MAMMALIA (MAMMALS). 



The name Mammalia is applied to all those forms which, 

 like the mouse, cow, and man, have warm blood, a body 

 covered with hair, and which bring forth living young, 

 nourished during the early stages by milk secreted by the 

 mother. These characters at once distinguish any mammal 

 from any other animal, but other features of equal or 

 greater importance occur. 



Hair occurs in the young of all mammals, and is usually 

 found also in the adult; but in the case of the whales it is 

 absent in the fully grown animal, and even in the young it 

 is only found near the mouth. Hair is a product of the 

 outer or epidermal layer of the skin. At places this layer 

 dips down into the deeper layer (dermis), forming a pit or 

 follicle from the bottom of which the hair grows, continual 

 additions being made at this point, commonly known as the 

 " root." The hair itself is a solid column, varying consid- 

 erably in shape in different animals, from the delicate fur of 

 the fur-seal, to the bristles of the pig or the spines of the 

 porcupine. There are usually glands present which open 

 into the follicle and which secrete a fluid, the object of 

 which is to keep the hair moist ; and besides, each follicle 

 is provided with muscles which serve to erect the hair at 

 times of fright (as in cats and dogs), or in cold weather. 



Closely related to hair are nails, claws, hoofs, and horn.* 



* Here is intended such horns as those of the cow, sheep, antelope, 

 and rhinoceros ; the horns of the deer are true bone. 



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