50 BRAINS OF RATS AND MEN 



near the beginning of the history of the elaboration 

 of the nervous mechanisms of correlation and integra- 

 tion. We find highly developed sense organs which 

 analyze the environmental agencies in very specific 

 ways. At the opposite or motor end of the reflex 

 apparatus we find again a system of very specific 

 organs of response, but so organized as to permit 

 very few kinds of movement. Between these there 

 is interpolated a nervous adjustor of simple pattern 

 so arranged that all sensory impulses may be con- 

 verged into a few motor paths. The specificity of the 

 receptive apparatus is quickly lost or generalized. 

 All kinds of sensory impulses converge upon a few 

 neurons whose efferent discharge is the resultant of 

 the interplay of all of the entering energies. 



With advancing specialization in higher animals 

 the lower primary reflex centers acquire more specific 

 characteristics, and fixed systems of correlation fibers 

 are laid down between them, thus establishing pro- 

 gressively more complex patterns of stable behav- 

 ior. But the convergence and interplay of diverse 

 sensory impulses and their interaction in determining 

 the nature of the resulting movement is only moved 

 farther back from the sensory surfaces. Progressively 

 more complex correlation centers are elaborated in 

 the midbrain, thalamus, and cerebral cortex, and here 

 the specificity of the sensory systems sooner or later 

 disappears in a common meeting ground. The associ- 

 ation centers of the human cerebral cortex present 



