274. The Spinal Nerves. From the gray matter on each side of the spinal cord 31 spinal nerves are given off and distributed chiefly to the muscles and the skin. They pass out at regular intervals on each side of the canal, by small openings between the vertebræ. Having escaped from the spine, they pass backwards and forwards, ramifying in the soft parts of the body. The first pair pass out between the skull and the atlas, the next between the atlas and the axis, and so on down the canal. The eighth pair, called _cervical_, pass out in the region of the neck; twelve, called _dorsal_, in the region of the ribs; five are _lumbar_, and five _sacral_, while the last pair leave the cord near the coccyx. Each spinal nerve has two roots, one from the anterior, the other from the posterior portion of the cord. These unite and run side by side, forming as they pass between the vertebræ one silvery thread, or nerve trunk. Although bound up in one bundle, the nerve fibers of the two roots remain quite distinct, and perform two entirely different functions. After leaving the spinal cord, each nerve divides again and again into finer and finer threads. These minute branches are distributed through the muscles, and terminate on the surface of the body. The anterior roots become motor nerves, their branches being distributed to certain muscles of the body, to control their movements. The posterior roots develop into sensory nerves, their branches being distributed through the skin and over the surface of the body to become nerves of touch. In brief, the spinal nerves divide and subdivide, to reach with their twigs all parts of the body, and provide every tissue with a nerve center, a station from which messages may be sent to the brain. Illustration: Fig. 118.—Side View of the Spinal Cord. (Showing the fissures and columns.) A, anterior median fissure; B, posterior median fissure; C, anterior lateral fissure; D, posterior lateral fissure; E, lateral column; F, anterior column; G, posterior column; H, posterior median column; K, anterior root; L, posterior root; M, ganglion of N, a spinal nerve. 275. The Functions of the Spinal Nerves. The messages which pass along the spinal nerves to and from the brain are transmitted mostly through the gray matter of the cord, but some pass along the white matter on the outer part. As in the brain, however, all the active powers of the cord are confined to the gray matter. The spinal nerves themselves have nothing to do with sensation or will. They are merely conductors to carry messages to and fro. They neither issue commands nor feel a sensation. Hence, they consist entirely of white matter.