The Excitable Cortex of tho Chimpanzee, Orang-Utan, and Gorilla 171 



in some of our .specimens to be larger even than that for thumb. Exten- 

 sion was a much more common movement than llexion, but isolated flexion 

 was sometimes evoked. Extension of index as an isolated UKJVement with- 

 out any motion in thumb or other fingeis was on very many occasions 

 readily obtained. A not infrequent respon.se from cortex when the resting 

 posture happened to be one with adducted thumb and semi-flexed index 

 was simultaneous abchiction of tinnnb and .straiirhteninir of index as if to 



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FlG. l.^. — Diagrams to illustrate the size and shape of the "motor" cortex of the 

 chimpanzee as determined by faradic stimulation. A, shows the extent as observed 

 on the free surface; B, shows the extent and sliape as observed wlion the portions 

 buried in the sulci are added to that on free surface. The line running through B 

 marks the position of the sulcus centralis; all the shaded area beliind that line 

 rej)resents the part of the "motor" surface buried in tliat fissure. The stippling 

 to left of the shaded area in each diagram indicates the gradual fading off of excitable 

 area in the anterior direction, making the actual demarcation of the motor field in 

 that direction a somewhat arbitrary one. The smallest squares on the map represent 

 mm.^ of actual cortex surface in the specimen mapi)ed. 



let go an object that had been picked up. Occasionally isolated extension 

 of the terminal and 2nd phalanx only of index was evoked. In some 

 chimpanzees we noted the using the singly extended index finger for 

 various purposes to be habitual ; thus, for picking the teeth after eating, 

 for getting up a rice or maize-grain between boards of the cage floor. 

 The marked individuality of this finger's representation in cortex stands 

 in harmony with such habituation. As an occasional response from cortex 

 we saw extension of index finger accompanied by flexion of the three other 

 lingers, no movement at all occurring in thumb. 



Other Fino-ers. — Isolated movements of other individual finorers were, 

 as might be expected, much less elicitable. I.solated flexion of middle finger 



