EXTERNAL APPENDAGES OF THE SKIN. 205 



of the outer skin in an outward direction. The nails (un- 

 gues), which are important protective formations over the 

 hind surface of the most sensitive parts of our limbs — the 

 tips of the fingers and toes — are horny products of the 

 epidermis, common to us with the Apes. In their place, 

 the lower Mammals generally possess claws, and the 

 Hoofed Animals (Ungulata) hoofs. The parent-form of 

 Mammals undoubtedly had claws, such as appear in a 

 rudimentary state in the Salamander. The hoofs of the 

 Hoofed Animals and the nails of Apes and of Man originated 

 from the claws of more ancient Mammals. In the human 

 embryo the first rudiment of the nails first appears (between 

 the horn-layer and the mucous layer of this outer skin) 

 in the fourth month. Their edges do not, however, project 

 until the end of the sixth month. 



The most interesting and important appendages of the 

 outer skin are the hairs, which, on account of their peculiar 

 structure and mode of origin, must be regarded as very 

 characteristic of the whole Mammalian class. Hairs, it is 

 true, appear widely distributed in many lower animals, e.g., 

 in Insects and Worms. But these hairs, like those of plants, 

 are thread-like processes of the outer surface, and differ 

 from Mammalian hairs in their characteristically finer 

 structure and in their mode of development. Hence Oken 

 rightly called Mammals " hairy animals." The hairs of 

 Man, as of all other Mammals, consist simply of epidermic 

 cells peculiarly differentiated and arranged. In their first 

 state, they appear in the embryo as solid plug-shaped pro- 

 cesses of the epidermis which penetrate into the underlying 

 leather-skin (corium), as do the sebaceous and the sweat 

 glands. As in the latter, the simple plug consists originally 



