CHAPTER XCVI 

 CEREBRAL LOCALIZATION (Cont'd) 



SENSORY CENTERS 



That the motor centers are located in the areas which we have just 

 described does not indicate that the nerve cells of the centers normally 

 dominate the reflex movements which their stimulation elicits. The motor 

 centers, strictly speaking, are the anterior horn cells of the spinal cord ; 

 and the so-called motor centers of the cerebral cortex must really repre- 

 sent nothing more than internuncial neurons between the entering and 

 leaving paths concerned in reflex movements. They are only links in 

 the long cerebral chain way-houses on the reflex cerebral pathway. 

 According to this view we should expect that these centers would be 

 the ultimate recipients of sensation, as well as the distributors of motor 

 impulses; sensorimotor, they have been called. Such, however, is not 

 the case, for Sherrington has shown that the centers most directly con- 

 cerned in the reception of sensory impulses are not located in front of 

 the Rolandic fissure but immediately behind it in the ascending parietal 

 or postcentral convolution. Electrical stimulation in this region does 

 not evoke any immediate response, at least if the stimulus is not too 

 strong. A movement indirectly due to the receipt of a sensation may 

 be elicited by a strong stimulus, just as is the case when the visual cen- 

 ter in the occipital lobe is strongly stimulated, producing secondary 

 movements of the eyes. 



Histologic, experimental and clinical evidence has been furnished to 

 support this location of the chief sensory center. The clinical evidence 

 was furnished by Harvey Gushing, 14 who induced two patients in whom 

 this part in the brain was exposed to allow him to stimulate it while they 

 were in a conscious state. As the result of the stimulation of the post- 

 central convolution definite sensory impressions were experienced, consist- 

 ing of a sensation of numbness or deadness to tactual impressions, but 

 no muscular groups underwent movement unless the precentral con- 

 volution was stimulated. During these movements, moreover, no sen- 

 sations were experienced by the patient except those which accompanied 

 the change in the position of the part that was moved. The sensations 

 which arc thus represented on the cortex are those of touch discrimina- 



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