190 THE FROG CHAI 



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, speciall; 

 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 o 

 which is necessary for the performance of the sensor] 

 function, so that the particular sense in question may b 

 destroyed, not only by destruction of the sense-organ 

 in the strict sense, but also by section of the afferen 

 nerve or by destruction of the brain. 



General Physiology Summary. Before going on t( 

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

 at the physiological processes of the frog as a whole (com 

 pare Fig. 23). The enteric canal is the manufactory ii 

 which the raw material of the food is worked up into ; 

 form in which it can be used by the various parts of th< 

 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 wast< 

 matters are collected from all parts and finally ejecte( 

 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 o 

 gaseous food. The central nervous system forms a sor 

 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 fron 

 headquarters are executed. And finally the sense-organ: 

 may be looked upon as the various branches of ai 

 intelligence department by which the headquarters an 

 informed of what is going on outside. 



