The Axial Skeleton: Structure and Function

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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)
Bones associated with the skull: 6 auditory ossicles, 1 hyoid bone
Thoracic cage: Sternum and 24 ribs
Vertebral column: 24 vertebrae, sacrum, coccyx

Labeled diagram of the human skeleton, highlighting the axial and appendicular skeleton

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.

Lateral view of the skull showing the brain case and facial bones

Cranial Bones

Parietal bones (2): Form most of the upper lateral sides of the skull and are joined at the top by the sagittal suture.
Temporal bones (2): Form the lower lateral sides of the skull. Notable features include the mastoid process (muscle attachment), 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. The foramen magnum allows the spinal cord to exit the skull, and the occipital condyles articulate with the first cervical vertebra.
Sphenoid bone (1): Forms part of the floor of the cranium, acts as a cross-bridge, and unites cranial and facial bones. The sella turcica houses the pituitary gland.
Ethmoid bone (1): Forms part of the floor of the anterior cranium, the roof of the nasal cavity, and part of the nasal septum. The cribriform plate contains olfactory foramina for the sense of smell, and the crista galli serves as an attachment for brain-stabilizing membranes.

Lateral view of the skull with labeled cranial bones and sutures Inferior view of the temporal bone showing mastoid process, styloid process, and mandibular fossa Posterior view of the skull showing occipital bone, foramen magnum, and occipital condyle Superior view of the cranial base showing the sella turcica and hypophyseal fossa Ethmoid bone with crista galli and cribriform plate

Sutures of the Skull

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

Coronal suture: Attaches the frontal bone to the parietal bones.
Sagittal suture: Attaches the right and left parietal bones.
Lambdoid suture: Attaches the occipital bone to the parietal and temporal bones.
Squamous suture: Attaches the temporal bone to the parietal bone.

Posterior view of the skull showing major sutures

Facial Bones

Maxillary bones (2): Form the upper jaw, most of the hard palate, the medial floor of the orbit, and the lateral base of the nose.
Palatine bones (2): Form the posterior portion of the hard palate.
Zygomatic bones (2): Cheekbones; form much of the 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 part of the nasal septum.
Mandible (1): Forms the lower jaw; only moveable skull bone. Features include the body, ramus, coronoid process (muscle attachment), condylar process (articulates with the temporal bone), alveolar process (anchors teeth), mental protuberance (chin), mental foramen (nerve passage), and mandibular foramen (blood vessels and nerves for lower 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, warming and humidifying inhaled air.

Cranial Fossae

Anterior cranial fossa: Contains the frontal lobes of the brain.
Middle cranial fossa: Contains the temporal lobes; includes openings such as the optic canal, foramen rotundum, and foramen ovale.
Posterior cranial fossa: Contains the cerebellum; includes the internal acoustic meatus, hypoglossal canal, and jugular foramen.

Superior view of the cranial base showing cranial fossae and foramina

Paranasal Sinuses

Paranasal sinuses are air-filled spaces within the frontal, sphenoid, ethmoid, and maxillary bones. They are lined with mucous epithelium, connected to the nasal cavities, and help warm, humidify, and filter inhaled air.

The Hyoid Bone

The hyoid bone is a U-shaped bone in the upper neck that does not articulate with any other bone. It serves as an attachment site for muscles of the larynx, pharynx, and tongue.

The Vertebral Column

Structure and Regions of the Vertebral Column

The vertebral column, or spine, is composed of 26 bones: 24 vertebrae, the sacrum, and the coccyx. It supports the head, protects the spinal cord, and provides attachment points for ribs and muscles.

Cervical vertebrae (7): Located in the neck (C1–C7). C1 (atlas) articulates with the occipital condyles; C2 (axis) allows head rotation.
Thoracic vertebrae (12): Located in the upper back (T1–T12); articulate with ribs.
Lumbar vertebrae (5): Located in the lower back (L1–L5); largest and bear the most weight.
Sacrum: Formed by fusion of 5 vertebrae; part of the pelvis.
Coccyx: Formed by fusion of 3–5 vertebrae; tailbone.

General Structure of a Vertebra

Vertebral body: Weight-bearing portion.
Vertebral arch: Forms the posterior part; includes pedicles (lateral walls) and laminae (roof).
Vertebral foramen: Passageway for the spinal cord; collectively form the vertebral canal.
Intervertebral foramen: Passageway for spinal nerves.
Intervertebral discs: Fibrocartilage pads between vertebrae; absorb shock.
Processes: Transverse (lateral), spinous (posterior), superior and inferior articular processes (articulation points).

Special Features of Vertebrae

Cervical vertebrae: Small bodies, transverse foramina for blood vessels, atlas (C1) and axis (C2) specialized for head movement.
Thoracic vertebrae: Larger bodies, costal facets for rib articulation.
Lumbar vertebrae: Largest bodies, support most body weight.
Sacrum: Fusion of five vertebrae; features include transverse ridges, median and lateral sacral crests, sacral canal, sacral hiatus, and sacral foramina for nerves.
Coccyx: Fusion of typically four vertebrae; articulates with the sacrum.

The Thoracic Cage and Ribs

Structure and Function of the Thoracic Cage

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

Sternum: Composed of the manubrium (articulates with clavicles and first ribs), body (articulates with ribs 2–7), and xiphoid process (muscle attachment).
Ribs: Elongate, curved bones that articulate posteriorly with thoracic vertebrae.
True ribs (1–7): Connect directly to the sternum via costal cartilage.
False ribs (8–10): Connect to the sternum indirectly via the cartilage of rib 7.
Floating ribs (11–12): Do not attach to the 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	Protects brain, forms face, houses sensory organs
Vertebral Column	Cervical (7), Thoracic (12), Lumbar (5), Sacrum, Coccyx	Supports body, protects spinal cord, allows movement
Thoracic Cage	Sternum, Ribs (24)	Protects heart and lungs, supports upper limbs
Associated Bones	Hyoid, Auditory Ossicles (6)	Muscle attachment, hearing