CHAPTER X. 



THE FROG {continued) : the nervous system. 



In a machine of human construction, such as the engine of 

 a steamer, the proper working of the whole depends, pro- 

 vided the parts of the machine itself are in good order, upon 

 two things— the stoking or regulation of the fires, and the 

 turning of certain cocks and levers by the engineer. In that 

 very complex machine the frog, we have already studied 

 what corresponds to stoking, viz., feeding and breathing. 

 We must now direct our attention to what may be considered 

 roughly to correspond with the work of the engineer — the 

 means by which the whole complex machinery is kept under 

 control, and its various parts made to work together to a 

 common end. 



How does it come about, for instance, that the various 

 digestive glands begin to secrete actively as soon as food is 

 taken into the enteric canal ? How is it that a touch on any 

 part of the body, or even the sight of an enemy, is followed 

 instantaneously by a series of vigorous muscular movements 

 so ordered as to facilitate escape from the source of danger? 



In the fourth chapter (p. 62) we got so far as to learn 

 that muscular contractions are induced b)' nervous impulses 

 travelling Irom the Ijrain or spinal cord, along the nerves, to 

 to the muscles. It may therefore be inferred that the con- 



