210 Ley ton and Sherrington 



iterated faradisation of a point, e.g. close in front of one regularly yielding 

 thumb movement or finger movement, will ultimately sometimes yield that 

 movement, especially if faradised directly after the point yielding it regu- 

 larly^ has responded. The posterior boundary of the motor area lies buried, 

 as we have shown, within sulcus centralis. In some specimens, and in 

 some parts of the length of the sulcus, the limit seems to correspond pretty 

 exactly with the floor of the fissure ; but it seems to vary, and in some 

 specimens it lies up the posterior wall of the fissure. Points on the free 

 face of the post-centralis and near to the lip of the fissure are, therefore, 

 not very distant from the posterior limit of motor area itself. 



In two cases, one of them including both hemispheres, we ablated the 

 arm area back to sulcus centralis, and in none of these, neither immediately 

 after the ablation nor subsequently at periods amounting in the longest case 

 to four months from the ablation, were we able to evoke motor responses 

 by any means from the face of centralis posterior for that length of it 

 corresponding with the centralis anterior ablation. In two cases we 

 ablated that portion of leg area which lies on the external face of the 

 hemisphere back to sulcus centralis, and in those cases also neither immedi- 

 ately after the ablation nor twenty-one and thirty-three days later re- 

 spectively did faradisation of the corresponding portion of gyrus post- 

 centralis evoke any motor response. In one case (ablation-experiment 8) 

 we ablated the facial area of precentralis back to the sulcus centralis, and 

 in that case likewise faradisation failed to elicit either immediately after 

 the ablation or twenty days later any motor response from the correspond- 

 ing region of centralis posterior. 



2. Ablation. 



We have made three experiments (ablation-experiments 9, 10, 11) on 

 ablation of gyrus post-centralis, all partial, all on the chimpanzee, and all 

 in the left hemisphere. In one the gyrus was removed with the knife from 

 opposite the superior genu above to a little below the inferior genu below. 

 The anterior limit of the ablation was sulcus centralis itself, and the whole 

 width of the gyrus was removed. Prior to the ablation the gyrus was 

 faradised from point to point systematically in the usual way, and yielded 

 no motor or other detected results. 



On recovering from the operative chloroformisation the animal became 

 somewhat excited, and observation of its movements revealed no paresis ; 

 it climbed about the cage apparently with normal facility, and picked up 

 grapes and other food with the right hand as with the left, and carried 

 the food to its mouth seemingly equally well with either hand. Nor was 

 there any obvious sign of asymmetry of the face or paresis of the lips or 

 eyelids. No abnormality of skin sensitivity was detected, but such ex- 

 amination was difficult, as the animal was not tame. 



A week later, no paresis having in the meantime been manifest, the 

 animal was again chloroformed, and the whole of tlie precentral gyrus, 



