140 Leytoii and Sherrington 



veniniT whicli nia\- cuhniuate in complete reversal of the sense of the 

 movement of the response, e.g. extension of a joint may become flexion of 

 that joint. Deviation of response is a change which alters the character 

 of the response, so that instead of the original movement appearing, some 

 other mo\'ement, e.g. of another joint or part, appears in place of the 

 orio-inal. All the.se changes are temporary. They may be taken as 

 expressions of what has been termed the functional instability of a cortical 

 motor point (5). 



1. Facilitation of Response. — Facilitation of response has been a 

 usual accompaniment of our observations on the anthropoid motor cortex. 

 Comparing our experience of it there with our experience of it in macaque 

 and calothrix, facilitation seems to be somewhat more extensive in the 

 anthropoid than in the lower forms of monkey. Thus, as was remarked 

 in our preliminar}^ communication, it affects the delimitation of the whole 

 of the anterior border of the motor field. That border is not of sharp 

 and abrupt edge, but seems to fade off forward rather gradually. Facili- 

 tation makes it extend farther forward than it does without facilita- 

 tion. Thus if the anterior border is delimited by stimulating series of 

 cortical points in succession from behind forward, the anterior limit of 

 the field is found to lie farther anterior than if determined by stimulating 

 a series of points starting well in front of the limit and followed from 

 before backward. In a similar way the boundary of the area for any 

 particular movement may by facilitation be extended beyond its average 

 limit ; in this latter case, deviation of response comes in as well. 



2. Reversal of Response. — The mutual influence exerted by points 

 moving the same joint but in opposite directions was dealt with in the 

 paper by T. Graham Brown and one of us (5). 



3. Deviation of Response. — A cortical point can also influence the 

 motor response of another whose response is neither diametrically opposed 

 to nor identical with or very closely similar to its own. Thus : chimpanzee 

 (19), left hemisphere (fig. 14), leg area. Point 319 gave regularly as 

 response plantar flexion of ankle, follow^ed by flexion of all toes except 

 hallux, followed further by adduction of hallux. That was its response 

 when first stimulated in the experiment. It was the fifth point stimu- 

 lated in the experiment, and was then stimulated next after a point 41 

 in face area, which yielded movement 41 (see list) ; and point 41 had 

 been stimulated next after one in arm area, which yielded movement 232. 

 From time to time in the course of the experiment point 319 was i-eturned 

 to from distant points, and gave as response regularly movement 319 and 

 no other. 



Point 342, similarly stimulated, was giving with like regularity move- 

 ment 342, flexion of hip followed by adduction of hip. When point 319 

 was stimulated immediately after point 342 had been stimulated, time being 

 allowed, however, for the movement evoked from 342 to subside completely, 

 the movement given by 319 was no longer movement 319. It then gave 



