236 STUDIES IN PHYSIOLOGY 



nective and elastic tissue. Beneath the dermis, too, there is 

 a large amount of these tissues, in the meshes of which fat 

 is deposited in considerable quantity. This fatty layer 

 serves, as already stated (p. 50), to retain the heat of the 

 body, and it is also used for fuel when needed for the pro- 

 duction of energy. The wrinkles of old age are due to the 

 fact that this fat has been drawn upon to such an extent 

 that the skin fits loosely over the underlying tissues. 



Nails. The horny layer of the epidermis becomes espe- 

 cially developed in the nails of the fingers and toes. Except 

 at their projecting ends these nails 

 lie upon and are closely attached 

 to the dermis, and their edges and 

 bases are covered over by a roll of 

 the epidermis (Fig. 110). The nail 

 itself, like all epidermis, is not sup- 



FIG. no. -Section of Nail plied with blood vessels, its general 

 and Parts beneath. pink color being due to the rich 



1, 2, 4 = horny cuticle or epi- supply of blood in the papillae of 

 ^ dermis. the dermis beneath. Near the base 



9 12 = dermis. ' ^ the na il? however, these papillae 



are less numerous and the nail it- 

 self is more opaque ; these facts explain the presence of the 

 whiter area known as the lu'nu-la (Latin, luna = moon + 

 ula = little). 



Nails increase both in thickness and in length by a 

 growth from the living cells on the under surface of the 

 nails and at their base. If the nail is accidentally torn off, 

 a new one is produced, provided these deeper cells are not 

 injured. 



Hair. A second modification of the epidermis is the 

 hair. To help understand the way hairs are placed in the 

 epidermis, one might imagine the head of a fine pin to be 

 pushed diagonally against a thin sheet of rubber in such a 

 way as to form a deep pit without breaking the surface. 

 The sheet of rubber would then represent the epidermis, 



