The Excitable Cortex of the Chimpanzee, Orang-Utan, and fJorilla 169 



been nitne comnion than a partly upward one. To evoke these eyeball 

 movements, whether purely lateral or not, requires, in our experience, 

 stron«;er stimuli than are required for motor responses from precentralis 

 motor area, and the responses even under these stronger stimuli are not 80 

 regularly obtainable as are the responses from precentralis area (33). Nor 

 do the points yielding them form, in our experience, a seemingly continuous 

 field of excitable surface either in the occipital or in the frontal regions. 

 The movements when obtained have further a slow deliberate development 

 (39) usually, distinguishing them somewhat from the limb, face, and eye- 



FiG. 14. — Examples of "deviation of response" from a chimpanzee 

 hemisphere, chimpanzee 19. Some only of the responses obtained 

 are mapped. Left hemisphere. 



lid-closure movements evocable from precentralis region. The movement 

 is usually bilaterally symmetrical, but not rarely the ipsilateral eyeball 

 lags somewhat behind the contralateral. Very occasionally we have seen 

 convergence of the eyeballs occur, sometimes very markedly, and as 

 though to fixate a point approximately in a plane continuous with the 

 sagittal plane of the head. After the lateral conjugate deviation has 

 been obtained, it has been usual for the eyes to remain for some time in 

 the posture thus assumed, and to return very slowly toward the primary 

 straight-forward posture after the stimulus has been withdrawn. 



Quite exceptionally we have seen movement of the eyeballs produced by 

 stimulation of the precentralis motor field. The movement has not been 

 primary ; it has accompanied turning of the neck, carrying the face to the 

 opposite side, and the region which has yielded it has been that of genu 



