THE SPINAL CANAL
This is the conduit through which the spinal cord passes. It is formed by the backs of the vertebral bodies and the vertebral arches, and protects the spinal cord. The cord is surrounded by cerebrospinal fluid, and encased in the dural tube which is made up of three tubular membranes, one inside the other. The dural tube descends all the way to the sacrum, though the spinal cord ends in the upper lumbar region.
The spinal cord is an extension of the brain, and is the trunk road of the nervous system, conveying information from the brain to all regions of the body and back again, by means of nerve roots which branch out from it.
At the level of each vertebra, the nerve roots emerge through chinks, known as intervertebral foramina, between adjacent pedicles, one on each side of each vertebra. Each nerve root is enclosed in a dural sleeve. From all these pairs of nerve roots a vast network of nerves branches out throughout the whole body.
Since the spinal cord stops short at the first lumbar vertebra, there is a sheaf of pairs of nerve roots passing downwards from the lower segments of the spinal cord to reach their respective foramina. This sheaf is called the cauda equina, the horse’s tail, because that is what it looks like.
The network of nerves which originate in the nerve roots has a pattern of distribution which is much the same for everyone, with only minor individual deviations. It is therefore possible to trace a pain sensation in any part of the body, caused by a compressed nerve root, back to its point of origin in the spine. For example, a pain in the big toe that is caused by a compressed nerve must originate in the last two lumbar vertebrae or the upper part of the sacrum – nowhere else in the spine.
The spinal cord is not easily injured, except through fracture or dislocation of the spine or when the cord is penetrated by sharp instruments, bullets or shrapnel. In normal circumstances, the spinal canal and its contents are protected from injury by bone and ligaments and muscle. Occasionally the blood supply to the spinal cord is damaged and nerve function can be affected.
The spinal canal changes its length with spinal movement. When you bend sideways, it becomes longer on one side than the other. On bending forwards and flexing the spine, the whole canal lengthens, more so behind than in front. The change in length in the cervical and lumbar regions may be as much as 25 per cent, and the contents of the spinal canal adapt accordingly. When the spine is bent back and arched, the intervertebral discs tend to bulge forwards and the ligaments at the back of the spinal canal slacken.
In the course of vertebral movements, the gaps between each pair of vertebrae (the intervertebral foramina) open and close, as the neighbouring vertebral arches move closer together or separate, and the nerve roots move inside the foramina.
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