2 . MAMMALIA. 



are frequently found in the epidermis, corium. and Malpighian layer. 

 The Cetacea approach the fishes in the texture of their corium, 

 which is composed of an interlacement of very loose fibres, the in- 

 tervals between which are filled with fluid fat. The pigmentary 

 layer in this order is remarkably dense, often several lines in thick- 

 ness, and lies directly beneath a thin, usually smooth and hairless 

 epidermis. The layers of the epidermis frequently attain a consid- 

 erable thickness, and form what are called callosities. In many of 

 the Rodentia and Carnivora, and in the Camels, these callosities are 

 developed into thick pads under the feet ; in the Apes they form 

 the cushions upon the buttocks. In the huge Pachydermata \ 

 similar structure prevails in connexion with the whole epidermis. 

 True scales are met with in the tail of many animals, as in the 

 Beaver. A horny tissue, consisting of coarse fibres, is exhibited 

 in the structure of nails and claws, but more especially in that of 

 hoofs and horns. Thus the horn of the Rhinoceros consists of cor- 

 neous fibres, like bristles, which have coalesced so as to form a hol- 

 low cone ; the individual fibres, however, have a fine cellular texture 

 like hairs. 



The most common of the horny covering of the Mammalia occurs 

 in the form of hairs. A distinction of these can be made, as in the 

 feathers of birds, into woolly hairs corresponding to down, and ordi- 

 nary hair or far. The first are very soft and slender, frequently 

 curled, and are situated next the skin. The second kind are longer, 

 stiffer, usually running to a fine point, and may be developed into 

 bristles, vibrissae, and spines. The spinous hairs are mixed with 

 the others ; they are coarser, more rigid, and generally slender at 

 the base, bulge out externally. The fine silken sort of hairs are 

 the connecting link with wool. The roots, follicles, and stems of 

 hairs, have the same structure in the mammiferous class of animals 

 as in Man. The follicles of the hairs are, however, very large in 

 the vibrissae of the upper lip and corners of the mouth in some 

 Mammalia, as the Seal, where they receive nervous twigs of consid- 

 erable size. 



The minute structure of hair presents great differences, accord- 

 ing to the class, order, genus, and species, to which the animal 

 belongs. In different parts of the body even the hairs have not the 

 same structure. They consist, as in Man, of cells, and are invested 

 by a thin cellular layer of epithelium. As a general rule, we can 

 distinguish a cortical and a medullary substance, which exhibit 

 differences in the color, thickness, form, and size of their cells 



