454 



GENERAL BIOLOGY 



stimuli shall be needed, and toward what ends all the activi- 

 ties of the body shall be directed. If there can be a head- 

 quarters to which all stimuli of this nature shall be referred, 

 and from which the dominating impulses shall go out to the 

 muscles, the resulting action may be more efficient. 



The mechanism for such control appears to have been 

 found in the derived circuits within the nerve centers. We 

 have already noted that besides the nerve cells composing 

 the reflex arcs, there are other cells within the centers, that 

 are situated as intermediaries between the primary circuits. 

 These have developed secondary circuits of their own; and 

 we have also noted that these are the media of distribution 

 of stimuli in case of multiple responses. A single reflex arc 

 is sensitive to but one kind of stimulus and tends of itself 

 alone always to give but one kind of response; but these 

 secondary circuits are so situated that they have to respond 

 to varied stimuli coming from every quarter sometimes 

 augmenting each other, sometimes interfering with each 

 other. We may not be able to say how they control action, 

 but we may well believe that their intermediary position is 

 advantageous in determining what shall be the totality of 

 the organic response. 



Among these secondary circuits are innumerable lines of 

 possible communication (through points of mutual contact), 

 and action is determined by the paths the stimuli take. If 

 the stimuli follow paths that lead to useful and beneficial 

 action the animal may live to have the like occur again and 

 again, until a main-travelled road for such stimuli is estab- 

 lished, and the action becomes habitual. If on the other 

 hand, the stimulus should follow unprofitable paths, leading 

 up to the wrong action, it might, if the mistake were serious, 

 lead to death (if, for example, a skunk seeing an approach- 

 ing train, should fail to get off the track) . Thus the struc- 

 ture of these secondary circuits is such as to furnish a means 



