RESPONSIVE LIFE OF ORGANISMS 



455 



of controlling and coordinating responses that may be quite 

 as automatic as the action of the reflex arcs themselves. 

 When varying or conflicting stimuli come in, which shall 

 have the right of way is determined by the availability of the 

 paths these secondary circuits furnish. 



Cephalization. — It has come about in the development of 

 all the higher animals that the most specialized of the sense 

 organs have been located at the front end of the body; and 

 doubtless, this fact has determined the location of the con- 

 trol center. The end of an animal that goes ahead first 

 comes into contact with the new features of environment. 

 Here then should be located the organs that are the best 

 agencies of discovery; here, the organs that are responsive 

 to stimuli coming from a distance; the eye, the ear, the nose 

 and the antennae of the arthropods. Such organs as these 

 can give knowledge of the presence of an enemy before their 

 possessor has fallen into its grasp; they can locate food or 

 shelter or kindred, that might be passed by blindly. It is 

 most natural, therefore, that the part of the central nervous 

 system with which these "distance receptors" are most 

 directly connected should gradually assume the regulative 

 function. 



The brain is primarily an aggregate of neurones. These 

 consist of the same parts as the other neurones — cell bodies, 

 dendrites at the receiving end, and axones at the other end 

 — and they are combined together in circuits, and connected 

 more or less directly with all the other circuits of the body. 

 They have been from the beginning most directly connected 

 with the organs of special sense in the head; hence they are 

 well used to receiving important stimuli from these organs 

 of outlook, and of discharging impulses to every part of the 

 nervous system. Certain of these stimuli have always been 

 of paramount importance to the organism and well worn 

 paths have been established for them. So, actions that are 



