Spine

The spine is the column of bones that runs down the back. Each of those bones is a vertebra, and humans have 33 of them, grouped by region: 7 cervical vertebrae in the neck, 12 thoracic vertebrae at chest level, 5 lumbar vertebrae in the lower back, and at the bottom 5 sacral and 4 coccygeal vertebrae.

That column supports the body's framework and also the nervous system. It is the load-bearing axis: it holds up the head and transfers the weight of the trunk and abdomen down to the legs. It works like the center pole of a tent, which bears what sits on top of it and carries that weight down to the ground.

And it is not straight. In adults it normally has gentle curves, each with its own name: a lordosis in the neck, a kyphosis in the chest and another lordosis in the lower back. Lordosis and kyphosis are curves that bend in opposite directions. They do not all appear at the same time. The thoracic curve is already formed before birth; the neck curve forms when a baby starts holding up its head, and the lumbar curve when a child starts to stand.