490 



ZOOLOGY 



SECT. 



longer than that which lies embedded in the follicle. At the same 

 time the follicle grows downwards into the dermis. During its 

 growth the hair is nourished by the blood-vessels in the dermal 

 hair-papilla, which projects into its base. 



Modifications of the hairs are often found in certain parts. 

 Such modified hairs are the elongated hairs of the tails of some 

 Mammals, e. g., most Ungulates ; the eye-lashes of the eye-lids, 

 which are stronger than the ordinary hairs ; and sensitive hairs 



or vibrissce about the 

 snout. In some Mam- 

 mals the hairs in 

 part assume the form 

 of spines, viz., in 

 Echidna, the Hedge- 

 hogs, and the Porcu- 

 pines. 



The coating of hairs 

 is scanty in some 

 Mammals, and is vir- 

 tually absent in the 

 Cetacea and Sirenia. 

 In such cases the 

 skin is greatly thick- 

 ened, as in the Ele- 

 phants, &c. ; or, as in 

 the Cetacea, an un- 

 derlying layer of fat 

 performs the function 

 as a heat- 



FIG. 1102. Four diagrams of stages in the development of a 

 hair. A, earliest stage in one of those Mammals in which 

 the dermal papilla appears first ; B, C, D, three stages in 

 the development of the hair in the human embryo. W6. 

 hair-bulb ; cm. horny layer of the epidermis ; foil, hair- 

 follicle ; grm. hair-germ ; h. extremity of hair projecting on 

 the surface ; muc. Malpighiau layer of epidermis ; pp. dermal 

 papilla ; seb. developing sebaceous glands ; sh l . sh 2 , inner 

 aths. 



and outer root-sheat 



(After Hertwig.) 



preserving covering. 



In Manis (Fig. 

 1115) the greater 

 part of the surface is 

 covered with large, 

 rounded, overlapping 

 horny scales of epi- 

 dermal origin, similar 

 in their mode of development to those of Reptiles. A similar 

 phenomenon is seen in the integument of the tail of Anoma- 

 lurus a Flying Rodent. The Armadillo (Fig. 1114) is the 

 only Mammal in which there occurs a bony dermal cxoskeleton 

 (vide infra). 



Also epidermal in their origin are the horny structures in the 

 form of nails, claws, or hoofs, with which the terminations of the 

 digits are provided in all the Mammalia except the Cetacea. And 

 the same holds good of the horny portion of the horns of Ruminants. 

 The horns of the Rhinoceros are also epidermal, and have the 



