THE main function of the spine is to support the head and to act as a shock absorber to prevent jarring of the brain and the spinal cord. It also forms a rube which protects the delicate nervous elements of the spinal cord from injury. In the child the vertebral column is composed of thirty-three separate bones, but in the adult, the lowest four vertebra;, which were originally the tail, have become fused together to form the coccyx and the next five have joined together to form the sacrum.

In the adult, therefore, there are twenty-six separate bones. Each bone is joined to the one above and below it by a thick disc composed of gristle, which acts as a shock absorber. The discs are so thick that altogether they form nearly one-quarter of the whole length of the spinal column. Each vertebra is connected with its neighbour by ligaments so that very little movement is allowed at each joint, although when all the movements are added together, the possible movement of the spine as a whole is very considerable.

When viewed as a whole, the spine will be found to have several curves in its different regions. The neck has a convexity forwards, the chest is concave forwards, the region of the abdomen has a convexity forwards and the sacrum is hollowed out towards the front. Again the reason for this is to prevent jarring, for it enables the vertebral column to coil and uncoil like a spring in bearing the weight of the head and shoulders.

Each vertebra is shaped like a ring, the front of the ring being much larger than the back and forming a body which articulates (I.e. is connected by a joint) with the one above and below it. There are also small joints between the back of the rings. All these rings together form a tube in which is contained a continuation of the brain known as the spinal cord. Between each vertebra there is a small hole through which nerves leave or enter the cord. Delicate ligaments

stretch from the side of the cord and connect it with the insides of the vertebral tube, while the cavity is filled with a fluid called the cerebro-spinal fluid. Both these mechanisms act as a buffer and prevent jarring.