PHYSIOLOGY OF THE NERVOUS SYSTEM 313 



some one fact must be singled out from among the mental 



processes excited. This fact becomes the exciting stimulus 



and leads to action. It 



follows, therefore, that the 



action which finally occurs 



is not necessarily the result 



of an immediate external 



stimulus, but of a selected 



stimulus one which is the 



result of choice. 



Not only does the element 

 of choice enter into the selec- 

 tion of the proper stimulus, 

 but it also enters into the 

 time, nature, and intensity 

 of the action. For these 

 reasons it is frequently im- 

 possible to trace voluntary 

 actions back to their actual 

 stimuli. The pupil will rec- 



M 



FIG. 141. Diagram of voluntary 



ognize the element of choice action pathways including reflex path- 



... . . ways, 



in such simple acts as pick- 

 ing up some object from the street, complying with a 

 request, and purchasing some article from a store. 



Reflex and Voluntary Action Compared. Certain like- 

 nesses and differences, already suggested in these two 

 forms of action, may now be more fully pointed out. 

 Reflex and voluntary action are alike in that the primary 

 cause of each is some outside force or condition which has 

 impressed itself upon the nervous system. They are also 

 alike in the general direction taken by the impulses in 

 producing the action. The impulses are, first, from the 

 surface of the body to the central nervous system ; second, 



