The Excitable Cortex of the Chimpanzee, Orang-Utan, and Goiilhi IGo 



draws a sharp distinction between eyelid movements of closure and of 

 openinjT respectively. This is the more strikini^, because movements of 

 opposite sense implicatin;^^ one and the same part, e.<^. thumb, index, ankle, 

 wrist, tonfjue, lips, even elbow and Jaw, are not easily or even commonly 

 8eparal)le in the cortex by reason of their foci of representation lying con- 

 siderably remote one from the other. But the fields for eye-opening and 

 eye-closure respectively do lie considerably separated apart. 



The great field of excitable cortex which lies open to examination in 

 the free surface of gyrus precentralis may be termed the precentralis motor 

 field ; and we may include under tliat term the whole of the seemingly 

 continuous field of motor points wliich occupies as well as the free face of 

 gyrus precentralis the adjoining portions of sulcus centralis and of sulci 

 precentrales and parts of the free faces of the gyri annectantes connecting 

 gyrus precentralis with the frontal convolutions. Among all the numerous 

 and varied movements which faradic stimulation applied at the appropriate 

 points evokes from this great field, opening of the eye does not appear to 

 be included, neither do movements of the eyeball. But closure of the eye 

 is well and definitely included among the movements elicitable from 

 precentralis field. 



Closure of Eye. — The place in that field wliich yields eye-closure lies at 

 the level of, and extends a little above, and to a wider extent below, genu 

 inferius of sulc. centralis. It meets, as examined by the electrode, 

 the lowest points of hand area (thumb) above ; it is intimately adjunct to 

 areas for ear, nostril, neck, and lip, occupying part of the upper portion of 

 face area, and is traceable with the electrode into tongue area. The eye- 

 closure is obtainable with faradic stimuli of the same strength as suffice 

 for other motor responses from precentralis and with the s<ame readiness. 

 The movement may be (e.g. with weak stimuli) restricted to closure of 

 opposite eye only, or even to isolated movement of the upper or lower lid 

 only of that eye. With moderate stimuli the closure is of both eyes, but 

 practically always is more vigorous in the opposite eye. The closure 

 sometimes has the appearance when the animal is not too deeply narcotised 

 of being executed against the animal's will, for it occurs while the other 

 eye remains almost open, and on withdrawal of the cortical faradisation 

 the contralateral eye, as also the less closed ipsilateral, re-opens again 

 immediately and quickly. 



Opening of the Eye. — This movement is observable under stimu- 

 lation of the cortex in various widely separated regions. It may occur, so 

 to say, in a desultory manner, and, in our experience, is prone to crop 

 up unexpectedly. But in two regions it occurs fairly regularly ; these 

 regions are a frontal area anterior to the lower half of precentralis gyrus, 

 and an occipital region including the calcarine area and the occipital pole. 



Taking the second region first, the opening of eyes elicitable thence is 

 clearly associated with a turning movement of the eyeballs toward the 

 opposite side. The eye-opening, like the eye-turning which it accompanies 



