488 



ZOOLOGY 



SECT. 



Systematic Position of the Exatfiple. 



The genus Lepus, to which the common Rabbit belongs, com- 

 prises a number of other species, the common Hare being among 

 the number, distinguished from one another by slight differences 

 in the proportions of the parts and in other general features. Lepus 

 is the only genus of the family Leporidm, which is associated 

 with the family Lagomyidm or Picas under the designation Dtoplici- 

 dentaia, owing to the presence in these two families, and in these 

 two alone of the entire order Rodentia to which they belong, of a 

 second pair of incisors in the upper jaw. The chief distinctive 

 features of the family Leporidas are the elongated hind-limbs, the 

 short recurved tail, the long ears, and the incomplete clavicles. 



3. General Organisation. 



Integument and General External Features. — Nearly all 

 Mammals are covered with hairs (Fig. 1100) developed in hair- 

 follicles. Each hair (Fig. 

 1101) is a slender rod, and 

 is composed of two parts, 

 a central part ov pith (M) 

 containing air, and an 

 outer more solid part or 

 cortex (B) in which air 

 does not occur ; its outer- 

 most layer may form a 

 definite cuticle (0). Com- 

 monly the cortical part 

 presents transverse ridges 

 so as to appear scaly. In 

 one case only, viz.. Sloths, 

 is the hair fluted longi- 

 tudinally. The presence 

 of processes on the sur- 

 face, by which the hairs 

 when twisted together in- 

 terlock firmly, gives a 

 special quality to certain 

 kinds of hair (wool) used 

 for clothing — the felting quality as it is termed. A hair is 

 usually cylindrical ; but there are many exceptions : in some 

 it is compressed at the extremity, in others it is compressed 

 throughout; the latter condition is observable in the hair of 

 negroid races of men. The fur is usually composed entirely of one 

 kind of hair ; but in some cases there are two kinds, the hairs of 



Fig. 1100.— Section of human skin. Co, dermis ; B, 

 sebaceous glands ; F, fat in dermis ; G, vessels in 

 dermis ; GP, vascular papillas ; H. hair ; N. nerves in 

 dermis ; iVP. nervous papillas ; Se, horny layer of 

 epidermis ; SD, sweat-gland ;i SI>', duct of sweat- 

 gland ; SM, Malpighian layer. (From Wiedersheim's 

 Comparative Anatomy,) 



