At birth, the human spine is composed of 33 interlocking bones that stack upon each other at a slight angle to form the spine’s S-shaped curve. This distinctive curvature helps the spine act as a shock absorber while still supporting the body’s weight when we move. The vertebrae grow progressively larger and stronger from top to bottom. By adulthood, most people have only 24 vertebrae because some fuse together.
The seven cervical (neck) vertebrae support the head; the 12 larger thoracic (chest) vertebrae bear the weight of the arms and trunk; and the five thickest and sturdiest lumbar (lower back) vertebrae carry the weight of the entire body. Because the lumbar region endures the greatest stress, it is the most vulnerable to strain or other problems. At the base of the spine are nine fused vertebrae that make up the sacrum (the back wall of the pelvis) and coccyx (tailbone). Each vertebra consists of a large oval block of bone (the vertebral body); pedicles and lamina that surround the canal through which the nerves run; facets that articulate with the vertebrae above and be low; two transverse bony projections (processes) that are points of attachment of spinal muscles; and a single spinous process that provides attachments for the muscles, ligaments and tendons that connect one vertebra to another.
Between each vertebral body and the next is an intervertebral disk, a flexible pad of cartilaginous tissue that cushions the vertebrae as the body moves. The high water content of the disks makes them very elastic—as we bend, twist and move, they can expand and contract, and then return to their original shape. Thus, the disks, working in conjunction with the interlocking facet joints, give the spine its tremendous flexibility.
Yet another crucial function of the spine is to form a protective shell around the delicate spinal cord and nerves. At the back of each vertebral body is a hole; when the vertebrae are stacked upon each other, these holes create the spinal canal, a channel through which the spinal cord runs. Spinal nerves, which branch off from the spinal cord through spaces (foramina) between adjacent vertebrae, extend to all parts of the body. The spinal cord ends at about waist level, dividing into a bundle of nerves that continues downward—the cauda equina (Latin for horse’s tail, since its structure resembles the tail of a horse). Because of this intricate interweaving of bone and nerve tissue, problems with the vertebrae can cause symptoms from pressure on the spinal nerves or the spinal cord.