The Axial Skeleton: Structure and Function

The Axial Skeleton

Overview of the Axial Skeleton

The axial skeleton forms the central supporting axis of the body and consists of 80 bones, accounting for approximately 40% of the bones in the human body. It includes the skull, vertebral column, and thoracic cage. The axial skeleton provides protection for the brain, spinal cord, and thoracic organs, and serves as an attachment site for muscles involved in movement and posture.

• 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

The Skull

Structure and Function

The skull is a complex structure that protects the brain and forms the framework 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 by the sagittal suture. Bounded anteriorly by the frontal bone, inferiorly by the temporal bone, and posteriorly by the occipital bone.

• 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 extends posteriorly to form 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 sella turcica (houses the pituitary gland).

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

Sutures of the Skull

Sutures are immobile joints between adjacent 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 (on both sides)

Facial Bones

• Maxillary bones (2): Form the upper jaw, most of the hard palate (roof of the mouth), medial floor of the orbit, and 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): Lower jaw; only moveable skull bone. Contains the body, ramus, coronoid process (muscle attachment), condylar process (articulates with temporal bone), 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, warming and humidifying inhaled air.

Cranial Fossae

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

• Middle cranial fossa: Contains temporal lobes; includes optic canal, foramen rotundum, and foramen ovale (passageways for nerves and vessels).

• Posterior cranial fossa: Contains cerebellum; includes internal acoustic meatus, hypoglossal canal, and jugular foramen.

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

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

• Cervical vertebrae (7): Neck region (C1–C7). C1 (atlas) articulates with the occipital condyles; C2 (axis) allows head rotation.

• Thoracic vertebrae (12): Upper back (T1–T12); each articulates with ribs.

• Lumbar vertebrae (5): Lower back (L1–L5); largest and bear the most weight.

• Sacrum: Fusion of 5 vertebrae; forms part of the pelvis.

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

General Vertebral Structure

• Vertebral foramen: Passageway for the spinal cord; collectively form the vertebral canal.

• Intervertebral foramen: Passageway for spinal nerves.

• Vertebral body: Anterior, weight-bearing portion.

• Vertebral arch: Posterior portion; includes pedicles (lateral walls) and laminae (roof).

• Intervertebral discs: Fibrocartilage pads between vertebrae; absorb shock.

• Processes: Transverse (lateral), spinous (posterior), superior and inferior articular (articulation with adjacent vertebrae).

Specialized Vertebrae

• Cervical vertebrae: Small body, transverse foramen for blood vessels to the brain. Atlas (C1) lacks a body/spinous process; axis (C2) has the dens for head rotation.

• 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 (fusion sites), median/lateral sacral crests, sacral canal, sacral hiatus, and sacral foramina (nerve passageways).

• Coccyx: Typically four fused vertebrae; may fuse to sacrum in old age.

The Thoracic Cage and Ribs

Structure and Function

The thoracic cage protects the heart and lungs and supports the shoulder girdles and upper limbs. 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): Vertebrosternal; direct connection to sternum via costal cartilage.

• False ribs (8–10): Vertebrochondral; indirect connection to sternum via cartilage of rib 7.

• Floating ribs (11–12): Vertebral; do not attach to the sternum.

Summary Table: Major Bones of the Axial Skeleton

Region	Major Bones	Key Features
Skull	Frontal, Parietal, Temporal, Occipital, Sphenoid, Ethmoid, Maxilla, Mandible, Zygomatic, Nasal, Lacrimal, Palatine, Vomer, Inferior Nasal Concha	Protects brain, forms face, houses sensory organs
Vertebral Column	Cervical (7), Thoracic (12), Lumbar (5), Sacrum, Coccyx	Protects spinal cord, supports head/trunk
Thoracic Cage	Sternum, Ribs (24)	Protects thoracic organs, supports respiration
Hyoid	Hyoid bone	Attachment for tongue/larynx muscles