Axial Skeleton

Overview of the Axial Skeleton

The axial skeleton forms the central axis of the human body and provides support and protection for the brain, spinal cord, and thoracic organs. It consists of the skull, vertebral column, and thoracic cage.

• Major Components: Skull, vertebral column, ribs, and sternum.

• Functions: Protection of vital organs, support for the body, and attachment for muscles.

Skull Anatomy

Cranial Bones and Sutures

The skull is composed of several bones joined by immovable joints called sutures. These bones protect the brain and form the structure of the face.

• Parietal Bone: Forms the superior and lateral aspects of the skull.

• Frontal Bone: Forms the forehead and the superior part of the eye sockets.

• Temporal Bone: Located on the sides of the skull, houses structures of the ears.

• Occipital Bone: Forms the posterior and base of the skull.

• Sphenoid Bone: Forms part of the base of the skull and parts of the eye socket.

• Ethmoid Bone: Located between the eyes, forms part of the nasal cavity and eye orbits.

• Zygomatic Bone: Cheekbone, forms part of the orbit.

• Nasal Bone: Forms the bridge of the nose.

• Maxilla: Upper jawbone, forms part of the orbit, nose, and mouth.

• Mandible: Lower jawbone, the only movable bone of the skull.

Sutures: Immovable joints connecting the bones of the skull.

• Coronal Suture: Between the frontal and parietal bones.

• Sagittal Suture: Between the two parietal bones.

• Lambdoid Suture: Between the parietal and occipital bones.

• Squamous Suture: Between the parietal and temporal bones.

Other Features:

• External Auditory Meatus: Canal leading to the eardrum and middle ear.

• Mastoid Process: Projection of the temporal bone behind the ear.

Vertebral Column

Structure and Curvatures

The vertebral column, or spine, is a flexible structure composed of individual vertebrae. It supports the head, encloses the spinal cord, and allows for movement.

• Regions: Cervical (7), Thoracic (12), Lumbar (5), Sacrum (5 fused), Coccyx (4 fused).

• Curvatures: The spine has four main curvatures: cervical, thoracic, lumbar, and sacral. These curves help absorb shock and distribute weight.

Types of Curvatures:

• Primary Curvatures: Thoracic and sacral (present at birth).

• Secondary Curvatures: Cervical (develops as the infant lifts the head) and lumbar (develops as the child begins to walk).

Vertebrae Types and Features

Each region of the vertebral column has vertebrae with distinct characteristics.

• Cervical Vertebrae (C1–C7):

  • Small body, bifid (split) spinous process, transverse foramina for vertebral arteries.

  • Atlas (C1): Supports the skull, allows nodding motion.

  • Axis (C2): Has the dens (odontoid process), allows rotation of the head.

  • Vertebra Prominens (C7): Has a prominent spinous process.

• Thoracic Vertebrae (T1–T12):

  • Heart-shaped body, long spinous process, facets for rib articulation.

• Lumbar Vertebrae (L1–L5):

  • Large, kidney-shaped body, short and thick spinous process, triangular vertebral foramen.

• Sacrum and Coccyx:

  • Sacrum: Five fused vertebrae, forms the posterior wall of the pelvis.

  • Coccyx: Four fused vertebrae, forms the tailbone.

Abnormal Spinal Curvatures

Abnormalities in spinal curvature can affect posture and health.

• Scoliosis: Lateral curvature of the spine.

• Kyphosis: Exaggerated posterior curvature, usually in the thoracic region.

• Lordosis: Exaggerated anterior curvature, usually in the lumbar region.

Thoracic Cage

Ribs and Sternum

The thoracic cage protects the heart and lungs and supports the shoulder girdles and upper limbs.

• True Ribs (1–7): Attach directly to the sternum via costal cartilage.

• False Ribs (8–12): Attach indirectly or not at all to the sternum.

• Floating Ribs (11–12): Do not attach to the sternum.

Intervertebral Discs

Structure and Function

Intervertebral discs are fibrocartilaginous pads between vertebrae that act as shock absorbers and allow flexibility in the spine.

• Annulus Fibrosus: Outer ring of fibrocartilage.

• Nucleus Pulposus: Gel-like center that absorbs compressive forces.

Disc Herniation: Occurs when the nucleus pulposus protrudes through the annulus fibrosus, potentially compressing spinal nerves.

Summary Table: Vertebral Regions and Key Features

Key Terms and Definitions

• Axial Skeleton: The part of the skeleton that includes the skull, vertebral column, and thoracic cage.

• Suture: An immovable joint between skull bones.

• Vertebra: One of the small bones forming the backbone.

• Intervertebral Disc: A fibrocartilaginous pad between vertebrae.

• Herniation: Protrusion of an organ or tissue through an abnormal opening.

Example Applications

• Clinical Relevance: Understanding spinal curvatures is essential for diagnosing conditions like scoliosis, kyphosis, and lordosis.

• Injury Prevention: Knowledge of intervertebral discs helps in understanding back injuries and herniated discs.