The Axial Skeleton

THE AXIAL SKELETON

The axial skeleton forms the central axis of the human body and provides support and protection to vital organs. It consists of the skull, vertebral column, ribs, and sternum. The skull protects the brain and supports the face, the vertebral column protects the spinal cord and maintains posture, and the rib cage (ribs and sternum) protects the heart and lungs. The axial skeleton mainly helps in protection, support, and posture.

The axial skeleton forms the central bony axis and includes:

• Skull (cranium + facial bones)
• Vertebral column (spine)
• Thoracic cage (ribs and sternum)

It supports and protects the central nervous system, thoracic organs, and forms the general framework of the trunk.

1) The Skull

The skull has two main parts:

• Cranium — encloses and protects the brain; formed by eight bones.
• Facial skeleton — fourteen bones forming the face, nasal cavity and mouth structures.

The Cranium (8 bones) — brief overview

• Frontal (1) — forms forehead and superior orbit; contains frontal sinuses (help reduce skull weight and add resonance).
• Parietal (2) — form sides and roof of cranium; meet at sagittal suture.
• Temporal (2) — form lower sides of skull; contain structures of the ear (petrous part) and mastoid process. Articulate with mandible (temporomandibular joint).
• Occipital (1) — forms posterior skull; contains foramen magnum for spinal cord passage.
• Sphenoid (1) — “keystone” that joins cranial and facial bones; contains the sella turcica that houses pituitary gland.
• Ethmoid (1) — light, spongy bone at anterior skull base; contributes to medial orbital walls and nasal septum; cribriform plate allows olfactory nerve fibers to pass.

The Face (14 bones) — brief overview

• Maxillae (2) — form upper jaw, floor of orbit and parts of nasal cavity; contain maxillary sinuses.
• Zygomatic bones (2) — cheekbones; part of lateral orbit.
• Mandible (1) — lower jaw; only movable skull bone (via temporomandibular joint); bears lower teeth.
• Nasal bones (2) — bridge of the nose.
• Lacrimal bones (2) — small bones forming medial orbit walls; contain nasolacrimal canal for tears.
• Palatine bones (2) — form posterior hard palate and part of nasal walls.
• Inferior nasal conchae (2) — scroll-shaped bones that increase nasal cavity surface (warming & humidifying air).
• Vomer (1) — forms lower part of nasal septum.
• Hyoid bone (1, not facial) — U-shaped bone in neck (above larynx), not directly articulated to other bones; provides attachment for tongue and neck muscles.

2) The Vertebral Column

The vertebral column (spine) is a flexible, strong column that supports the trunk and protects the spinal cord. It is made of 26 vertebrae in adults (some fused):

• Cervical (7) — neck region (C1–C7). First two are specialized:
• Atlas (C1) — supports the skull, has no body; permits nodding.
• Axis (C2) — has the odontoid (dens) process allowing rotation (shaking head «no»).
• Thoracic (12) — each articulates with a pair of ribs; heart-shaped bodies, long downward spinous processes.
• Lumbar (5) — large, robust bodies to bear weight; short thick spinous processes.
• Sacrum (1 — 5 fused) — formed by fusion of five sacral vertebrae; connects spine to pelvic girdle. Has sacral foramina for nerve passage and a sacral promontory.
• Coccyx (1 — 4 fused) — tailbone; small fused vertebrae at the base.

Functions of the Vertebral Column

• Protects the spinal cord inside the vertebral canal.
• Supports the skull and trunk; transfers weight to the pelvis.
• Provides attachment points for ribs, muscles and ligaments.
• Intervertebral discs between vertebrae absorb shock and allow flexibility.
• Vertebral foramina and intervertebral foramina allow passage of spinal cord and spinal nerves.