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 bones in the body. It includes the skull, vertebral column, and thoracic cage, providing 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

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 form the cranial cavity, jaws, nasal cavities, and orbits.

• Brain case: Encases and protects the brain

• Facial bones: Form the upper and lower jaws, nasal cavities, and orbits

Cranial Bones

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

• Parietal bones: Form most of the upper lateral side of the skull

• Temporal bones: Form the lower lateral side of the skull; include the mastoid process (muscle attachment site)

• 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)

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

• Ethmoid bone: Forms part of the anterior cranial floor, roof of the nasal cavity, and medial orbit wall; includes cribriform plate (olfactory nerves), perpendicular plate (nasal septum), and crista galli (brain membrane attachment)

Facial Bones

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

• 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 portion of the 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

Sutures of the Skull

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

• Coronal suture: Attaches frontal bone to parietal bones

• Sagittal suture: Attaches right and left parietal bones

• Lambdoid suture: Attaches occipital bone to parietal and temporal bones

• Squamous suture: Attaches temporal bone to parietal bone

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

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; optic canal, foramen rotundum, foramen ovale

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

Paranasal Sinuses

Paranasal sinuses are air-filled spaces within certain skull bones, lined with mucous epithelium. They help warm, humidify, and filter inhaled air.

• Sinuses: Frontal, sphenoid, ethmoid, maxillary bones

• Function: Mucous secretions trap dust and microbes

Hyoid Bone

The hyoid bone is a U-shaped bone in the upper neck, not connected to 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, consists of 26 bones: 24 vertebrae, the sacrum, and the coccyx. It provides structural support and protects the spinal cord.

• 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 and weight-bearing

• Sacrum: Fusion of 5 vertebrae; part of pelvis

• Coccyx: Fusion of 3–5 vertebrae; tailbone

General Vertebra Structure

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

• Vertebral foramen: Passageway for spinal cord

• Intervertebral foramen: Passageway for spinal nerves

• Intervertebral disc: Fibrocartilage, shock absorbing

• Transverse and spinous processes: Muscle attachment sites

• Articular processes: Superior and inferior, for vertebral articulation

Specialized Vertebrae

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

• Atlas (C1): No body or spinous process; allows nodding

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

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

• Lumbar vertebrae: Largest, thickest body

• Sacrum: Fusion of sacral vertebrae; sacral canal, sacral hiatus, sacral foramina

• Coccyx: Fusion of coccygeal vertebrae; articulates with sacrum

The Thoracic Cage and Ribs

Structure and Function

The thoracic cage protects vital organs and supports respiration. 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 to sternum via cartilage of rib 7

• Floating ribs (vertebral): Rib pairs 11–12, no connection to sternum

Key Functions

• Protection: Shields heart, lungs, and major vessels

• Respiration: Provides attachment for muscles involved in breathing

Summary Table: Axial Skeleton Components