The human spine consists of 33 vertebrae, but some of them grow together in adults. There are 7 cervical (neck), 12 thoracic (chest region), 5 lumbar (lower back), 5 sacral (hip region), and 4 coccygeal (tailbone region) vertebrae. The vertebrae are held in place by muscles and strong connective tissue called ligaments. Most vertebrae have fibrous intervertebral disks between them to absorb shock and enable the spine to bend.

The spine normally has a slight curve. Abnormal curvatures may be present at birth. They may also result from disease, poor posture, or a strain on the muscles attached to the spine. Scoliosis occurs when the spine curves sideways. Kyphosis, or hunchback, is a forward bending of the thoracic vertebrae that often affects elderly people. Lordosis, or swayback, is an exaggerated curvature of the lumbar vertebrae. It usually affects overweight people and pregnant women.

Damage to the spine often occurs in the cervical and lumbar regions. Fractured cervical vertebrae may injure the spinal cord, resulting in a loss of sensation, paralysis, or even death. Whiplash is an injury to the muscles and ligaments attached to cervical vertebrae. It occurs when a sudden force—such as a rear-end car accident—throws the head backward. As people age, the inner part of an intervertebral disk is likely to stick out through the outer part. This is called a slipped disk. A slipped disk in the lower back may pinch nerves, causing lumbago (low back pain) or sciatica (pain shooting down the leg).