The Excitable Cortex of the Chimpanzee, Orang-Utan, and Gorilla 177 



region examined at one time and by one series of stimulations may not 

 ajn'ec in detail with that obtained from the same motor reirion at another 

 time and under another series uf stimulations. But the ditlerences bt-twecn 

 the charts fi'om ditierent individuals in our experiments seem too wide in 

 most instances to be accounted for by merely temporal fluctuations of 

 response, such as localisation experiments somewhat ditlerently conducted 

 micrjit evoke from one and the same hemisphere. Inspection of the charts 

 reproduced exhibits the scale of difierence observed better than can verbal 

 description. We regard them as indicating that individual variation of the 

 functional topography of the motor cortex, as found by Franz (17) in 

 Macacus rhesus, is demonstrable in the anthropoid species examined by 

 us, and in at least as li])eral measure. 



The list of motor responses taken as a whole shows that a very con- 

 siderable number of different movements are obtainable from the motor 

 cortex of the anthropoid, far more than can be obtained from the dog or 

 macaque. Although of these a very large proportion may crop up in any 

 single systematically conducted point-to-point examination of a single 

 hemisphere, many of them do not. From our experience we imagine that 

 had our experiments extended to a larger number of hemispheres, the list, 

 which continually slowly grew in length as our experiments proceeded, 

 might have grown a great deal farther. Another point obvious from the 

 bare memoranda in the list, but still more obvious to inspection of the 

 movements as they occurred at the time, is that the individual movements, 

 elicited by somewhat minutely localised stimulations, are, broadly speaking, 

 fractional, in the sense that each, though co-ordinately executed, forms, so 

 to say, but a unitary part of some more complex act, that would, to attain 

 its purpose, involve combination of that unitary movement with others to 

 make up a useful whole. In evidence of this " fractional " character it is 

 only necessary to note the predominantly unilateral character, as elicited 

 from the cortex, of movements that under natural circumstances are 

 symmetrically bilateral. Thus under cortical stimulation even such move- 

 ments as contraction of the fauces, adduction of the vocal cords, closing 

 and opening of the jaws, protrusion of anus, were often, indeed usually, 

 detectibly asymmetrical, the execution being chiefly or wholly in some 

 cases by the muscles of the contralateral side. A further point evident 

 from the list is the considerable variety of combination into which these 

 fractional movements were welded in the movement-sequences noted. Our 

 main purpose being " localisation " of the primary movement, we did not 

 usually, by pressing and prolonging any single stimulation, develop these 

 sequences in our observations. Had that been done, the listed variety of 

 them would doubtless have been greatly increased. Their variety, how- 

 ever, even in the list obtained, indicates that a property possessed by the 

 cortex is the combining of a large, though exhaustible, number of move- 

 ments, belonging to this and that restricted portion of limb, face, or other 

 motile part, into sequences of very great variety, sequences in which 



