EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 305 



combined into reflex systems or otherwise correlated. The 

 afferent stimuli which reach the cerebral cortex are not crude 

 sensory impressions, but purposeful reflex combinations, often 

 including sensory data from several different sense organs. 



The nerve-centers of the spinal cord and brain stem in general 

 are of this more rigid type, the internal adjustments of the sys- 

 tem being, for the most part, as mechanically determined as are 

 those of an automatic telephone exchange. The cerebellum is 

 the highest member of this series, exerting a regulatory and re- 

 inforcing influence upon all of the other members. Nevertheless 

 the cerebellum adds no new types of reaction or combinations of 

 reactions to those of the brain stem; its cortex shows little de- 

 monstrable localization of different functions, and its efferent 

 tracts are physiologically related to a limited number of pre- 

 established systems of motor coordination in the brain stem and 

 spinal cord. In all of these respects the contrast between the 

 cerebellar cortex and the cerebral cortex is very striking. 



The variable or individually modifiable type of reaction is 

 served chiefly by the cerebral cortex and its immediate depend- 

 encies, though some capacity of this sort is found in the brain 

 stem, as shown by the behavior of lower vertebrates which lack 

 the cerebral cortex. This type of reaction is genetically related 

 with that modifiability arising from variable internal physio- 

 logical states which we have mentioned as present in the reflex 

 centers. There is no proof that the simpler forms of this indi- 

 vidually modifiable behavior are conscious, though the higher 

 forms are certainly so. 



The cerebral cortex can in no case act independently of the 

 reflex centers of the brain stem, but always through the agency 

 of these centers. It is superposed upon them much as the cere- 

 bellum is, though the control exerted is of a very different type. 

 Here there is a very elaborate regional differentiation of the 

 cortex with an infinite complexity of associational connections. 

 The efferent pathways, moreover, are not physiologically homo- 

 geneous; but they are so diversified that any possible combina- 

 tion of the organs of response may be effected by associations 

 within the cortex. The various afferent functional systems enter 

 sharply circumscribed cortical areas (the sensory projection 

 centers); and the efferent fibers likewise leave the cortex from 

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