720 WITHOUT CEKEBRAL HEMISPHEEES. [BOOK m. 



few and so limited as to raise the question whether they can 

 fairly be called volitional. Such a frog, for instance, after being 

 kept alive for some time and made to exhibit the phenomena of 

 which we are about to speak, has been placed on a table with a 

 line drawn in chalk around the area covered by its body, and 

 left to itself has subsequently been found dead without having 

 stirred outside the chalked circle. 



We must here however repeat the caution laid down in 

 457, 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. Still even in their most complete 

 development such movements do not absolutely negative the view 

 that the frog in the absence of the cerebral hemispheres is want- 

 ing in what we ordinarily call a 4 will.' 



474. We have seen that a frog from which the whole 

 brain has been removed and the spinal cord alone left appears 

 similarly devoid of a 4 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 fun- 

 damentally from those of an entire frog in the following import- 

 ant feature ; they inevitably follow when the stimulus is applied ; 

 they come to an end when the stimulus ceases to act. By con- 

 tinually 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 



