intracapsular tendon Bursa Bursitis Septic arthritis Fat Pads Mobility Stability Passive assistance Roll Slide Spin Bony Ligamentous Muscular Dynamic ligaments’ Stretch reflexes Close packed Loose packed Dislocation Subluxation Vascular Circle


Skeletal Muscle Striated Somatic Non-Striated Autonomic Cardiac Muscle Collective unit Endomysium Perimysium Epimysium Fleshy Tendinous Roughening Line Crest Tubercle Origin Insertion Biceps Triceps Adductor magnus Muscle belly Tendon Musculotendinous junction Fusiform Digastric Tendinous intersections Flat Circular Palmaris longus Plantaris Vestigial Regressive Atavistic Avulsed Gastrocnemius

Tendons Aponeurosis Raphe Deep Superficial Retinaculum Septa Sheets Sheaths Fascial Intermuscular Interosseous membrane Fibrous tendon sheaths Fibro-osseous tunnels Synovial sheaths Tenosynovitis Tendinitis Tenovaginitis Power Fulcrum Load Line of pull Parallel Obliquely Pennate Uni Bi Multi Isotonic Eccentric Isometric Flexor Extensor Agonist Antagonist Fixator Dynamic ligament Synergists Peripheral nerve Motor unit Proprioceptive Hypertrophy Disuse atrophy’ Denervation atrophy Myotome Pedicles


Striae Skin cleavage lines Mucocutaneous junctions Alignment Tension Disfigurement Indirect Direct Inter-nervous line Midsagittal Welcoming Position Dermatome maps Herpes zoster Shingles Vesicles Unpaired Paired Mobile Fixed Lymphangitis Lymphotome


Hollow Solid Serosa Muscularis Mucosa Orifices Folds Thickenings Visceral obstruction Impaired Passage Distension Pain Constipation Abdominal distension Pain Bowel sounds Duct Exocrine Orifice Endocrine Hormones Midline Non-midline Sac Invaginate Mesenteries Posterior Peritoneal Subperitoneal Mobility Fixation Suspended on a mesentery Motility Sphincter Distal Reservoir Functional sphincter Folds Junction zones Fusion Mucocutaneous Junctions Mucocutaneous junctions Strangulation


Axon Supporting cells Axoplasm Neurons Schwann cell Neurilemma Nerve Synapse Synaptic cleft Sensory Motor Receptor Effector Reflex Negative feedback’ Somatic Visceral Neural crest Neuroglia Cortex Tracts Dermatome


246

Myotome Plexuses Branchial arches Gill clefts Ganglion Parasympathetic ganglia White rami communicans Grey ramus communicans Splanchnic nerves Thoracic pain line Pelvic pain line Plexus Coronal morphological plane’ Peripheral Segmental Muscular Cutaneous Articular Vasomotor Muscular Articular Cutaneous Joint Dislocation Angiosome Choke vessels Blood brain barrier Laceration Traction Compression Meningitis Neurogenic pain Neuralgia Herpes zoster Shingles Varicella zoster Phantom pain Phantom limb


Rete mirabile Pulmonary Systemic Haemorrhage RICE Rest Ice Compression Elevation Arteriovenous End artery Anatomical end artery Hemiplegia Cardiac arrhythmia Vasoconstriction Thrombus Embolus Thromboembolus


Communicating veins Varicose vein Emissary veins Thrombosis Thrombus Thromboembolus Tumour metastases Septicaemia Prostate cancer Septic thrombosis

Peripheral oedema Pulmonary Oedema Ascites Portal hypertension Oesophageal varices


Venom Lymphoedema Antigens Cervical Axillary Inguinal Accessory spleens



Region Position Relations Module


Paired Unpaired Bony Soft tissue Apertures


Unpaired Ventral Dorsal Thoracic Abdomino-pelvic Cranial Vertebral Paired Bilateral symmetry Rotate Boundaries Apertures Compartments Boundaries


Prime movers Fixators Compartment syndrome Laminectomy Fibrous septa


Body wall Parietal Hernia Compression Obstruction Strangulation Serous sac Mesothelium Parietal Visceral Mobility Motility Prolapse


Neurovascular bundle Axial artery

Neurovascular hilum Boundaries Apertures Direct relations External haemorrhage Internal haemorrhage Lacerations Fracture Dislocation Entrapment External compression



Normal variations Atypical Anatomical variations Abnormal Normal Function Anomaly Partial Complete Single Multiple Unilateral Bilateral Reciprocal Compensatory Pathological changes Impaired function Congenital Acquired


Growth Development Prenatal Embryonic Foetal Zygote Morula Blastocyst Embryoblast Trophoblast Bilaminar germ disc Ectoderm Endoderm Trilaminar germ disc Mesoderm Organogenesis Longitudinally Transversely Neural tube Somites Crown-rump length Amniotic fluid Gubernaculum Crown-heel length Oxygenated Deoxygenated Ductus venosus Ductus arteriosus Foramen ovale Ligamentum venosum Ligamentum arteriosum Ligamentum teres Medial umbilical ligaments Neonate


247

Pre-term Premature Fontanelles Sutures Primary curvatures Infancy Primary dentition Secondary curvatures Childhood Early Late Secondary dentition Adolescence Adulthood Adolescent Growth spurt


Normal Variation Maturity Involution Old Age Menopause Atrophy Postmenopausal Osteoporosis Hypertrophy Spermatogenesis Andropause Senescence Osteoporosis Gingivitis Arteriosclerosis Genetic Hormonal Environmental Heavy Medium Light Adipose Somatotyping Mesomorphs Endomorphs Ectomorphs Obesity Body Mass Index Functional differences Posture Respiration Pregnancy


Mobile Expansile Exercise Contents Activation Vestigial Atavistic Cranial Caudal Supernumerary Accessory


Excessive mobility Direction Aberrant Aberrant Accessory

Dysphagia Dysphagia lusoria


Congenital malformations Anatomical variation Pathological changes Malformation syndrome Down's Syndrome Trauma Ulceration Laceration Contusion Strain Sprain Fracture-dislocation First Degree Second Degree Third Degree Inflammation Physical Chemical Organismal Autoimmune Acute Inflammation Resolution Spread Suppuration Fibrosis Chronic Inflammation Abscess Scar Degeneration Cell Damage Necrosis Regeneration Calcification Lysis Gangrene Apoptosis Infiltrations Congestion Oedema Haemorrhage Shock Haematoma Aneurysm Thrombus Embolus Ischaemia Infarction Compression Collapse Obstruction Dilatation Hernia Prolapse



Surface Anatomy Functional Anatomy Radiographic Anatomy Plain Radiography Contrast Studies Sectional Anatomy Computed Tomography Magnetic Resonance Imaging Ultrasound Imaging

Endoscopic Anatomy Endoscopy Procedures Autopsy Postmortem Dissection Cadaver Predissected Wet Specimens Plastinated Specimens Bones Forensic Osteology Odontology


Stressing Passive assistance Hypertrophy Disuse atrophy Denervation atrophy Somatic Visceral Gluteus Maximus


X-Rays X-Ray tube X-Ray film Radiograph X-Ray image Digital Radiography Image Intensifier tubes Attenuation Tissue radiodensity Radiolucent Radio opaque ‘End-on’ effect Views Projections Anteroposterior (A-P) Posteroanterior (P-A) Standard Oblique Penetration Sharpness Geometric Unsharpness Motion Unsharpness Spatial Resolution Image Noise Contrast Resolution Magnification Distortion Superimposition Summation Trabeculae Subchondral Bone Compact Bony Tables Diploe Primary centres Secondary centres Epiphysial line Bony articular surfaces Radiological joint space Congruence Alignment Mammography Contrast radiograph Contrast material Positive Negative Direct


248

Indirect Pharyngogram Barium Swallow Barium Meal Small bowel series Single contrast Double contrast Barium Enema Oral Cholecystography Endoscopic Retrograde Cholangiopancreatography

Intravenous Urography Retrograde Pyelography Cystography Hysterosalpingography Myelography Contrast Arthrography Peritoneography Angiography Arteriography Arterial Capillary Venous Venography Lymphography Digital Subtraction Angiography


Subtracted Image Computed Tomography Windowing Narrow Window Wide Window Spatial Resolution Contrast Resolution High-Resolution Computed Tomography

Multislice CT Helical CT Volume Scanning Magnetic Resonance Imaging MRCP


Ultrasonography Ultrasound Transducer Reflection Absorption Scatter Acoustic Impedance Echogenicity Echotexture Probes Doppler effect Colour Doppler Duplex Scanning Pulsed Doppler


Incision Relaxed skin tension lines Keloid Incisional hernia Wound Incision Laceration Interrupted Continuous Subcuticular

Glue Clips Strips Anatomical Landmarks Lacerations Debrided Synovial Cavity Synovial Cavity Puncture Body Cavity Body CavityPuncture Intradermal Subcutaneous Intramuscular Nerve block Intercostal nerve block Digital nerve block Arterial puncture Radial artery Brachial artery Femoral artery Venepuncture Tourniquet

Appendix 4: List of Derivations

249

1 Ectoderm G. ‘outside skin’ Mesoderm G. ‘middle skin’ Endoderm G. ‘inside skin’ L. ‘area’ L. 'wall' L. ‘rupture’ L. ‘serum’ a watery fluid L. ‘thin skin’ L. ‘falling’ G. ‘not’ + ‘type’ L. ‘away’ + ‘rule’ G. ‘irregular’ G. ‘in’ + ‘grow’ G. ‘yolk’ L. ‘mulberry’ G. ‘germ’ + ‘bladder’ G. ‘nutrition’ + ‘germ’ L. ‘rudder’ L. ‘new’ + ‘birth’ G. ‘month’ + ‘beginning’) L. ‘grown up’ L. ‘to wrap up’ G. ‘month’ + ‘pause’ G. ‘man’ + ‘pause’ L. ‘growing old’ G. ‘bone’ + ‘porous’ L. ‘fatty’ G. ‘body’ + ‘form’ L. ‘ancestor’ L. ‘above’ + ‘number’ L. ‘straying’ G. ‘bad’ + ‘eat’ L. ‘a sport of nature’ L. ‘with’ + ‘born’’ G. 'running together' L. 'to go away' 2 L. ‘arrow’ L. ‘crown’ L. ‘nearest’ L. ‘distant’ L. ‘belly’ L. ‘back’ G. ‘skull’ L. ‘tail’ L. ‘beak’ L. ‘begin’ as in born first L. ‘bend’ L. ‘stretch’) L. ‘face down’ L. ‘back down’ 'breath' G: ‘hollow’ G ‘outside skin’ ‘middle skin’ ‘inside skin’ G. 'cord' (G. ‘back’ + ‘cord’ G. 'throat' G. 'gill' G. 'muscle parts' G. ‘body’ G. ‘hard’ + ‘cut’ G. ‘skin’ + ‘muscle cut’ L. 'sewer' G: 'skin' + cuts' G: 'muscle' + 'cuts'

L. 'jointed') L. 'dried up' L. ‘four’ + “footed’ L.'breast' L. ‘womb’ L. ‘navel’ L. ‘cake’ L. 'first' L. ‘arm’ L. 'man form' L. 'man' L. 'wise man' G. 'voice' L. ‘sticky’ G: ‘bone + germ’ G. ‘bone + break’ G. ‘between growth’ L. ‘seams’ G. ‘together + bone’ G. ‘together + band’ G. ‘bolt’ G. ‘together + grow’ L. ‘to bend’ L. ‘joint’ L. ‘egg-like’ L. ‘saddle’ L. ‘lip’ G. ‘bone + growths’ L. ‘box’ L. ‘with + egg’ G. ‘blood + joint’ L. ‘bind’ L. ‘with + side’ L. ‘tear away’ L. ‘lip’ L. ‘little half-moons’ L. ‘purse’ L. ‘mouse’ G. ‘within muscle’ ‘around muscle’ Upon muscle’ G: ‘two + heads’ L. ‘spindle + shape’ G. ‘double + stomach’ L. forefather’ L. ‘tear away’ L. ‘stretch out’ L. ‘from + tendon’ L. ‘seam’ L: ‘tether’ L. ‘partition’ L ‘feather’ G. ‘equal stretch’ ‘equal length’ G. ‘contest’ G. ‘with + work’ L. ‘one’s own receiver’; G. ‘over-nourishment’ G. ‘muscle + cut’ G. ‘creep’ + ‘girdle’ L. ‘outside + secrete’ L. ‘opening’ G. ‘inside + secrete’ G. ‘rouse’ L. ‘serum’ G. ‘middle intestine’ G. ‘strangle’) L. ‘slit’ L. ‘choke’

L. ‘axis’ L. ‘tendons’ G. ‘nerve’ + ‘husk’ L. ‘string’ G. ‘touch’ L. ‘receive’ L. ‘bend backwards’ G. ‘nerve’ + ‘glue’ L. ‘shell’ G. ‘gill’ G. ‘swelling’ G. ‘viscus’ L. ‘braid’ G. ’nerve’ + ‘pain’ G. ‘creep’ + ‘girdle’ L. ‘net’ + ‘wonderful’ L. ‘lung’ G. ‘blood + gush’ G. ‘clot’ G. ‘plug’ L. ‘out + send G. ‘bag’ G. ‘produce against’ direct G. ‘disease’ L. ‘small fountains L. ‘ring’ G. ‘absent’ + ‘formation’ reciprocal ‘endangered’ compensatory fail defectively redness swelling heat pain direct lymphatic blood G. 'dropping off' G. ‘new’ + ‘moulding’ L. 'crab' G. ‘beyond’ + ‘standings’ G. 'cancer' + 'swelling' G. 'flesh' + 'swelling' G. ‘blood’ + ‘swelling’ G. ‘widening’ G. ‘clot’ G. ‘plug’ G. ‘keep back’ + ‘blood’ L. ‘stuffing’ L. ‘rupture’ L. ‘falling’ G. ‘within’+’look’ G. ‘self + view’ L. ‘after death’ L. ‘apart’ + ‘cut’ L. ‘fallen’ G. ‘over-nourishment’ deltoid gluteus maximus vastus lateralis Ontogeny recapitulates

Phylogeny' L. 'drum' L. 'monthly' L. ‘build’ G. ‘tool’

Appendix 4: List of Derivations

250

L: ‘organised whole’ L. ‘body’ L: ‘vessel’ G: ‘dried up’ L: ‘beams L: ‘marrow’ G: ‘around bone’ G. ‘within bone’ G: ‘blood + make’ G: ‘air’ L. ‘spaces’ G: ‘sesame seed-like’ L. ‘joint’ L: ‘thorn’ G. ‘knuckle’ Fr. ‘little face’ L. ‘ditch’ L. ‘pit’ L. ‘furrow’ L. ‘bore’ L. ‘passage’ (L. ‘aperture’ G. ‘glass’ G. ‘around +cartilage’ ‘changing growth’ ‘upon growth’ L. 'covering' L. ‘break’ L. ‘skin’ L. ‘vessel’ + ‘body ’ 3 L. ‘yellow’ L. ‘in + sheath’ L. ‘middle + tendons’ L. ‘slit’ G. ‘tension’ G. ‘skin’ G. ‘upon + nipple’ L. ‘scale’ G. ‘black’

L. ‘nipples’ L. ‘white’ L: ‘hair’ + ‘grease’ L. ‘small bags’ ‘hair’ ‘sweat’ L. ‘wax’ L. ‘breast’ L. ‘under’ + ‘nail’ L. ‘fat’ G: ‘skin + cut’ L. ‘sticky’ L. ‘light’ L. ‘flask’ L. ‘acorn’ L. ‘shell’ L. ‘middle’ L. ‘lead’ G. ‘middle G. ‘wall’ L. ‘slippery + thin skin’ L. ‘plate + special’ L. ‘nipples’ L. ‘sewer’ G. ‘visceral’ G. ‘marrow’ G. ‘within nerve’ ‘around nerve ‘upon nerve’ G. ‘trees’ L. ‘carry to’ G. ‘self+ law’ G. ‘intestine’ G. ‘membranes’ L. ‘hard’ + ‘mother’ G. ‘spider web-like’ + ‘mother’ L. ‘tender’ + ‘mo G. ‘circles’ L. ‘furrows’ L. ‘nuts’ L. ‘little cords’

L. ‘branch’ L. ‘beak’ L. ‘tail’ G. ‘lines L. ‘vessels of nerves’ G. ‘air’ + ‘carry’ L. ‘light’ G. ‘within’ + ‘nipple’ L. ‘minute hairs’ L. ‘space-like’ G. ‘eat + cells’ L. ‘little balls of thread’ ‘with + contract’ between + contract’ G. ‘artery’ + ‘hardness’ ‘gruel + hardness’ G. ‘widening’ G. ‘through’ + ‘mouth’ L. ‘balls of thread’ G. ‘keep back + blood’ L. ‘stuffing’ L. ‘vessels of vessels’) G. ‘not yoked’ G. ‘gateway’ + ‘liver’ G. ‘under + growth L. ‘flaps’ L. ‘veins’ + ‘accompanying’ L. ‘hollow’ L. ‘tough mother’) L. ‘bring together’ G. ‘swelling’ L. ‘clear fluid’ L. ‘milk’ L. ‘juice’ ‘reservoir’ + ‘juice’ L. ‘knots’ L. ‘flat’ L. ‘middle + carry’ G. ‘tension’

Index

251

Eizenberg's General Anatomy

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