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.
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.
276. Functions of the Spinal Cord. The spinal cord is the principal channel through which all impulses from the trunk and extremities pass to the brain, and all impulses to the trunk and extremities pass from the brain. That is, the spinal cord receives from various parts of the body by means of its sensory nerves certain impressions, and conveys them to the brain, where they are interpreted.
The cord also transmits by means of its motor nerves the commands of the brain to the voluntary muscles, and so causes movement. Thus, when the cord is divided at any point, compressed, as by a tumor or broken bone, or disorganized by disease, the result is a complete loss of sensation and voluntary movement below the point of injury. If by accident a man has his spinal cord injured at some point, he finds he has lost all sensation and power of motion below that spot. The impulse to movement started in his brain by the will does not reach the muscles he wishes to move, because traveling down the spinal cord, it cannot pass the seat of injury.
So the impression produced by pricking the leg with a pin, which, before pain can be felt, must travel up the spinal cord to the brain, cannot reach the brain because the injury obstructs the path. The telegraph wire has been cut, and the current can no longer pass.
277. The Spinal Cord as a Conductor of Impulses. The identity in structure of the spinal nerves, whether motor or sensory, and the vast number of nerves in the cord make it impossible to trace for any distance with the eye, even aided by the microscope and the most skillful dissection, the course of nerve fibers. The paths by which the motor impulses travel down the cord are fairly well known. These impulses originate in the brain, and passing down keep to the same side of the cord, and go out by nerves to the same side of the body.