The 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).

A pair of spinal nerves branches at each vertebral level (one to the left and one to the right), providing sensation and movement to all parts of the body. Three large bone processes arise from the vertebrae's arch-one to each side (transverse) and one straight toward the back of the body (spinous). Strong ligaments and muscles attached to the vertebrae both support the spine and further protect the delicate spinal cord and nerves inside.

Figure 1.

Lateral (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.

Figure 2.

Detailed views of a vertebra and vertebral segment. The drawing to the right represents a top view of a lumbar vertebra. The drawing below is a lateral (side) view of a segment of three lumbar vertebrae.