WITHOUT CEREBRAL HEMISPHERES. 637 



adjuncts, and this, as we shall see, constitutes a nervous machinery capable 

 of carrying out exceedingly complicated acts. 



ON THE PHENOMENA EXHIBITED BY AN ANIMAL DEPRIVED OF ITS 

 CEREBRAL HEMISPHERES. 



550. 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 ani- 

 mals it is possible to remove the cerebral hemispheres and to keep the ani- 

 mal 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 

 behavior 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, leav- 

 ing behind intact and uninjured the optic thalami with 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 con- 

 veniently call the cerebral structures lying between the cerebral hemispheres 

 and the bulb) insures the healthy action of the vascular, respiratory, and 

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

 lowed ; 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 appar- 

 ently 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 out- 

 side the chalked circle. 



We must here, however, repeat the caution laid down in 495, as to the 

 ultimate effects of an operation on the central nervous system. The longer 

 the frog is kept alive and in good health after the removal of the cerebral 

 hemispheres, the greater is the tendency for apparently spontaneous move- 

 ments to show themselves. For days, or even weeks, after the operation 

 there may be no signs whatever of the working of any volition ; but after 

 the lapse of months, movements, previously absent, of such a character as 

 to suggest that they ought to be called voluntary, may make their appear- 

 ance. To this point we shall return, but may, in the meanwhile, state that 

 even in their most complete development such movements do not negative 

 the view that the frog in the absence of the cerebral hemispheres is want- 

 ing in what we ordinarily call a " will." 



551. We have seen that a frog from which the whole brain has been 

 removed, and the spinal cord only left, appears similarly devoid of a "will;" 

 but the phenomena presented by a frog possessing the middle portions of the 

 brain differ widely from those presented by a frog possessing a spinal cord 

 only. We may, perhaps, broadly describe the behavior of a frog from which 

 the cerebral hemispheres only have been removed by saying that such an 



