Chapter 7: Axial Skeleton

Skull (22 bones)• Cranial bones (8 bones)

- form the large cranial cavity to protect and enclose the brain- form several smaller cavities (nasal cavity, orbits, paranasal sinuses, hearing,

and equilibrium)

• Frontal Bone- forms the forehead, the roofs of the orbits, and most of the anterior portion of

the cranial floor- after birth, the left and right sides of the frontal bone fuse by metopic suture

that disappears between ages 6-8- contain the frontal paranasal sinuses- articulates with the parietal bones with the coronal suture

• Parietal Bone (2)- form the greater portion of the sides and roof of the cranial cavity- contain many protrusions and depressions that accommodate blood vessels- articulates with each other by the sagittal suture

• Temporal Bone (2)- form the inferior lateral aspects of the cranium and part of the cranial floor- temporal squama forms the anterior and superior portion of the temple- zygomatic process articulates with the zygomatic bone of the face- mandibular fossa helps form the temporomandibular joint (TMJ)- mastoid process is a rounded projection of the temporal bone posterior to the

external auditory meatus; it is a point of attachment for several neck muscles- external auditory meatus - also called the ear canal; directs sound waves into

the ear- styloid process projects inferiorly from the inferior surface of the temporal

bone and serves as an attachment point for muscles and ligaments of the tongue and neck- articulates with parietal bones with the squamous suture

• Occipital Bone- forms the posterior part and the base of the cranium- foramen magnum is the large hole in the inferior part of the skull which con-

nects the medulla oblongata with the spinal cord- occipital condyles are oval processes on either side of the foramen magnum- occipital protuberance is a midline projection on the posterior surface of the

bone- articulates with the parietal bones with the lambdoid suture

• Sphenoid Bone

- lies in the middle part of the base of the skull and is the keystone bone of the cranial floor since it articulates with the other cranial bones holding them to-gether- contains the sphenoidal paranasal sinuses (drains into the nasal cavity)- sella turcica - bony saddle shape structure on the superior surface of the sphe-

noid; houses the pituitary gland- greater wings of sphenoid bone project laterally from the body; forms the lat-

eral wall of the skull anterior to the temporal bone- lesser wings of sphenoid bone are smaller and form the ridge of bone anterior

and superior to the greater wings which form the posterior portion of the orbit of the eye

• Ethmoid Bone- light spongy bone located in the middle of the anterior part of the cranial floor- anterior to the sphenoid and posterior to the nasal bones forms:

* part of the anterior portion of the cranial floor* medial wall of the orbits* superior portion of the nasal septum (divides into right and left halves)* most of the superior sidewalls of the nasal cavity

- perpendicular plate forms the superior portion of the nasal septum- cribriform plate lies in the anterior floor of the cranium and forms the nasal cav-

ity roof- two thin scroll-shaped projections: superior and middle nasal concha- crista galli - attachment part for the meninges of the brain

Facial Bones (14 bones)- form the face (growth ceases at 16 years of age)

• Nasal Bones (2)- meet at the midline and form part of the bridge of the nose

• Maxillae (2)- form the upper jawbone- articulates with every bone of the face except the mandible- forms part of the floor of the orbits, lateral and floor of nasal cavity and most of

the hard palate- contains maxillary paranasal sinuses

• Zygomatic Bones (2)- cheekbones- articulates with maxilla, frontal, sphenoid, and temporal bones

• Mandible- only movable joint in the skull other than the auditory ossicles

- forms the temporomandibular joint (TMJ) with the mandibular fossa and articu-lar tubercle of the temporal bones- coronoid process which provides attachment for temporalis muscle- mandibular foramen located on the ramus of the mandible- mandibular notch is located between the coronoid process and the condylar

process- mental foramen located below the second premolar tooth

• Lacrimal Bones (2)- smallest bones of the face- contain lacrimal fossa houses the lacrimal sac or “tear sacs and ducts”

• Palatine Bones (2)- are l-shaped and form the posterior portion of the hard palate- also form part of the floor and lateral wall of the nasal cavity- horizontal plates form the posterior portion of the hard palate (separates nasal

cavity from oral cavity)

• Inferior nasal conchae bones (2)- inferior to the middle nasal conchae of the ethmoid bone- scroll-like bones that project into the nasal cavity- help swirl and filter air before it passes through the lungs

• Vomer- triangular bone on the floor of the nasal cavity that articulates with the perpen-

dicular plate of the ethmoid bone- part of the nasal septum

• Hyoid (1 bone)- unique component of the axial skeleton- does not articulate with any other bone- helps support the tongue and the corresponding muscles- muscles and ligaments attach to the lesser and greater horns

• Auditory ossicles (6 bones)

• Vertebral Column (26 bones)- 2/5ths total height of the body- the cervical, thoracic, and lumar vertebrae are movable, whereas the sacrum

and coccyx are not movable- has four slight bends or curvatures named after the vertebrae in that specific re-

gion- each vertebrae starting with the second cervical vertebra are separated by in-

tervertebral discs

- PARTS OF A TYPICAL VERTEBRA:* body is the thick disk-shaped anterior portion that bears the weight of

the column* vertebral arch extends posteriorly from the body and surrounds the

spinal cord‣ pedicles are short processes that form the vertebral arch‣ vertebral foramen lies between the arch and the body

* processes arise from the vertebral arch‣ transverse process extends laterally from each side‣ spinous process projects posteriorly from the junction of the laminae‣ superior articular processes articulate with the inferior articular processes of the vertebra immediately above them‣ articulating surfaces of the processes are called facets

- 7 cervical vertebrae* smaller than thoracic vertebrae but have a larger vertebral arch* 3 foramina: on vertebral foramen and 2 transverse foramina* first cervical vertebra is called the atlas which supports the head

‣ no body or spinous process, but has an anterior & posterior arch‣ superior articular facets articulate with occipital condyles to form atlanto-occipital joints, which permit movement of the head, “yes”

* second cervical vertebra is called the axis which allows head to pivot‣ has a body and a peglike process called a dens‣ the dens allows the atlas and head to pivot and rotate such as to say “no”‣ atlanto-axial joint is the articulation between the articular facets and the anterior arch of the atlas

* the third through sixth cervical vertebrae follow a typical pattern* the seventh cervical vertebra has a single large spinous process that

can be felt at the base of the neck- 12 thoracic vertebrae

* larger and stronger than cervical vertebrae* longer and larger transverse processes than cervical vertebrae* articulate with the ribs through joints called vertebrocostal joints* contain facets and demifacets* intervertebral discs are thinner between the thoracic vertebrae

- 5 lumbar vertebrae* largest and strongest vertebrae with thick and broad spinous processes

for attachment of back muscles* bone surface markings are similar to the generic description

- 1 sacrum* triangular bone formed by fusion of five sacral vertebrae* fusion takes place between ages 16-18 and complete by age 30* serves as strong foundation for the pelvic girdle

* concave anterior side faces the pelvic cavity and contains transverse lines. at the end of these lines are the anterior sacral foramina

* posterior side (convex) contains a median & lateral sacral crest and posterior sacral foramina

- 1 coccyx* triangular and formed by the fusion of four coccygeal vertebrae* fusion takes place between ages 20 and 30* articulates with the apex of the sacrum* dorsal surface of the body contains two long coccygeal cornua that

connect ligaments to the sacral cornua* lateral surfaces contain transverse processes

Thorax (25 bones)• Sternum (1)

- flat narrow bone that consists of three parts:* manubrium which is the superior portion‣ contains suprasternal notch which is a depression on the superior sur-face‣ clavicular notches articulate with the clavicle to form the sternoclavic-ular joints‣ articulates with the costal cartilage to form the sternocostal joints

* body which is the middle and largest portion‣ body of the sternum directly or indirectly articulates with the 2nd through 10th ribs

* xiphoid process which is the inferior and smallest portion‣ made of hyaline cartilage in infancy and does not ossify completely until age 40

• Ribs (24)- bone surface markings of ribs: inferior facet, superior facet, nonarticular

part of tubercle, head, neck, body, and articular part of tubercle- 7 pairs of true ribs (connected directly to the sternum via costal cartilage)- 5 pairs of false ribs (not connected directly to the sternum; connects instead to

costal cartilage of true ribs)* 2 pair of false ribs are called floating ribs (not connected to any other

bone or cartilage)