A STUDY OF THE SKIN AND THE KIDNEYS 247 



acterized by the possession of feathers, so the highest group 

 of animals, the mammals, are distinguished by the presence 

 of hair. This varies in amount from the scattered bristles 

 on the body of a whale, and from the numerous thickened 

 quills that protect the porcupine, to the dense hairy cover- 

 ing of the bear or the sheep. In structure, however, all 

 these forms of hair agree more or less closely with that 

 already described for human hair. 



Many animals of this group are supplied with claws that 

 enable them to seize and tear their prey. Claws differ from 

 human finger and toe nails only in the fact that the former 

 grow on all sides of the end joint of the animal's append- 

 ages. The bone, therefore, forms the core of the claw, and 

 it is inclosed by the layer of horn. Hoofs of horses, cows, 

 and of deer are formed, like claws, by a horny layer that 

 incloses the end bone of the various digits (see Fig. 84). 

 This hoof is constantly growing, and if it is not worn away 

 by use, it has to be pared off, as is the case when the horse 

 is shod. 



The horns- of animals are of two kinds : namely, either 

 hollow or solid. The horns of an ox are formed by a conical 

 projection of the frontal bone, which is covered over by 

 dermis, and this in turn is incased in a hard layer of horny 

 epidermis. Hence, on removing the horny part, we find 

 that it is hollow. Successive layers are formed one within 

 another, and this kind of horn lasts throughout the life of 

 the animal. Deer, on the other hand, grow new horns or 

 antlers each year. In the early months two projections grow 

 out from the frontal bone and branch with great rapidity. 

 At first they are richly supplied with blood vessels and are 

 covered with a soft layer of epidermis that looks and feels 

 like velvet. Later in the year this disappears, the antlers 

 harden, a line of separation is formed between them and the 

 skull, and they finally drop off. The next year the process 

 is repeated. Horns tvhich are hollow are therefore perma* 

 nent; solid antlers are deciduous. 



