Spinal and Cranial Nerves.—The nerves from the spinal cord go out through notches between the vertebræ. Since there are thirty-one pairs of spinal nerves (Fig. 109) and only twenty-four vertebræ, some of the nerves go out through holes in the sacrum. The cranial nerves (to eyes, ears, tongue, nose, face, etc.) leave the brain through holes in the cranium, or skull. There are twelve pairs of them.
Relation of the Cerebrum to the Lower Centers.—As already stated, nerve activities are of four kinds,—reflex, automatic, coördinate, and voluntary. A manufactory has more complex work than a shop. A man with a shop may enlarge it into a factory and leave trained assistants in charge of the different shops, keeping only the general management for himself. If he should cease to control his assistants entirely, the work of the factory would soon be in disorder. If the manager should try to direct everything, he would become exhausted. So the cerebrum, the seat of the will and the reason, leaves the reflex centers in the spinal cord, medulla, and cerebellum to do most of the work. If the mind wishes the hand to move and grasp the hand of a friend, the motor center in the cerebrum sends a message to the cerebellum; and if the cerebellum has been well trained, the act is accurately performed.
A less imperfect wisdom than that of the mind is in the lower nerve centers. The reason and will control the lower centers through the cerebrum, but the control is very limited. It is well that this is so, not only for the relief of the cerebrum, but for the safety of the body. Can you change the rate of the heart beat by the exercise of the will? Can you blush at will, or prevent the flushing of the capillaries when you are embarrassed, or when you go close to a hot fire? It is impossible for a person to commit suicide by holding the breath. What change in the blood would soon force a breath to be taken? Repeat the two examples of reflex action triumphing over the will which have already been given. We shall next take up a system of nerves almost independent of the will.
The ganglionic or sympathetic portion of the nervous
system controls the viscera (vis'sē-ra), or internal organs,
e.g. peristalsis of food tube, tone of arteries. The nerves
that go to the viscera branch off from the spinal nerves
not far from the spinal column, and enter a row of ganglia
on each side of the spine (see Fig. 115). Each ganglion
is connected by nerves with the one above and below it,
so that they appear like two knotted cords suspended one
