CHAP. XVI 



THE NERVOUS SYSTEM 



221 



numerous telegraph wires running in all directions, by 



means of which the trains moving anywhere on the line 



are controlled. The train dispatcher / in the central 



station receives messages from every train on the road, 



so that he knows just where every train 



is located and how .it is moving. He 



sends messages out to the conductors, 



giving them their orders so that their 



trains may not run into each other. A 



single mistake of either train dispatcher 



or conductor in sending a wrong message 



or in failing to obey orders, might result 



in serious loss of life and property. One 



man must plan and operate the entire 



system. 



In much the same way the brain per- 

 forms its duties, which are, however, very 

 much more complicated. If the muscles 

 did not receive orders from the brain and 

 obey them, we could not walk or speak, 

 the lungs and heart could not work, the 

 foods could not be digested, in fact an- 

 archy would reign supreme in the human FIG. n6. The 



brain and spi- 



body. We shall now study this wonder- nal cord from 

 ful system in detail. behind ( Pratt )- 



154. Nerve Tissue. The nerve tissue consists of 

 cells which in size and shape are very unlike those 

 found in any other tissue. They have a cell body and 

 a large nucleus, made up of a very soft, delicate sub- 

 stance. Each nerve cell sends out a principal projec- 



