CHAPTER X 



THE FROG (continued) : THE NERVOUS SYSTEM 



IN a machine of human construction, such as a steam- 

 engine, the proper working of the whole depends, 

 providing 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 vigor- 

 ous 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 by nervous im- 

 pulses travelling from the brain or spinal cord, along the 



