CHAPTER XIX 



THE NERVES AND THE SENSE ORGANS 



398. The Nervous System. In studying a complex 

 structure like the human body, we need to consider it 

 part by part, and system by system. We must remem- 

 ber, however, that the body is not simply one structure 

 added to another, but that it is a complete organism, with 

 many parts acting together harmoniously. The eye, 

 hand, and foot have each a separate function, but all can 

 combine to do one thing, say, to make a "drop kick" in 

 football, or to run a sewing machine. The movements 

 of the eye, hand, and foot are under the control of the 

 will, but there are many activities of the body that pro- 

 ceed without our notice and without any effort of the 

 will. Examples of these are the beating of the heart, the 

 movements of the ribs and diaphragm in respiration, the 

 secretion of saliva, gastric juice, and bile when they are 

 needed. Our food is under our control until the muscles 

 of the throat have contracted in swallowing; after that it 

 is in the control of involuntary muscles. How does each 

 organ of the body know when to perform its function, 

 how much effort to put forth, and when to stop? The 

 answer is that the different organs are in communication 

 with one another, like the stations of a telegraph system, 

 or the houses on a telephone line. When there is a need 

 for the performing of a function, the organ concerned 

 receives notice of the need, and sets about doing its par- 



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