722 WITHOUT CEKEBKAL HEMISPHERES. [BOOK m. 



we have here no occasion to deal, it is evident that in the frog 

 deprived of its cerebral hemispheres all these factors are pres- 

 ent, 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 there- 

 fore which the cerebral hemispheres take in executing the move- 

 ments 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 in- 

 crease, or to curb, or to stop activity already present. We might 

 almost speak of the will as an intrinsic stimulus. Its opera- 

 tions 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 cere- 

 bral hemispheres by applying such stimuli as shall throw pre- 

 cisely 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. When 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 off the board like a lifeless mass. Though, 

 as we have seen, the various parts of the spinal cord of the f r< >g 

 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. 





