90 THE HUMAN MECHANISM 



Cooperation, adjustment, and coordination are thus brought 

 about in the body by two means : first, through the chemical 

 action of hormones ; and, second, through the mechanisms of 

 the central nervous system. The first provides for situations 

 where no great delicacy of adjustment is required; in the 

 secretion of the pancreatic juice, for example, it is not neces- 

 sary that a definite quantity, no more and no less, be secreted ; 

 in such a muscular movement as writing, on the other hand, 

 it is necessary that each muscle taking part shall act in a very 

 exact manner. For such coordinations the action of the nerv- 

 ous system is generally necessary. Finally, as suggested by 

 our consideration of the conditioned reflex, the nervous system 

 is the chief means whereby we can acquire new mechanisms 

 of coordination, thereby increasing our power of adjustment 

 to new conditions of life. 



