CHAPTER VI.
THE BACK.
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Fig. 31.—The spinal column. (Church and Peterson.)
The Spine.—The trunk may be roughly divided into the back, the chest or thorax, the abdomen, and the pelvis. By the back is denoted the spinal column with its muscles, blood-vessels, etc., and the spinal cord already described. The spine or vertebral column, which serves the double purpose of holding the body erect and of protecting the cord, is usually about two feet, two inches in length. In its course there occur several curves, which serve to give springiness and strength and, with the intervertebral cartilages, to mitigate the force of concussion from blows and falls. The curve is convex forward in the cervical region, convex backward in the dorsal, forward in the lumbar, and backward again in the sacral region. There is most freedom of motion in the cervical region.
As is the case with the other bones, the vertebræ are specially adapted in shape and size to the needs they are called upon to fill. Strength and flexibility, with a minimum bulk, a channel for the cord, and passages for the numerous nerves and blood-vessels are some of the requirements which, in combination, they meet to an astonishing degree. They are thirty-three in all, and are divided into groups according to the region in which they occur: seven cervical in the neck, twelve dorsal or thoracic, five lumbar, five sacral, and four coccygeal.
