CHAP. IL] THE BRAIN. 1001 



the wood, crawl upon it, and so come to rest. If disturbed from 

 its natural posture, as by being placed on its back, it immediately 

 struggles to regain that posture; only by the application of 

 continued force can it be kept lying on its back. 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 upon like a musical, or at least an acoustic instrument. 

 Moreover, provided that the optic nerves and their arrangements 

 have not been injured by the operation, the movements of the 

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

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

 obstacles, at least such as cast a strong shadow ; it turns its course 

 to the right or left or sometimes leaps over the obstacle. In fact, 

 even to a careful observer the differences between such a frog and 

 an entire frog which was simply very stupid or very inert, would 

 appear slight and unimportant except in this, that the animal 

 without its cerebral hemispheres is obedient to every stimulus, 

 and that each stimulus evokes an appropriate movement, whereas 

 with the entire animal it is impossible to predict whether any 

 result at all, and if so what result, will follow the application of 

 this or that stimulus. Both may be regarded as 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 

 indeed, as we have already urged, 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. Muscular movements 

 are determined by afferent influences proceeding from the muscles 

 and constituting the foundation of the muscular sense ; they are 

 also directed by means of afferent impulses passing centripetally 

 along the sensory nerves of the skin, the eye, the ear, and other 

 organs. Independently of the particular afferent impulses, which 

 acting as a stimulus call forth the movement, very many 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. 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 



