SEC. 4. ON THE PHENOMENA EXHIBITED BY AN 

 ANIMAL DEPRIVED OF ITS CEREBRAL HEMISPHERES. 



637. The cerebral hemispheres, as we have more than once 

 insisted, seem to stand apart from the rest of the brain. In the 

 case of some animals it is possible to remove the cerebral hemi- 

 spheres and to keep the animal not only alive, but in good health 

 for a long time, days, weeks, or even months after the operation. 

 In such case we are able to study the behaviour of an animal 

 possessing no cerebral hemispheres and to compare it with that of 

 an intact animal. Such an experiment is best carried out on a 

 frog. In this animal it is comparatively easy to remove the 

 cerebral hemispheres, including the parts corresponding to the 

 corpora striata, leaving behind intact and uninjured the optic 

 thalami with the optic nerves, the optic lobes (or representatives 

 of the corpora quadrigemina), the small cerebellum and the bulb. 

 If the animal be carefully fed and attended to, it may be kept 

 alive for a very long time, for more than a year for instance. 



The salient fact about a frog lacking the cerebral hemispheres, 

 is that, as in the case of a frog deprived of its whole brain, the 

 signs of the working of an intelligent volition are either wholly 

 absent or extremely rare. The presence of the bulb and the 

 middle parts of the brain (for so we may conveniently call the 

 cerebral structures lying between the cerebral hemispheres and 

 the bulb) ensures the healthy action of the vascular, respiratory 

 and other nutritive systems ; food placed in the mouth is readily 

 and easily swallowed ; the animal when stimulated executes various 

 movements ; but if it be left entirely to itself, and care be taken to 

 shield it from adventitious stimuli, either it remains perfectly and 

 permanently quiescent, or the apparently spontaneous movements 

 which it carries out are so few and so limited as to make it very 

 doubtful whether they can fairly be called volitional. Such a frog, 

 for instance, after being kept alive for some time and made to 

 exhibit the phenomena of which we are about to speak, has been 

 placed on a table with a line drawn in chalk around the area 

 covered by its body, and left to itself has subsequently been found 

 dead without having .stirred outside the chalked circle. 



