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About the spine 
The vertebral column is built up from alternating vertebrae (there are 33) and discs, which are made of cartilage and act as shock absorbers. The disc is composed mainly of fibres (the annulus) surrounding a central pulpy substance (nucleus pulposus) which attracts water into the centre of the disc, making it resilient (in a young adult, 90% of the disc content is water, reducing to 70% later on). The vertebrae, which are made of bone, are joined by ligaments which are intended to protect the spine from extremes of movement.
Of the 33 vertebrae, only 24 are independent, namely those above the sacrum. Each vertebra consists of a body at the front, and the vertebral arch at the back, which contains the all-important spinal cord, the object being to provide a mechanically strong structure capable of protecting the cord from all but the most severe trauma. The vertebrae are divided into levels: 7 cervical, 12 thoracic (or dorsal), 5 lumbar, 5 sacral and 4 coccygeal. They are abbreviated by the initial of their area and their numbering from the top, for example C6 or T3. The cord is the line of communication between the brain and the rest of the body, and it ends near the base of the back, where it becomes the Cauda Equina (literally, horse's tail, due to its shape). Each vertebra has two facet (apophyseal) joints linking one vertebra to the next by articular processes.
The sacro-iliac joints link the sacrum to the back of the pelvis. At each segmental level of the spinal cord, a nerve root leaves the cord on either side, passing through spaces between adjacent vertebrae (the intervertebral foramen).
