CHAPTER Oil 



THE INTEGRATIVE ACTION OF THE CEREBRUM 



The motor areas of the cerebral cortex are connected through the 

 pyramidal tracts with the various lower motor neurons of the body. 

 We have seen that the excitation of a localized area in the motor cor- 

 tex may bring about a response of some localized group of muscles 

 which is coordinated, involves recriprocal inhibition, and develops 

 through an orderly march in much the same way as does the reflex 

 figure which arises from a cutaneous stimulation. These responses 

 utilize the same motor mechanisms as the spinal reflexes do, and con- 

 sequently come in competition with them for use of the final common 

 paths. Consequently cerebral influences may modify profoundly re- 

 flex responses, reinforcing them when both affect the motor mechanism 

 in the same way, inhibiting them when their actions are antagonistic. 

 The inhibitory aspects of the cerebral influence are particularly prom- 

 inent, and consequently many reflexes are elicited with greater cer- 

 tainty in animals from which the cerebrum has been removed. In this 

 competition probably much the same factors determine which influence 

 shall control the common path as govern which of two antagonistic 

 spinal reflexes shall prevail. Cerebral influences are apparently prepo- 

 tent over all but the most intense reflex responses to harmful stim- 

 uli and those which result from strongly affective sensations. We can, 

 for example, inhibit the reflex withdrawal of the hand from hot water 

 unless the pain is particularly intense. On the other hand it is diffi- 

 cult to refrain from winking when the cornea becomes irritated. Within 

 limits the respiratory reflex may be controlled by the will, but when 

 the stimulus to the respiratory center becomes intense, the breath 

 can no longer be held. A large group of reflex arcs concerned with 

 the regulation of the visceral organs are not connected with the paths 

 from the cerebrum and consequently cannot be brought under vol- 

 untary control. 



In describing the motor areas in the cortex it was pointed out that 

 these were points at which many neurons from widely separated parts 

 of the brain converge upon paths which lead to the lower motor neu- 

 rons and their muscles. The pyramidal fibers consequently are com- 

 mon paths used in many diverse volitional responses. For their con- 

 trol many different influences come into competition and the unpre- 

 dictable nature of volitional response is no doubt due to the impos- 



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