190 THE FROG CHAP. 



Notice that the general plan of all the sensory organs, 

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

 sist of certain peculiarly modified epithelial cells, specially 

 sensitive to impulses of particular kinds, and in com- 

 munication, 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 (com- 

 pare 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 con- 

 trolled, 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. 



