190 THE FROG CHAP. 



those of the skin, eye, and ear, is the same. They 

 consist of certain peculiarly modified epithelial cells, 

 specially sensitive to impulses of particular kinds, and in 

 communication, by means of an afferent nerve, ^with nerve- 

 cells of the brain. The three things sensory cell, afferent 

 nerve, and brain form a chain, every link of which is 

 necessary for the performance of the sensory function, so 

 that the particular sense in question may be destroyed, not 

 only by destruction of the sense-organs in the strict sense, 

 but also by section of the afferent nerve or by destruction 

 of the brain. 



General Physiology-Summary. Before going on to the 

 next chapter it will be as well to take a final glance at the 

 physiological processes of the frog as a whole (compare 

 Fig. 23). The enteric canal is the manufactory in which 

 the raw material of the food is worked up into a form in 

 which it can be used by the various parts of the body. The 

 circulatory organs are the communicating system by which 

 the prepared food is taken to all parts ; and they also form 

 a drainage system by which waste matters are collected 

 from all parts and finally ejected by the three main sewers, 

 the skin, lungs, and kidneys. The skin and lungs, besides 

 getting rid of waste matters, serve for the supply of oxygen 

 a necessary form of gaseous food. The central nervous 

 system forms a sort of headquarters' staff by which the 

 entire body is controlled, the means of communication 

 being the nerves, and the muscles the executive by which 

 the orders from headquarters are executed. And finally 

 the sense-organs may be looked upon as the various branches 

 of an intelligence department by which the headquarters 

 are informed of what is going on outside. 



