CHAP, vi.] THE BRAIN. 609 



begins at once to swim about in the most regular manner, and will 

 continue to swim till it is exhausted, if there be nothing present on 

 which it can come to rest. If a small piece of wood be placed on 

 the water the frog will when it comes in contact with the wood 

 crawl upon it, and so come to rest. Such a frog, if its flanks be 

 gently stroked, will croak ; and the croaks follow so regularly and 

 surely upon the strokes that the animal may almost be played up- 

 on like a musical instrument. Moreover, the movements of the 

 animal appear to be influenced by light ; if it be urged to move in 

 any particular direction, it seems to avoid in its progress objects 

 casting a strong shadow. In fact, even to a careful observer the 

 differences between such a frog and an entire frog which was 

 simply very stupid or very obstinate, would appear slight and 

 unimportant except in one point, viz. that the animal without its 

 cerebral hemispheres was obedient to every stimulus, and that 

 each stimulus evoked an appropriate movement, whereas with the 

 entire animal it would be impossible to predict whether any result 

 at all, and if so what result, would follow the application of this or 

 that stimulus. Both are machines ; but the one is a machine and 

 nothing more, the other is a machine governed and checked by a 

 dominant volition. 



Now such movements as crawling, leaping, swimming, and in- 

 deed, to a greater or less extent, all bodily movements, are carried 

 out by means of coordinate nervous motor impulses, influenced, 

 arranged, and governed by coincident sensory or afferent impulses. 

 We have already seen that muscular movements are determined 

 by the muscular sense ; they are also directed by means of sensory 

 impulses passing centripetally along the sensory nerves of the skin, 

 the eye, the ear, and other organs. Independently of the afferent 

 impulses, which acting as a stimulus call forth the movement, all 

 manner of other afferent impulses are concerned in the generation 

 and coordination of the resultant motor impulses. Every bodily 

 movement such as those of which we are speaking is the work of a 

 more or less complicated nervous mechanism, in which there are 

 not ^only central and efferent, but also afferent factors. And, 

 putting aside the question of consciousness, 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. The share therefore which the cerebral hemispheres 

 take in executing the movements of which the entire animal is 

 capable, is simply that of putting this machinery into action. The 

 relation which the higher nervous changes concerned in volition 

 bear to this machinery is not unlike that of a stimulus. We 

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

 operations are limited by the machinery at its command. The 

 cerebral hemispheres in their action can only give shape to a 



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