The Excitable Cortex of the Chimpanzee, Orang-Utan, and Gorilla 209 



was obtained. In many experiments the whole length of post-central is was 

 systematically explored, in some only a part of it, namely, that corresponding 

 with face area or arm or leg area of the precentralis. Stronger faradisa- 

 tion of post-centralis also failed to give motor responses; indeed fre(iuentl}- 

 the strength of faradism applied to post-centralis was carried far beyond 

 the strength ordinarily permissible for reliable physiological observations, 

 and still (juite failed to evoke any detectable effect. 



Faradism of the cortex of the centralis posterior convolution, though 

 not itself eliciting movement, does, however, when employed at certain 

 places, facilitate sometimes, as was mentioned in our preliminary communi- 

 cation, the elicitation of movement by faradisation at certain points in 

 about the same horizontal level of the precentral convolution. In other 

 words, from certain parts of the post-central gyrus a facilitating influence 

 can be exerted upon somewhat adjacent parts of the precentral gyrus. 

 This observation has been confirmed for the human brain by C. K. MilU 

 (27, 28, 29). In our observations on the anthropoid we met the phenomenon 

 especially in the region below the inferior genu ; for instance, stimulation 

 of a point of centralis posterior close to the central sulcus facilitated 

 response of a point opposite to it in precentral gyrus yielding movement 

 of the contralateral side of the lips. 



Under certain special circumstances faradism may at times evoke, as 

 was mentioned in our first preliminary communication, reactions from the 

 post-central gyrus itself, though the conditions are sufficiently different 

 from those which obtain for elicitation of responses from the centralis 

 anterior to exclude the centralis posterior being accepted as equivalent 

 to centralis anterior cortex. When the centralis posterior near to the 

 central fissure is faradised immediately after elicitation of a motor response 

 from centralis anterior at a point in the latter lying about opposite the 

 point faradised in centralis posterior, the motor response obtained from 

 the centralis anterior may reappear, and this even a few times in succes- 

 sion, though not for many unices centralis anterior be restimulated. This 

 " echo-response " is a phenomenon of considerable constancy. Our observa- 

 tions on it were made chiefly in the region of the inferior genu and below 

 that, and with motor responses in lips, thumb, or index finger. Graham 

 Brown (4, 5) ^ has, independently of us, observed the phenomenon in 

 regard to flexion of the arm, and in small monkeys macacus and cerco- 

 pithecus as well as in chimpanzee. We met with it in all of the three 

 anthropoid apes. We have been inclined to regard it as analogous to a 

 phenomenon met with along the anterior confines of the motor region. 

 The anterior limit of the excitable region as examined by faradism seems 

 to merge somewhat graduatim into the inexcitable surface beyond it. 

 Towards the end of a lengthy stimulation-experiment, when the region 

 has been repeatedly stimulated at many points, the anterior limit of it 

 seems not rarely to extend forward farther that it had done at first. Re- 

 1 Journ. of Physiol., 1914, xlviii. p. xxx ; Quart. Journ. of Exper. Physiol., 1915, ix. 82. 



