172 THE P'ROG CHAi'. 



varied according to circumstances ; if one leg is prevented 

 from rubbing off the irritating substance, the other imme- 

 diately 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 nerve 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. \\'ithout such external stimulus, how- 

 ever, the spinal cord of a brainless 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 in- 

 evitable movements and no others ; but a single stimulus 

 may produce varied movements, the nature and direction of 

 which cannot be predicted ; the animal will probably give a 

 series of leaps, but the number and extent of these varies 

 according to circumstances. 



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 communication — 

 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, nt.c, eg, c.cort). In certain of these 

 brain-cells (c.cort), voluntary impulses originate and exercise 

 a controlling effect upon the cells of the spinal cord, so that 

 these latter do not constitute, as in the brainless frog, a 

 machine every movement of which can be accurately pre- 

 dicted. 



