The Axial Skeleton

THE SKELETAL SYSTEMThe Axial Skeleton

• The skeleton consists of– Bones (206)– Cartilages– Joints – also called articulations, are the junctions

between skeletal elements– Ligaments – connect bones

• Divided into axial and appendicular

• Axial skeleton - forms long axis of body– Skull– Vertebral column– Thoracic cage

• Appendicular skeleton – appendages and what they attach to– Upper limbs (arms)– Pectoral girdle (shoulder)– Lower limbs (legs)– Pelvic girdle

Axial skeletonSkullVertebral columnThoracic cage

Axial skeleton is shown in green

The Skull• Cranial bones (or cranium)– Enclose the cranial cavity, which supports and protects the

brain– Attachment sites for some head and neck muscles

• Facial bones (anterior aspect of skull)– Form framework of face– Form cavities for sense organs of sight, taste and smell– Provides openings for passage of air and food– Hold the teeth– Anchor the muscles of the face

Cranium• Vault – “calvaria” = skullcap– Forms superior, lateral and

posterior aspects of skull, and forehead

• Base or floor: inferior part– Prominent bony ridges divide

cranial base into 3 “fossae” (steps) – anterior, middle and posterior

Anterior cranial fossa

Middle cranial fossa

Posterior cranial fossa

(looking down on the floor of the skull)

Cranial bones

• Frontal bone• Parietal bones (paired)• Occipital bone• Temporal bones (paired)• Sphenoid bone• Ethmoid bone

Cranial bones

parietal parietalfrontal

temporal

parietal

occipital

_______sphenoid_____ethmoid

occipital

Temporal bones

this is the right temporal bone looking at it from the right side

Sphenoid

Ethmoid Small cranial bones…

Sutures

• Immovable, interlocking joints of flat bones of skull• Irregular, saw-toothed appearance

• Largest 4 skull sutures: where bones articulate with parietal bones– Coronal– Sagittal– Squamous– Lambdoid

Find: coronal, squamous and lamboid sutures

Find: sagittal and lambdoid sutures

• Cranial “cavity” – houses brain

• Smaller cavities– Housing middle and inner ear– Nasal cavity– Orbits– Sinuses

• Openings (foramina, canals, fissures) for:– Spinal cord– Blood vessels– Twelve cranial nerves: I-XII

Remember, the skull is composed of:1. Cranial bones (or cranium)

and

2. Facial bones (anterior aspect of skull)– Form framework of face– Form cavities for sense organs of sight, taste and

smell– Provides openings for passage of air and food– Hold the teeth– Anchor the muscles of the face

Facial bones• Mandible• Vomer• Maxillae (paired)• Zygomatics (paired)• Nasal (paired)• Lacrimal (paired)• Palatines (paired)• Inferior nasal conchae (paired)

MandibleVomerMaxillae (paired)Zygomatics (paired)Nasal (paired)Lacrimal (paired)Palatines (paired)Inferior nasal conchae (paired)

Facial bones:

Mandible (lower jaw)

Maxilla (there are 2 which fuse, forming the upper jaw)

Nasal cavity

• Of bone and cartilagenasal bone

maxilla___________

OrbitCone-shaped bony cavities holding the eyes, muscles that move the eyes, some fat and tear-producing glands

optic nerve passes out through it

Paranasal sinuses• Air-filled sacs in the bones• “Paranasal” because they cluster around and

connect to the nasal cavity

Hyoid bone

• Only bone which does not articulate with any other bone

• Moveable base for the tongue

• Points of attachment for neck muscles that raise and lower the larynx during swallowing

SkullVertebral columnThoracic cage

Axial skeleton is shown in green

The Vertebral Column

• Fetus and infant: 33 separate bones or vertebrae

• Adult: 24 vertebrae– Inferior 9 have fused forming• The sacrum (5) and• The coccyx (4)

Vertebrae

• Cervical – 7• Thoracic - 12• Lumbar - 5• Sacrum (5 fused)• Coccyx (4 fused)

Spinal curvatures

• Cervical and lumbar are concave posteriorly* (lordosis)

• Thoracic and sacral are convex posteriorly* (kyphosis)

• Abnormal: – Too much of either– Scoliosis (more than 10 degrees of

lateral curvature)

*when viewed from the side

Abnormal curvatures

Disorders of the axial skeleton

• Scoliosis (over 10% curvature)

• Kyphosis

• Lordosis

• Vertebral compression fractures

• Spinal stenosis

Non-bony parts• Intervertebral

discs – anulus fibrosis

and nucleus pulposus)

• Anterior longitudinal ligament

• Posterior longitudinal ligament

• Ligamentum flavum

Anterior longitudinal ligament: wide, strong and attaches to vertebrae as well as discs (prevents hyperextension)

Posterior longitudinal ligament: narrow and relatively weak, attaching only to discs

*

Structure of a typical vertebra

Cervical vertebrae (C1-C7)C1 (atlas)

C2 (axis)

• Smallest• Lightest• Most flexible• Triangular vertebral

foramen• Transverse processes

have foramina (transverse foramen)

• Spinous process bifid (forked) except for C7

Cervical Vertebrae

• Heart shaped body• Additional small

costal facets (costal=ribs)

• Round or oval vertebral foramen

• Form posterior part of rib cage

Thoracic Vertebrae T1-T12

• Massive blocklike bodies

• Short, thick hatchet-shaped spinous processes

• Limited mobility

Lumbar Vertebrae L1-L5

The SacrumShapes posterior wall of pelvis

Composite bone of 5 fused vertebrae

Sacral foramina allow passage of vessels & nerves

Coccyx(the tailbone)

Remember that the Axial skeleton includes:

SkullVertebral columnThoracic cage

Axial skeleton is shown in green

The Thoracic Cage

Sternum Ribs

• Manubrium

• Body

• Xiphoid process

• True ribs 1-7

• False ribs 8-12

• Floating ribs 11,12