The Excitable Cortex of the Chimpanzee, Orang-Utan, and Gorilla 189 



the previous two experiments. The right was now exposed at the level of 

 inferior genu of sulcus centralis. The field of cortex exposed embraced 

 about a third of the length of both precentral and post-central gyri, and 

 the exposed portions of these were carefully faradised from point to point. 

 From post-centralis the faradisation never evoked any responsive move- 

 ment, despite repeated trials, often with very strong currents applied both 

 with the unipolar and with the bipolar electrodes. Precentralis, on the 

 contrary, was an " excitable " field continuously throughout the whole 

 portion of its length exposed to view, i.e. from 5 mm. below superior genu 

 to about 6 mm. below inferior genu. The topography of a number of the 

 points yielding the movements and the movements so obtained were re- 

 corded and mapped (fig. 5, B). It is noted that in proceeding with the 

 application of stimuli to a series of points taken in successive order from 

 above downward, i.e. passing away from mid-line responsive, movement of 

 elbow could be evoked as a " leading " (primary) movement quite down to 

 genu inferius; whereas when the successive stimuli to the points pro- 

 ceeded in the opposite direction (upward), movement of index and thumb 

 was evoked as the "leading" (primary) movement, even when the stimulus 

 reached points up in the shoulder area of the cortex near genu superius. 

 From a narrow area just below genu inferius there was elicited closure 

 of the lids of the opposite eye unaccompanied by any other movement. 

 Careful search was made for evidence of movement in the right arm on 

 stimulation of this the cortical area for the left arm, in order to test the 

 supposition that the recovery of the right arm movements might be ex- 

 plicable by supplementary functions for right arm taken over by the 

 cortical field for left arm. Even with very strong and diff'use (widespread 

 bipolar electrodes) stimulation, let alone moderate and weak with the uni- 

 polar electrode, never was any trace of movement of right arm evoked by 

 excitation of this the motor arm area of the right hemisphere. The move- 

 ments elicited in left arm were, however, very various and vigorous. 

 Finally, the whole of the area which under faradisation had provoked 

 "leading" (primary) movement in fingers, thumb, wrist, and elbow was 

 then extirpated by the knife to a depth of about 8 mm., and the floor of 

 the ablated area cauterised superficially with the electro-cautery. The 

 wound was then closed with full aseptic procedure (fig. 5, B, limit lines 

 of lesion shown). 



On recovering from the operation narcosis the animal became some- 

 what excited and very active in its movements. Not the slightest re- 

 crudescence of symptoms of paresis and clumsiness in the right arm was 

 detected. In the left arm the opportunity for seeing impairment of move- 

 ment was excellent, because the animal (and the same condition was re- 

 peatedly noticed in other experiments on cortical ablation) was evidently 

 slow to realise that he could not use the limb as before. The animal spent 

 nearly five minutes in trying to pick up individual grape-berries placed on 

 the floor of his cao-e. There was evident inability to move thumb and 



