Back
The back is made from twenty-four bones, called vertebrae, which sit one on top of the other. The bottom vertebra sits on top of the sacrum, which is really another five vertebrae that have fused together. The sacrum forms the back part of the pelvis. The top vertebra is specially modified to allow the skull to sit on it, and swivel in all directions.
When looked at from behind, the vertebrae form a straight line. From the side though, the bones of the back are aligned in several smooth curves. The back curves in at the waist, out over the back of the chest, and in again at the neck. This careful alignment of bones is maintained by ligaments (which are stout bands of fibrous tissue) and muscles that run along the length of the back. Between each vertebra is a cushion of cartilaginous material known as the disc. This has a semi liquid centre (like a jelly filled balloon) and absorbs the shocks the body receives in walking, running and jumping.
The spinal cord runs through holes in the centre of each vertebra. This cord is an extension of the brain, and passes through every vertebra from the skull to waist level. Between each vertebra, the spinal cord sends out nerves that supply that section of the body. Nerves run out from the neck to supply the arms, and from the lower vertebrae to supply the legs.
The sacrum, that big bone at the bottom of the spine, is attached to the pelvis by a complex network of ligaments positioned just under the dimples that people have on either side of their back, just above their buttocks.
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