CHAP, ii.] THE BRAIN. 721 



the body of the frog is in equilibrium, the crawling ceases ; and 

 if the position be not disturbed the animal will remain impassive 

 and quiet for an almost indefinite time. When thrown into 

 water, the creature begins at once to swim about in the most 

 regular manner, and will continue to swim until 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. 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 musi- 

 cal, 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 particu- 

 lar 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 mus- 

 cles and constituting the foundation of the muscular sense ; they 

 are also directed by means of afferent impulses passing cen- 

 tripetally 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 genera- 

 tion 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 



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