Chapter 7: The Axial Skeleton--Structure and Function

The Axial Skeleton

Overview of the Axial Skeleton

The axial skeleton forms the central axis of the human body and is essential for protection, support, and movement. It consists of 80 bones, accounting for approximately 40% of the bones in the human body. The main components include the skull, vertebral column, and thoracic cage.

• Skull: 22 bones (8 cranial, 14 facial), plus 6 auditory ossicles and the hyoid bone

• Thoracic cage: Sternum and 24 ribs

• Vertebral column: 24 vertebrae, sacrum, and coccyx

The Skull

Structure and Function of the Skull

The skull protects the brain and forms the structure of the face. It is divided into the cranial bones (brain case) and facial bones. The cranial cavity houses the brain, while the facial bones form the upper and lower jaws, nasal cavities, and orbits.

Cranial Bones

• Parietal bones (2): Form most of the upper lateral sides of the skull, joined at the top, bounded by the frontal, temporal, and occipital bones.

• Temporal bones (2): Form the lower lateral sides of the skull. Notable features include the mastoid process (muscle attachment for head rotation), external acoustic meatus (ear canal), mandibular fossa (part of the temporomandibular joint), and articular tubercle.

• Frontal bone (1): Forms the forehead and the floor of the anterior cranial cavity.

• Occipital bone (1): Forms the posterior skull and base of the cranial cavity. Contains the foramen magnum (spinal cord exit) and occipital condyles (articulate with the first cervical vertebra).

• Sphenoid bone (1): Forms part of the cranial floor, acts as a cross-bridge, unites cranial and facial bones, and contains the optic canals and sella turcica (houses the pituitary gland).

• Ethmoid bone (1): Forms part of the anterior cranial floor, roof of the nasal cavity, part of the nasal septum, and medial orbit wall. Contains the cribriform plate (olfactory foramina for smell), crista galli (membrane attachment), and ethmoidal labyrinth (paranasal sinuses).

Sutures of the Skull

Sutures are immobile joints between skull bones. Major sutures include:

• Coronal suture: Frontal to parietal bones

• Sagittal suture: Right and left parietal bones

• Lambdoid suture: Occipital to parietal and temporal bones

• Squamous suture: Temporal to parietal bones

Facial Bones

• Maxillary bones (2): Form the upper jaw, most of the hard palate (roof of mouth), medial floor of the orbit, and lateral base of the nose.

• Palatine bones (2): Form the posterior hard palate.

• Zygomatic bones (2): Cheekbones, lateral wall and margin of the orbit.

• Nasal bones (2): Form the bridge of the nose.

• Lacrimal bones (2): Form the anterior, medial wall of the orbit.

• Inferior nasal conchae (2): Thin, curved bones projecting into the nasal cavity.

• Vomer (1): Forms the posterior-inferior nasal septum.

• Mandible (1): Lower jaw, only moveable skull bone, with body, ramus, coronoid process (muscle attachment), condylar process (temporomandibular joint), alveolar process (anchors teeth), mental protuberance (chin), mental foramen (nerve passage), and mandibular foramen (blood vessels and nerves for teeth).

The Orbit and Nasal Structures

• Orbit: Bony socket for the eyeball, formed by seven bones (frontal, zygomatic, maxilla, palatine, ethmoid, lacrimal, sphenoid).

• Nasal septum: Formed by the perpendicular plate of the ethmoid, vomer, and septal cartilage.

• Nasal conchae: Create air turbulence, warm and humidify inhaled air.

Cranial Fossae

• Anterior cranial fossa: Contains frontal lobes of the brain.

• Middle cranial fossa: Contains temporal lobes, with openings for optic canal, foramen rotundum, and foramen ovale.

• Posterior cranial fossa: Contains cerebellum, with openings for internal acoustic meatus, hypoglossal canal, and jugular foramen.

Paranasal Sinuses

• Air-filled spaces in the frontal, sphenoid, ethmoid, and maxillary bones

• Lined with mucous epithelium, connected to nasal cavities

• Functions: warm/humidify air, trap dust/microbes

The Hyoid Bone

• U-shaped bone in the upper neck, not connected to other bones

• Attachment for muscles of the larynx, pharynx, and tongue

The Vertebral Column

Structure and Regions of the Vertebral Column

The vertebral column (spine) supports the body, protects the spinal cord, and allows movement. It consists of 26 bones: 24 vertebrae, the sacrum, and the coccyx.

• Cervical vertebrae (7): Neck region, C1 (atlas) supports the skull, C2 (axis) allows head rotation

• Thoracic vertebrae (12): Upper back, articulate with ribs

• Lumbar vertebrae (5): Lower back, largest and weight-bearing

• Sacrum: Fusion of 5 vertebrae, part of the pelvis

• Coccyx: Fusion of 3–5 vertebrae, forms the tailbone

General Vertebral Structure

• Vertebral foramen: Passage for the spinal cord; all together form the vertebral canal

• Intervertebral foramen: Passage for spinal nerves

• Vertebral body: Anterior, weight-bearing part

• Vertebral arch: Posterior part, includes pedicles (walls) and laminae (roof)

• Intervertebral discs: Fibrocartilage, shock absorption between vertebrae

• Processes: Transverse (lateral, muscle attachment), spinous (posterior, muscle attachment), superior and inferior articular processes (articulation between vertebrae)

Specialized Vertebrae

• Cervical vertebrae: Small body, transverse foramen for blood vessels

• Atlas (C1): No body or spinous process, nodding movement

• Axis (C2): Dens (odontoid process) for head rotation

• Thoracic vertebrae: Larger body, costal facets for rib articulation

• Lumbar vertebrae: Largest, thickest bodies for weight support

Sacrum and Coccyx

• Sacrum: Fusion of five vertebrae, features include transverse ridges (fusion sites), median sacral crest (fused spinous processes), lateral sacral crest (fused transverse processes), sacral canal (continuation of vertebral canal), sacral hiatus (inferior opening), and sacral foramina (nerve passageways)

• Coccyx: Fusion of four vertebrae, articulates with sacrum, fusion may complete in late adulthood

The Thoracic Cage and Ribs

Structure and Function of the Thoracic Cage

The thoracic cage protects vital organs and supports respiration. It consists of the sternum, 12 pairs of ribs with costal cartilages, and thoracic vertebrae.

• Sternum: Three parts—manubrium (articulates with clavicles and first ribs), body (articulates with ribs 2–7), xiphoid process (muscle attachment, no rib articulation)

• Ribs: Elongate, curved bones articulating posteriorly with thoracic vertebrae

• True ribs (1–7): Directly connect to sternum via costal cartilage

• False ribs (8–10): Indirectly connect to sternum via cartilage of rib 7

• Floating ribs (11–12): Do not attach to sternum

Summary Table: Major Bones of the Axial Skeleton

Region	Main Bones	Key Features
Skull	Frontal, parietal, temporal, occipital, sphenoid, ethmoid, maxilla, mandible, zygomatic, nasal, lacrimal, vomer, palatine, inferior nasal concha, hyoid	Protects brain, forms face, supports sensory organs
Vertebral Column	Cervical (7), thoracic (12), lumbar (5), sacrum, coccyx	Supports body, protects spinal cord, allows movement
Thoracic Cage	Sternum, ribs (24), costal cartilages	Protects heart/lungs, supports breathing