KATABOLISM. 430 



or else that this function is performed in some other way. 

 On the other hand, if the skin be soft, moist, and mu- 

 cous, it is a sign that it is very active, and performs 

 many functions which have yet been but partially differ- 

 entiated and relegated to an infolded surface. 



2. The function of cooling the blood by evapora- 

 tion can only exist in air-breathing and land-inhabiting 

 animals. 



Mammals. — The structure and function of the skin 

 in mammals are similar to the same in man ; yet there 

 are many mammals that are nonsweating. Such, in 

 many cases at least, are panting animals. The most 

 familiar illustration of this is the dog. The dog is a very 

 hot-blooded animal, and yet in it there is no visible 

 sweat, although there is, of course, exhalation. The 

 cooling of the blood is largely through the lungs by pant- 

 ing. A dog pants not because he is tired and wants 

 more oxygen, but because he is hot. By panting he fans 

 his lungs. 



Birds are also very hot-blooded, and in hot weather 

 they also supplement the cooling of the blood through 

 the skin by panting. 



Reptiles have dry, scaly, inactive skin. But little or 

 no cooling of the blood is required in them, because the 

 internal fires burn low ; they are cold-blooded. 



Amphibians have the extreme opposite condition. 

 They have soft, moist, mucous, and very active skins, 

 not, however, because they require cooling of the blood, 

 for they are cold-blooded and live mostly in the water, 

 but because many other functions are to some extent 

 performed by the skin — for example, respiration and 

 even the absorption of food. 



Fishes live in water, and there can be no evapora- 

 tion from the skin ; also, like amphibians, the skin is soft 

 and slimy and active. 



