218 Ley ton and Sherrington 



were still responding well, the spinal cord was exposed at the 4th thoracic 

 segment, and the right lateral half carefully severed. Subsequent micro- 

 scopic examination of the semi-section proved it to have been an accurate 

 one, the whole right half being severed, with a slight trespass only into 

 the left side in the dorsal column. Stimulation of the left cortex evoked 

 after this lesion unaltered responses from face and arm area, but no 

 response at all fi-om trunk area or leg area. Responses from right cortex 

 as before the serai-section. A second right side semi-section of the cord 

 midway between 3rd and 4th cervical roots was then made. Microscopic 

 examination subsequently showed that in this semi-section the mesial part 

 of right ventral white column escaped severance. Stimulation of the left 

 hemisphere's face area after this second semi-section evoked facial move- 

 ments as before, but stimulation of the arm area evoked no intrinsic arm 

 movements, although responses in trapezius and rhomboids were obtained 

 from it. Responses from right hemisphere remained unaltered. 



X. Summary of Conclusions. 



1. The "motor" area of the cortex in the three species of anthropoid 

 examined (chimpanzee, orang-utan, and gorilla), as determined by faradisa- 

 tion, embraces almost all of the free surface and a large part of the sulcal 

 surfaces of gyrus centralis anterior ; it also extends over the mesial border 

 upon gyrus marginalis for a distance about half-way toward sulcus cinguli, 

 in agreement with Campbell's delimitation of his " precentral type" of 

 cortex in chimpanzee and orang. 



2. The proportion of motor area buried in the sulci is probably usually 

 about one-third of the whole area. 



3. Although the broad " localisation " of the responses of the various 

 main motor parts of the opposite half of the body follows a well-fixed 

 topographical scheme in this cortex, the minuter localisation, as examined 

 by faradisation, is subject to temporal instability. 



4. This instability is largely the expression of mutual influences exerted 

 transiently by the physiological states for the time being of different points 

 of the motor cortex, and of the sub-cortical centres they connect with, one 

 upon another. These influences make themselves felt as " deviation of 

 response," " reversal of response," and " facilitation," phenomena all seem- 

 ingly akin. 



5. Subject to this temporal instability, details of localisation of various 

 movement groups in chimpanzee, orang, and gorilla are described. Differ- 

 ences in the smaller details of the localisation were met with from in- 

 dividual to individual of the same species, and between the right and left 

 motor areas of the same individual. Making all allowance for experi- 

 mental error, these diflTerences, in some particulars, seem too large to be 

 accounted for fully by that or by temporal instability of the cortex ; they 

 may represent, as Franz urges for analogous differences he found in 



