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The axial skeleton is divided into three sections: the skull, the vertebral_column (spinal_column), and the thorax.

The vertebral column extends from the skull to the pelvis, and creates a central support system for the body. The bones of the vertebral column are called the vertebrae (sing: vertebra). There are 24 single vertebral bones (7 cervical, 12 thoracic, and 5 lumbar) and 9 fused bones (the sacrum has 5; the coccyx has 4).

The vertebral column is flexible because of pads of fibrocartilage that lie between each bone. These pads are called intervertebral_discs. Each disc has a fluid center called the nucleus_pulposus and an outer ring of fibrous tissue called the annulus_fibrosus.

The basic structure of a vertebra consists of:

centrum (body),





superior and inferior_articular_processes, and the


C-1 is called the atlas bone, because it holds up the skull (like the legendary Atlas, who is reputed to have supported the world). C-1 lacks a centrum and its transverse processes have large concave depressions to support the skull.

C-2 is the axis bone and it acts as a pivot for the atlas. It has a large vertical process known as the odontoid_process (dens) that serves as the pivot point.

The more typical cervical vertebrae (C-3 through C-7) are small, light, and have triangular vertebral_foramina. Their spinous process is short and often forks at the end. The spinous process of C-7 is visible through the skin and is called the vertebra_prominens. It is used as a reference point when counting vertebrae. Cervical vertebrae have wide transverse processes with foramina that accommodate the passage of the vertebral arteries (going towards the brain).

Spinous process Superior articular process Superior articular process Centrum or body Vertebral foramen Transverse foramen Transverse foramen Transverse process Transverse process Spinous process Inferior articular process Inferior articular process Vertebral foramen Centrum or body Transverse foramen Transverse foramen Transverse process Transverse process