CH 7 Axial Skeleton

Axial Skeleton

The part of the skeleton that consists of the skull, vertebral column, and thoracic cage.

Skull

Composed of 29 bones total, including 8 cranial bones and 14 facial bones.

Cranial Bones

8 bones that form the cranium: frontal (1), parietal (2), temporal (2), occipital (1), sphenoid (1), ethmoid (1).

Facial Bones

14 bones that form the face: mandible (1), maxilla (2), zygomatic (2), palatine (2), nasal (2), lacrimal (2), inferior nasal conchae (2), vomer (1).

Auditory Ossicles

6 total: malleus, incus, stapes (2 of each).

Hyoid Bone

A single bone that provides a moveable base for the tongue and attachment sites for tongue muscles and neck muscles.

Vertebral Column

Composed of 26 bones total: cervical vertebrae (7), thoracic vertebrae (12), lumbar vertebrae (5), sacrum (1, fused from 5), coccyx (1, fused from 4).

Thoracic Cage

Composed of 25 bones total: sternum (1) and ribs (24; 12 pairs).

Number of Bones in Adult Human

206 bones total.

Number of Bones in Axial Skeleton

80 bones.

Number of Bones in Appendicular Skeleton

126 bones.

Orbit

The bony cavity that contains the eye, formed by 7 bones: frontal, sphenoid, zygomatic, maxilla, palatine, lacrimal, ethmoid.

Paranasal Sinuses

Hollow air-filled cavities within the frontal, sphenoid, ethmoid, and maxillae bones, lined with mucous membranes.

Functions of Paranasal Sinuses

Humidify, warm, and filter air; lighten the skull's mass; enhance the quality of voice (resonance).

Function of Nasal Conchae

Increase surface area of the nasal cavity, create turbulence in airflow, trap dust and pathogens in mucus, warm and humidify incoming air.

Nasal Septum

A vertical partition that divides the nasal cavity into two, composed of vomer, perpendicular plate of ethmoid bone, and septal cartilage.

Middle Ear

Housed within the temporal bone, specifically in the tympanic cavity, containing the auditory ossicles.

Inner Ear

Housed within the temporal bone, specifically in the bony labyrinth, which includes the cochlea, vestibule, and semicircular canals.

Single Bones of the Skull

4 single bones: frontal, occipital, sphenoid, ethmoid.

Paired Bones of the Skull

4 paired bones: parietal (2), temporal (2).

Cervical Region

7 vertebrae (C1-C7), the smallest and lightest vertebrae, includes the atlas (C1) and axis (C2).

Thoracic Region

12 vertebrae (T1-T12) that articulate with the ribs.

Lumbar Region

5 vertebrae (L1-L5), the largest vertebrae that support most body weight.

Sacrum

1 bone formed by fusion of 5 sacral vertebrae.

Coccyx

1 bone formed by fusion of 3-5, usually 4, coccygeal vertebrae.

Total Bones in Adult Vertebral Column

26 bones total in the adult vertebral column.

Cervical Vertebrae Characteristics

Smallest vertebrae (except sacrum/coccyx), small oval-shaped body, often bifid spinous process (except C1 and C7), large triangular vertebral foramen, transverse processes contain foramina for vertebral artery and vein.

Atlas (C1)

No body, supports skull.

Axis (C2)

Has dens (odontoid process) for head rotation.

C7

Vertebra prominens, long spinous process easily felt at base of neck.

Thoracic Vertebrae Characteristics

Medium size, larger than cervical but smaller than lumbar, heart-shaped body, long pointed spinous process angled sharply downward, smaller rounder vertebral foramen, transverse processes have facets for rib articulation (except T11 & T12).

Defining Feature of Thoracic Vertebrae

Only vertebrae that articulate with ribs.

Lumbar Vertebrae Characteristics

Largest and heaviest vertebrae, massive kidney-shaped body (supports most body weight), short thick broad blunt spinous process (projects straight back), small triangular vertebral foramen, long slender transverse processes (no rib facets).

Defining Feature of Lumbar Vertebrae

Built for weight-bearing and stability.

Functions of Thoracic Cage

Encloses and protects organs in thoracic and superior abdominal cavities, provides support for upper limbs, provides attachment points for muscles of back, neck, chest, & shoulders, role in breathing (intercostal muscles).

Fontanelles

Soft, membranous gaps (fibrous connective tissue) between the cranial bones of an infant's skull where the sutures have not yet fused, commonly called the 'soft spots' on a baby's head.

Main Fontanelles

6 total in infants: Anterior fontanelle (largest, diamond-shaped, between frontal & parietal bones), Posterior fontanelle (triangular, between occipital & parietal bones), Sphenoidal fontanelles (2, at junction of frontal, parietal, temporal, sphenoid), Mastoid fontanelles (2, at junction of parietal, temporal, occipital).

Functions of Fontanelles

Allow flexibility of the skull during birth (skull bones can slightly overlap to fit through the birth canal), permit rapid brain growth during infancy, serve as clinical indicators (pulsation can be seen, dehydration or increased intracranial pressure can be assessed through them).