ADDITIONAL TOPICS IN HUMAN BIOLOGY 155 



229. CoSperation of the organs. — But a human being is more 

 than a mere collection of working organs, for all the various organs 

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

 tion (Latin, co = together + operari = to work). Suppose we 

 take a few instances from everyday experience to illustrate this 

 cooperation. 



When food is taken into the mouth, the salivary glands pour out 

 upon it an abundant supply of saUva. 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? 



If a blow is aimed at one's face, one's hands immediately fly up to 

 ward off the threatened injury. If the attack were pressed and one 

 were really compelled to defend himself, his heart would beat much 

 more rapidly, he would breathe faster, and the flow of perspiration 

 would become evident. During the contest certain feelings, also,, 

 would doubtless be aroused. 



230. Functions of the nervous system. — All the succession of 

 activities just described would be utterly impossible if some means 

 were not provided for making the organs work together for the com- 

 mon good. The arms could not see to strike at the antagonist; 

 nor could the heart, hxags, or skin respond to the sudden exertion of 

 the rest of the body. It is the nervous system that controls the 

 action of each of the organs in the body and brings about a coopera- 

 tion between them. All our sensations, too, and our will power are 

 doubtless correlated with the activities of the nervous system. 



231. Parts of the nervous system. — The nervous system con- 

 sists of nerve centers and nerve fibers (of which nerves are composed). 

 The principal nerve centers are the brain and spinal cord (Fig. 50). 

 These delicate organs are inclosed and wonderfully protected by the 

 bony cranium and spinal column. 



From the brain and spinal cord pass off nxmierous bundles of 

 nerves. As they approach the different organs of the body they 

 divide into branches, and thus the nerves become smaller and 

 smaller. Finally, the microscope is needed to trace the individual 

 nerve libers to their endings in muscle, gland, or sense organ. By 



