INTERVERTEBRAL DISCS
The vertebral bodies are the weight-carrying parts of the vertebrae. They are separated by intervertebral discs, of which there are 23 – one between every adjoining pair of vertebral bodies. In normal use, these discs are very efficient shock-absorbers; without them every step and movement would jar. The disc is a very tough structure. Inside a strong fibrous casing (annulus fibrosus) there is a pulpy gelatinous substance (nucleus pulposus), soft yet firm, and reinforced with strands of fibre. The disc has no blood and very little nerve supply, and consists mostly of water. During daily activity, the pressures on the spine force some fluid from the discs into the vertebral bodies. This is reabsorbed by the discs during relaxation, so that people actually become slightly shorter in the course of the day, and taller during the night. As people grow older, the discs lose some of their fluid content permanently and become thinner.
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