Study Guide: The Axial Skeleton (Chapter 7, Anatomy & Physiology)

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The Axial Skeleton

Overview of the Axial Skeleton

The axial skeleton forms the central axis of the human body and consists of 80 bones, accounting for approximately 40% of the total bones. It includes the skull, vertebral column, and thoracic cage, providing structural support and protection for vital organs.

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

Anterior and posterior views of the human skeleton highlighting axial and appendicular skeleton

The Skull

Structure and Function

The skull protects the brain and forms the structure of the face. It is divided into the brain case (cranial bones) and facial bones, which together create the cranial cavity, orbits, nasal cavities, and jaws.

Brain case: Encases and protects the brain
Facial bones: Form the upper/lower jaws, nasal cavities, and orbits

Lateral view of the skull showing brain case and facial bones

Cranial Bones

The cranial bones form the brain case and are joined by immobile joints called sutures. Key cranial bones include:

Parietal bones: Form most of the upper lateral side of the skull; joined at the top, bounded by frontal, temporal, and occipital bones
Temporal bones: Form the lower lateral side; include the mastoid process (muscle attachment), external acoustic meatus (ear canal), and mandibular fossa (part of the temporomandibular joint)
Frontal bone: Forms the forehead and floor of the anterior cranial cavity
Occipital bone: Forms the posterior skull and base; contains the foramen magnum (spinal cord exit) and occipital condyles (articulate with C1)
Sphenoid bone: Forms part of the cranial floor, acts as a cross-bridge, unites cranial and facial bones, contains optic canals and sella turcica (pituitary gland housing)
Ethmoid bone: Forms part of the anterior cranial floor, roof of the nasal cavity, part of the nasal septum, and medial orbit wall; includes cribriform plate (olfactory foramina), perpendicular plate, crista galli, and ethmoidal labyrinth

Lateral view of the skull with labeled cranial bones and sutures Posterior view of the skull showing parietal, occipital, and temporal bones Inferior view of the temporal bone with labeled processes and fossa Superior view of the cranial cavity with labeled bones and foramina Inferior view of the skull with labeled bones and foramina Superior and posterior views of the sphenoid bone with labeled features Ethmoid bone with labeled crista galli, cribriform plate, 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

Lateral view of the skull with labeled sutures

Facial Bones

The facial bones form the structure of the face, jaws, and nasal cavities. There are 14 facial bones, including:

Maxillary bones: Form the upper jaw, most of the hard palate, medial floor of the orbit, and lateral base of the nose
Palatine bones: Form the posterior hard palate
Zygomatic bones: Cheekbones; form lateral wall and margin of the orbit
Nasal bones: Form the bridge of the nose
Lacrimal bones: Form the anterior, medial wall of the orbit
Inferior nasal conchae: Project into the nasal cavity; create air turbulence
Vomer: Forms the posterior-inferior part of the nasal septum
Mandible: Forms the lower jaw; only moveable skull bone

Inferior view of the skull with labeled facial bones Lateral view of the skull with labeled facial bones

Orbit and Nasal Structures

The orbit is a bony socket housing the eyeball, formed by seven skull bones. The nasal septum is composed of bone and cartilage, and the nasal conchae create air turbulence for warming and humidifying inhaled air.

Orbit: Frontal, zygomatic, maxilla, palatine, ethmoid, lacrimal, sphenoid bones
Nasal septum: Perpendicular plate of ethmoid, vomer, septal cartilage
Nasal conchae: Superior, middle (ethmoid), and inferior conchae

Ethmoid bone with labeled nasal conchae and perpendicular plate

Cranial Fossae and Foramina

The floor of the cranial cavity is divided into three fossae: anterior, middle, and posterior. Each contains openings (foramina) for nerves and blood vessels.

Anterior cranial fossa: Contains frontal lobes
Middle cranial fossa: Contains temporal lobes; includes optic canal, foramen rotundum, foramen ovale
Posterior cranial fossa: Contains cerebellum; includes internal acoustic meatus, hypoglossal canal, jugular foramen

Superior view of the cranial cavity with labeled fossae and foramina

Paranasal Sinuses and Hyoid Bone

Paranasal sinuses are air-filled spaces within certain skull bones, lined with mucous epithelium. The hyoid bone is a U-shaped bone in the neck, not connected to other bones, serving as a muscle attachment site.

Sinuses: Frontal, sphenoid, ethmoid, maxillary bones
Functions: Warm/humidify air, trap dust/microbes
Hyoid bone: Attachment for muscles of larynx, pharynx, tongue

The Vertebral Column

Structure and Regions

The vertebral column, or spine, consists of 26 bones: 24 vertebrae, the sacrum, and the coccyx. It provides support, protects the spinal cord, and allows flexibility.

Cervical vertebrae (C1–C7): Neck region; C1 (atlas) and C2 (axis) allow head movement
Thoracic vertebrae (T1–T12): Superior back; articulate with ribs
Lumbar vertebrae (L1–L5): Inferior back; largest, bear most weight
Sacrum: Fusion of 5 vertebrae; part of pelvis
Coccyx: Fusion of 3–5 vertebrae; tailbone

General Vertebral Structure

Each vertebra has a vertebral body (anterior, weight-bearing), vertebral arch (posterior), vertebral foramen (spinal cord passage), and processes for muscle attachment.

Intervertebral foramen: Passageway for spinal nerves
Intervertebral disc: Fibrocartilage, shock absorbing
Transverse and spinous processes: Muscle attachment
Articular processes: Superior and inferior, for articulation between vertebrae

Specialized Vertebrae

Cervical vertebrae have small bodies and transverse foramina for blood vessels. The atlas (C1) lacks a body/spinous process and allows nodding. The axis (C2) has the dens for head rotation. Thoracic vertebrae have costal facets for rib articulation. Lumbar vertebrae are largest and thickest.

Sacrum and Coccyx

The sacrum and coccyx are formed by fusion of vertebrae. The sacrum has transverse ridges, sacral canal, sacral hiatus, and sacral foramina for nerves. The coccyx articulates with the sacrum and may fuse completely in old age.

The Thoracic Cage and Ribs

Structure and Function

The thoracic cage protects the heart and lungs and supports breathing. It consists of the sternum, 12 pairs of ribs, costal cartilages, and thoracic vertebrae.

Sternum: Manubrium (superior, articulates with clavicles and 1st ribs), body (articulates with ribs 2–7), xiphoid process (inferior, muscle attachment)
Ribs: Elongate, curved bones; posteriorly articulate with thoracic vertebrae
True ribs (vertebrosternal): Rib pairs 1–7, direct connection to sternum via costal cartilage
False ribs (vertebrochondral): Rib pairs 8–10, indirect connection via cartilage of rib 7
Floating ribs (vertebral): Rib pairs 11–12, no connection to sternum

Summary Table: Axial Skeleton Components

Region	Main Bones	Key Functions
Skull	Cranial (8), Facial (14), Auditory ossicles (6), Hyoid (1)	Protects brain, forms face, supports sensory organs
Vertebral Column	Cervical (7), Thoracic (12), Lumbar (5), Sacrum (1), Coccyx (1)	Supports body, protects spinal cord, allows movement
Thoracic Cage	Sternum (1), Ribs (24)	Protects heart/lungs, supports breathing