The Spine

Vertebrae. The spine is a column of small bones, or vertebrae, that support the entire upper body. The column is grouped into three sections of vertebrae:

• The cervical (C) vertebrae are the seven spinal bones that support the neck.
• The thoracic (T) vertebrae are the twelve spinal bones that connect to the rib cage.
• The lumbar (L) vertebrae are the five lowest and largest bones of the spinal column. Most of the body's weight and stress falls on the lumbar vertebrae.

Each vertebra can be designated by using a letter and number; the letter reflects the region (C=cervical, T=thoracic, and L=lumbar), and the number signifies its location within that region. For example, C4 is the fourth bone down in the cervical region, and T8 is the eighth thoracic vertebra.

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Below the lumbar region is the sacrum, a shield-shaped bony structure that connects with the pelvis at the sacroiliac joints. At the end of the sacrum are two to four tiny, partially fused vertebrae known as the coccyx or "tail bone."

The Spinal Column and its Curves. Altogether, the vertebrae form the spinal column. In the upper trunk the column normally has a gentle outward curve (its kyphosis) while the lower back has a reverse inward curve (its lordosis).

The Disks. Vertebrae in the spinal column are separated from each other by small cushions of cartilage known as intervertebral discs. Inside each disk is a jelly-like substance called the nucleus pulposus, which is surrounded by a tough, fibrous ring called the annulus. The disk is 80% water. This structure makes the disk both elastic and strong. The disks have no blood supply of their own, however, but rely on nearby blood vessels to keep them nourished.