THE NERVOUS SYSTEM \2J 



Spinal and Cranial Nerves. — The nerves from the spinal cord go 

 out through notches between the vertebrae. Since there are tliirty-one 

 pairs of spinal nerves (Fig. 109) and only twenty-four vertebras, 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, coordinate, 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 assist- 

 ants in charge of the different shops, keeping only the general man- 

 agement 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 capil- 

 laries 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'se-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 



