CHAPTER XIII 



A STUDY OF THE NERVOUS SYSTEM 



The Body as a Collection of Organs. In the preceding 

 chapters we have discussed the digestive, respiratory, and 

 circulatory systems and have seen that these organs furnish 

 all parts of the body with food and oxygen. We have 

 studied the process of oxidation whereby we keep warm 

 and get power to do work. And, finally, we have consid- 

 ered the bones and muscles as the organs that give support 

 to the body and provide the machinery for all our motions. 

 The fact has been continually emphasized that the body is 

 composed of a great many organs, each with its special 

 function or functions. 



Cooperation of the Organs. But a human being or any 

 other complex organism is more than a mere collection of 

 working organs. In our definition of an organism (p. 39) 

 we included the statement that all the various organs work 

 together for the common good. This is what we mean by 

 cooperation (Latin co- = together -+- operari = to work). Sup- 

 pose we take a few instances from everyday experiences to 

 illustrate this cooperation. 



When I take food into my mouth, my salivary glands 

 pour out upon it an abundant supply of saliva. Now, the 

 food never comes in contact with the glands. How is it, 

 then, that they send out their secretion at just the right 

 time and in the proper amount ? The same questions may 

 be asked with reference to the gastric and pancreatic secre- 

 tions. If any one attempts to strike me in the eye, my 

 eyelids instantly close, and my hands fly up in front of my 

 face to ward off the blow. 



253 



