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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


The spinal cord ends at L-2, so administration of spinal anesthesia (saddle_block) or a lumbar_puncture to obtain spinal fluid are done between L-3 and L-4 or L-4 and L-5 to minimize possible damage to the spinal (nerve) cord.

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