spinal column, or backbone, consists of 33 bones (vertebrae)
and can be divided into five segments (Fig. 1A). The uppermost
24 vertebrae are separated from one another by fibrous cartilage
pads, called intervertebral discs (Fig. 1B), which
allow the spine to bend and act
as shock absorbers during activity. In the lowest part of the
spine, the vertebrae are naturally fused to form the sacrum and
the coccyx (tai l bone).
Protruding from the back of
each block-like vertebral body is an arch of bone that helps to form
the large, vertical spinal canal, which surrounds the spinal cord
and nerve bundles (Fig. 1C, below). A fluid-filled protective
membrane, the dura, covers the contents of the spinal canal from
where the cord begins at the base of the brain to where it ends (in a
bundle of nerve fibers known as the cauda equina).
(side) view of a normal spine. The drawing shows the locations
of the five major spinal levels. The cervical region has seven
vertebrae (C1 through C7), the thoracic region has 12 vertebrae
(T1 through T12) and the lumbar region has five vertebrae (L1 through
L5). The sacral region consists of five vertebrae, all fused together
to form one continuous bone mass known as the sacrum. The coccygeal
region consists of four vertebrae, all fused together to form the
coccyx or tailbone.