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),

vertebral_arch,

vertebral_foramen,

transverse_process,

spinous_process,

superior and inferior_articular_processes, and the

intervertebral_foramina

The different categories of vertebrae (cervical, thoracic and lumbar) each have their own unique specializations.The first two cervical vertebrae (C-1 and C-2) have the most specialized shapes.

The five lumbar vertebrae (L-1 through L-5) have large, block-like centrums and short hatchet-shaped spinous processes.

Their superior facets face posteromedially and their inferior facets face anteriolaterally.

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.