1002 WITHOUT CEREBRAL HEMISPHERES. [BOOK in. 



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 behaviour 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. W T hen pinched, or otherwise stimulated, it does 

 not crawl or leap forwards ; 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 otf 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 purposeful 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. 



