The spine is a column of bone and cartilage that extends from the base of the skull to the pelvis. It encloses and protects the spinal cord and supports the trunk of the body and the head. The spine is made up of approximately thirty-three bones called vertebrae. A joint that stabilizes the vertebral column and allows it to move connects each pair of vertebrae. Between each pair of vertebrae is a disk-shaped pad of fibrous cartilage with a jelly-like core, which is called the intervertebral disk-or usually just the disk. These disks cushion the vertebrae during movement.
The entire spine encloses and protects the spinal cord, which is a column of nerve tracts running from every area of the body to the brain. The vertebrae are bound together by two long, thick ligaments running the entire length of the spine and by smaller ligaments between each pair of vertebrae. The anterior longitudinal ligament consists of strong, dense fibers, located inside the bodies of the vertebrae. They span nearly the whole length of the spine, beginning with the second vertebrae (or axis), and extending to the sacrum. The ligament is thicker in the middle (or thoracic region). Some of the shorter fibers are separated by circular openings, which allow for the passage of blood vessels. Several groups of muscles are also attached to the vertebrae, and these control movements of the spine as well as to support it.