WITHOUT CEREBRAL HEMISPHERES. 639 



complicated nervous mechanism, in which there are not only central and 

 efferent but also afferent factors. And, putting aside the question of con- 

 sciousness, with which we have here no occasion to deal, it is evident that in 

 the frog deprived of its cerebral hemispheres all these factors are present, 

 the afferent no less than the central and the efferent. The machinery for all 

 the necessary and usual bodily movements is present in all its completeness. 

 We may regard the share, therefore, which the cerebral hemispheres take in 

 executing the movements of which the entire animal is capable, as that of 

 putting this machinery into action or of limiting its previous activity. The 

 relation which the higher nervous changes concerned in volition bear to this 

 machinery may be compared to that of a stimulus, always bearing in mind 

 that the effect of a stimulus on a nervous centre may be either to start 

 activity or to increase or to curb or to stop activity already present. We 

 might almost speak of the will as an intrinsic stimulus. Its operations are 

 limited by the machinery at its command. We may infer that in the frog 

 the action of the cerebral hemispheres in giving shape to a bodily movement 

 is that of throwing into activity particular parts of the nervous machinery 

 situated in the lower parts of the brain and in the spinal cord ; precisely the 

 same movement may be initiated in the absence of the cerebral hemispheres 

 by applying such stimuli as shall throw precisely the same parts of that 

 machinery into the same activity. 



Very marked is the contrast between the behavior of such a frog which, 

 though deprived of its cerebral hemispheres, still retains the other parts of 

 the brain, and that of a frog which possesses a spinal cord only. The latter 

 when placed on its back makes no attempt to regain its normal posture ; in 

 fact, it may be said to have completely lost its normal posture, for even when 

 placed on its belly it does not stand with its fore feet erect, as does the other 

 animal, but lies flat on the ground. When thrown into water, instead of 

 swimming it sinks like a lump of lead. When pinched or otherwise stimu- 

 lated it does not crawl or leap forward ; it simply throws out its limbs in 

 various ways. When its flanks are stroked it does not croak ; and when a 

 board on which it is placed is inclined sufficiently to displace its centre of 

 gravity it makes no effort to regain its balance, but falls off the board like a 

 lifeless mass. Though, as we have seen, the various parts of the spinal cord 

 of the frog contain a large amount of coordinating machinery, so that the 

 brainless frog may, by appropriate stimuli, be made to execute various pur- 

 poseful coordinate movements, yet these are very limited compared with 

 those which can be similarly carried out by a frog possessing the middle and 

 lower parts of the brain in addition to the spinal cord. It is evident that a 

 great deal of the more complex machinery of this kind, especially all that 

 which has to deal with the body as a whole, and all that which is concerned 

 with equilibrium and is specially governed by the higher senses, is seated 

 not in the spinal cord but in the brain. We do not wish now to discuss the 

 details of this machinery ; all we desire to insist upon at present is that in 

 the frog the nervous machinery required for the execution, as distinguished 

 from the origination, of bodily movements even of the most complicated kind, 

 is present after complete removal of the cerebral hemispheres, though these 

 movements are such as to require the cooperation of highly differentiated 

 afferent impulses. 



552. In warm-blooded animals the removal of the cerebral hemi- 

 spheres is attended with much greater difficulties than in the case of the 

 frog. Nevertheless, in the bird the operation may be carried out with 

 approximate success. Pigeons, for instance, have been kept alive for five 

 or six weeks after complete removal of the cerebral hemispheres, with the 

 exception of portions of the crura and corpora striata immediately sur- 



