1000 WITHOUT CEREBRAL HEMISPHERES. [BOOK in. 



We must here however repeat the caution laid down in 

 582, as to the ultimate effects of an operation on the central 

 nervous system. The longer the frog is kept alive and in good 

 health after the removal of the cerebral hemispheres, the greater 

 is the tendency for apparently spontaneous movements to shew 

 themselves. For days or even weeks after the operation there 

 may be no signs whatever of the working of any volition ; but 

 after the lapse of months, movements, previously absent, of such 

 a character as to suggest that they ought to be called voluntary, 

 may make their appearance. To this point we shall return, but 

 may in the meanwhile state that even in their most complete 

 development such movements do not negative the view that the 

 frog in the absence of the cerebral hemispheres is wanting in 

 what we ordinarily call a 'will/ 



638. We have seen that a frog from which the whole brain 

 has been removed and the spinal cord only left appears similarly 

 devoid of a 'will;' but the phenomena presented by a frog 

 possessing the middle portions of the brain differ widely from 

 those presented by a frog possessing a spinal cord only. We may 

 perhaps broadly describe the behaviour of a frog from which the 

 cerebral hemispheres only have been removed, by saying that such 

 an animal, though exhibiting no spontaneous movements, can by 

 the application of appropriate stimuli be induced to perform all 

 or nearly all the movements 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 natural 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 posture. 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 impor- 

 tant 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 continue 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 



