SKIN. 641 



tion of an ingrowing epidermic papilla, surrounded at its 

 base by a moat-like follicle, and nourished during growth 

 by a vascular projection of the dermis. 



Each hair consists of a spongy central part and a denser cortex, but 

 there are many diversities of form and structure, such as short fur and 

 long tresses, the soft wool of sheep and the bristles of pigs, the spines of 

 hedgehog, porcupine, and Echidna, the cilia of the eyelids and the tactile 

 vibrissa of the lips and cheeks. 



It is generally believed that the hairs of Mammals are homologous 

 with the feathers of Birds and the scales of Reptiles, but Maurer 

 maintains that the facts of development upset the homology and point 

 rather to a resemblance between hairs and the sensory papillae of 

 Amphibians. But this is still under discussion. 



The hair keeps the animal dry and warm ; in the practically hairless 

 Cetacea the layer of fat or blubber underneath the skin also serves to 

 sustain the temperature of the body. Like feathers, hairs die away 

 and are cast off, being replaced by fresh growths. A few mammals, 

 such as the Arctic fox, the mountain hare, and the ermine, become white 

 in winter, harmonising with the snow. In the case of Ross's lemming, 

 we know that this change is due to the influence of the cold, and 

 depends in great part on the appearance of gas bubbles inside the 

 hairs. 



That the colouring is sometimes of protective advantage we have 

 already noticed ; but in many cases no utilitarian interpretation can be 

 read into the stripes and markings. Those of related species often 

 form regular series, and are superficial outcrops of constitutional changes 

 hardly to be analysed. Sometimes there is considerable change during 

 the lifetime of the animal, thus most young deer have spots, but only 

 the Fallow and Axis deer retain these when adult. To an excess of 

 pigment is due the variation known as melanism or blackness, e.g., in 

 black wolves and rabbits ; to a dearth of pigment albinism is due, as in 

 white mice and white elephants. In tropical countries the skin is some- 

 times very darkly coloured, as in Indian cattle, and many monkeys — 

 especially males — are notable for the bright colours of the bare parts of 

 the body. 



Among other tegumentary structures are the scales which 

 occur along with hairs on the pangolins {Manis) ; the scales 

 on the tails of rats and beavers and some other forms ; the 

 thickened skin-pads or callosities on the ischia of apes, 

 the breast of camels, the legs of horses ; the nails, claws, or 

 hoofs which ensheath the ends of the digits in all mammals 

 except Cetaceans. Unique is the armature of the armadillos, 

 for it consists of bony plates developed in the dermis, 

 overlaid by epidermic scales. The median solid horns of 

 the rhinoceros are epidermic outgrowths, comparable to 

 exaggerated warts ; the paired horns of the Ruminants con- 



41 



