=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. Although the vertebræ of the different groups differ more or less in size and shape in accordance with the various demands of their positions, they all have certain general characteristics. Each has a body, two laminæ, two pedicles, two transverse processes, and one spinous process. The _pedicles_ extend back from the body on either side and support two broad plates of bone, the _laminæ_, whose juncture at the back completes the _spinal foramen_ for the passage of the cord. At their juncture is the _spinous process_, which can be felt beneath the skin, while the _transverse processes_ project from the juncture of the laminæ with the pedicles. All the processes are for the attachment of muscles that move the spine. The _body_ is formed of cancellous bone with a compact layer outside. Transversely it is slightly oval, while its upper and lower surfaces are flat, except in the cervical region, where the upper surface is concave laterally and the under convex laterally and concave from before back. Between the bodies are disks of fibro-cartilage, which increases motion and springiness. The _spinous process_ or _spine_ is short in the cervical region, long and directed downward in the dorsal region, thick and projecting almost straight out in the lumbar region. The pedicles are notched above and below so that when articulated the notches of two vertebræ join to form the _intervertebral foramen_ for the outward passage of nerves and the inward passage of blood-vessels.