638 THE BRAIN. 



animal, though exhibiting no spontaneous movements, can by the application 

 of appropriate stimuli be induced to perform all, or nearly all, the move- 

 ments which an entire frog is capable of executing. It can be made to swim, 

 to leap, and to crawl. Left to itself, it assumes what may be called the natu- 

 ral posture of a frog, with the fore limbs erect and the hind limbs flexed, so 

 that the line of the body makes an angle with the surface on which it is 

 resting. When placed on its back it immediately regains this natural pos- 

 ture. When placed on a board it does not fall from the board when the 

 latter is tilted up so as to displace the animal's centre of gravity ; it crawls 

 up the board until it gains a new position in which its centre of gravity is 

 restored to its proper place. Its movements are exactly those of an entire 

 frog, except that they need an external stimulus to call them forth. They 

 differ, moreover, fundamentally from those of an entire frog in the following 

 important feature : they inevitably follow when the stimulus is applied ; they 

 come to an end when the stimulus ceases to act. By continually varying the 

 inclination of a board on which it is placed, the frog may be made to con- 

 tinue crawling almost indefinitely ; but directly the board is made to assume 

 such a position that 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 appli- 

 cation 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. Muscu- 

 lar 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 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 move- 

 ment, very many other afferent impulses are concerned in the generation 

 and coordination of the resultant motor impulses. Every bodily move- 

 ment, such as those of which we are speaking, is the work of a more or less 



