270 



Cereb 



Why Living 



Fig. 254 Tl3e central nervous system of man. 

 Only the largest nerves are shown. About how 

 many nerves arise jrom the spinal cord? Notice 

 how the nerves branch into all body parts. 



There is a complex nervous system in 

 you which makes your bilHons of cells 

 work together, making you one organ- 

 ism instead of a group of independent 

 cells. This nervous system consists of 



Thijjgs Behave As They Do unit v 



two parts, the cejitral nervous syste?n and 

 the aiitonofiiic Jiervous system. The auto- 

 nomic system is often called the involun- 

 tary nervous system. The two systems 

 of nerves work together closely. The 

 autonomic nervous system controls the 

 internal responses, such as the contrac- 

 tions of the heart and digestive tube, 

 secretion by digestive glands, and many 

 others. You are seldom aware of these 

 responses. You will learn more about the 

 autonomic nervous system and its activi- 

 ties later in this problem. Let us study 

 the central nervous system first. 



Nerves and nerve centers. The nerve 

 centers of the central nervous svstem are 

 the brai?i and its continuation, the sphial 

 cord. Attached to these are the nerves 

 which look like white cords stretching 

 all through the body. Study of Figure 

 254 shows that, while nerves are found 

 in every part of the body, they do not 

 interlace or form a network. All the 

 nerves are attached to the brain or spinal 

 cord. If you traced any nerve in the fig- 

 ure from its farthest point in any part of 

 the body you would always end up in 

 the brain or spinal cord. That is why 

 they are called the nerve centers. 



When a receptor is stimulated, a "mes- 

 sage" or impulse travels along a nerve to 

 the nerve center. If the response to this 

 stimulus is a muscular contraction, it 

 means that another impulse goes out 

 along a nerve to the muscles, which then 

 contract. An impulse never goes from 

 the receptor directly to the muscle. Our 

 central nervous system is somewhat like 

 a telephone system with a central office. 

 Ordinarily, you communicate with a 

 friend through the central office, not di- 

 rectly. 



