MAMMALIA 



327 



have a few bristle-like hairs on the upper lip. Sometimes hairs may- 

 be fused into scale-like or horn-like structures as in the scaly ant- 

 eaters and the rhinoceros. Again they may be more or less covered 

 or obscured as in the armor of the armadillos. The hair arises from 

 a shght thickening of the Malpighian layer of the epidermis, which 

 subsequently invaginates so as to form a deep pocket or follicle of the 

 epidermis and a dermal papilla pushes up into the bottom of the in- 

 vagination after the manner of a pulp cavity in a tooth. Thus the 

 origin and development of 

 the hair is totally different 

 from that of a scale or of a 

 feather. The hair is hke 

 nothing else; it is sui 

 generis. 



There are many kinds of 

 skin glands among mam- 

 mals, but they may all be 

 reduced to two fundamental 

 types: sudoriparous or sweat 

 glands and sebaceous glands. 

 Generalized sweat and seba- 

 ceous glands (Fig. 167) are 

 scattered over nearly the 

 entire skin, while local spe- 

 cializations of both types 

 occur in all mammals. The 

 mammary glands of the 

 monotremes are speciaUzed 

 sweat glands, while those 

 of the Eutheria are specialized sebaceous glands. A great many 

 mammals possess scent glands located in various regions. These 

 serve a variety of uses, principal among which are: to attract the 

 opposite sex; to enable gregarious forms to distinguish their kind; 

 and for defensive purposes, as in the skunk and his tribe. 



Either claws, hoofs, or nails are present in all mammals except the 

 whales; even in the latter rudiments of claws appear in the foetus and 

 are subsequently lost. These three distinct types, and the total ab- 

 sence of any such structures, serve to divide the mammals into four 

 great sections: the clawed, the nailed, the hoofed, and the whales, 



Fig. 167. — Section of human skin. Co, 

 dermis; D, sebaceous glands; F, fat in der- 

 mis; G, vessels in dermis; G. P, vascular pa- 

 pillae; H, Hair; N, nerves in dermis; N. P, 

 nervous papilte; Se, horny layer of epidermis; 

 S. D, sweat gland; S. Z)', duct of sweat gland; 

 SM, Malpighian layer. (From Wiedersheim.) 



