II AIR AND HORNti OF MAMMALS. 



561 



the limb from the body becomes more marked as we ap- 

 proach man. In the seal, only the wrist protrudes from 

 the skin, the limb of the otter slightly more ; the horse's 

 leg does not protrude beyond the elbow, that of the monkey 

 projects two thirds of its length, while in man the limbs 

 become wholly free from the trunk (Wyman). 



The hairs originate in minute sacs which extend from the 

 epidermis into the cutis ; from the bottom of this inpushing 

 of the epidermis grows up the shaft of the hair, which is 



Fig. 486. Diagram of the development of the nipple ; vertical section, a, periphery 

 of the glandular area (b) ; gl, glands. A, form of the gland in Echidna ; B, its form 

 in most mammals ; C, its form in some ungulates, as the cow, mare, etc. After 

 Gegenbaur. 



surrounded at the base by the cellular wall of the hair-sac 

 forming the root-sheaths. The spines of the porcupine, the 

 scales of the Manis, of the armadillo, of the tail of the rat, 

 are modified hairs, all developing in the same manner. 



Many mammals, especially the ruminants, as the deer, ox, 

 rhinoceros, etc., are armed with horns. There are two 

 kinds, those with a bony core surrounded with a horny 

 epidermal case as in the deer ; while in others, as the ante- 

 lopes, sheep, goats, and oxen, the horns are hollow ; hence 



