Spinal disks are rubbery pads between the vertebrae, the specialized bones that make up the spinal column. Doctors call them intervertebral disks. Each disk is a flat, circular capsule about an inch in diameter and one-quarter inch thick. They have a tough, fibrous, outer membrane (the annulus fibrosus), and an elastic core (the nucleus pulposus).
The disks are firmly embedded between the vertebrae and are held in place by the ligaments connecting the spinal bones and the surrounding sheaths of muscle. There is really little, if any, room for disks to slip or move. The points on which the vertebrae turn and move are called facet joints, which stick out like arched wings on either side of the rear part of the vertebrae. These facet joints are separate from the discs and keep the vertebrae from bending or twisting excessively, which could damage the spinal cord and the vital network of nerves that runs through the center of the spinal canal formed by the stack of vertebra.
The disk is sometimes described as a shock absorber for the spine, which makes it sound more flexible or pliable than it really is. While the disks do separate the vertebrae and keep them from rubbing together, they are far from spring-like. In children, they are gel- or fluid-filled sacs, but they begin to solidify as part of the normal aging process. By early adulthood, the blood supply to the disk has stopped, the soft inner material has begun to harden, and the disk is less elastic. By middle age, the disks are tough and quite unyielding, with the consistency of a piece of hard rubber. These changes related to aging make the outer protective lining weaker and the disks more prone to injury.