The spine consists of a series of vertebrae that are stacked on top of one another from the neck to the pelvis. These vertebrae are connected and kept in the correct position by the discs that are between the vertebrae and also the facet joints that interlock with one another. There are also supporting ligaments and tendons. The discs between the two adjacent vertebrae are composed of a central, soft, jelly-like substance, called the nucleus pulposus, and an outer, tougher fibrous part, called the annulus fibrosus. In some cases the tough annulus fibrosus becomes damaged and tears, leaving a defect in the strong outer layer.