HAIR AND HORNS 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 nimple ; vertical section. @, periphery 
of the glandular area (0); gi, 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 which are solid and bony, as in the deer ; while 
in others, as the antelopes, sheep, goats, and oxen, the horns 
are hollow, the horny case enveloping a bony core; hence 
