172 THE FROG CHAP. 



movements are of such a nature as to withdraw the part 

 stimulated from the irritating substance. Moreover, 

 as shown by the experiment of applying acid to various 

 parts of the body, the movements are varied according 

 to circumstances ; if one leg is prevented from rubbing 

 off the irritating substance, the other immediately comes 

 into play. Obviously, then, a simple stimulus reaching 

 the spinal cord may be transmitted to numerous motor 

 cells of the ventral horn, and through these to numerous 

 motor nerves, the particular nerves affected differing 

 according to circumstances (compare Fig. 55). The 

 spinal cord, therefore, is able, in response to a stimulus 

 reaching it by a sensory nerve, to originate motor 

 impulses causing complex muscular movements so 

 adjusted as to serve definite purposes. Without such 

 external stimulus, however, the spinal cord of a brain- 

 less frog is quite inactive, and the body of the animal 

 will remain without movement until it dries up or 

 decomposes. 



In the uninjured frog, i.e., the frog with its brain 

 intact, the case is very different. The animal no 

 longer acts like an unintelligent machine, each stimulus 

 producing certain inevitable movements and no others ; 

 but a single stimulus may produce varied movements, 

 the nature and direction of which cannot be predicted. 

 The frog will probably give a series of leaps, but the 

 number and extent of these vary according to circum- 

 stances ; its movements are voluntary, not reflex. 



This is explained by the fact that certain nerve-fibres 

 of the cord pass forwards to the brain, and that the 

 nerve-cells in the grey matter of the cord are in com- 

 munication owing to the interlacing of their branching 

 processes with those of the collaterals with similar 

 cells in the grey matter of the brain (Fig. 55, m. c, c. g, c. 

 cort). In certain of these brain-cells (c. cort}, voluntary 



