Mammalia: Skin, Muscles, Skeleton 



10 



Skin 



The skin has the same basic structure as in other vertebrates but 

 has some elaborately specialized features, although they include no 

 structure so complex as a feather (Fig. 442). 



Hair is as characteristic of mammals as feathers are of birds. A 

 hair is a comparatively simple filamentous horny structure, a modifica- 

 tion of the stratum corneum and therefore wholly epidermal (Fig. 443). 

 The root of a hair is contained in an epidermal follicle. Small strands 

 of nonstriated muscle-fibers attached to the deeper part of the follicle 

 make possible the erection of the hair — as in response to cold or fright. 

 The statement that all mammals possess hair comes dangerously near 

 to having some exceptions. Hair is scanty on such large thick-skinned 

 animals as the hippopotamus and elephant. In sea cows (Sirenia) and 

 the whalelike mammals (Cetacea), the adults are nearly or quite devoid 

 of hair. The fetus, however, usually possesses some hair, or at least a 

 few short, stiff bristles, especially about the mouth. It has been 

 claimed, however, that the fetus of the narwhal (Monodon) and of the 

 white whale (Delphinapterus) are quite hairless. 



Horny scales resembling those of reptiles occur in some mammals 

 — on the feet and tail of some marsupials, insectivores, and rodents; 

 over the entire body in the pangolin (Manis). In most mammals 

 horny claws on the digits are strongly developed, but in some the 

 stratum corneum at the tip of the digit is elaborated into a massive 

 hoof, while in man, apes, and usually in monkeys, it acquires the form 

 of a flat nail (Fig. 444). In many hoofed mammals (Ungulata) the 

 head bears so-called "horns," but only the outer layer of the "horn" 

 is horny, the core being dermal bone. Antlers in the deerlike ungulates 

 are, when fully developed, entirely bony (Fig. 445). 



In the armadillo (Dasypus: Fig. 554) there are vestigial horny 

 scales underlaid by massive dermal bony plates which constitute a 



573 



