434 



THE SKIN. 



a 



posterior edge of the nail, from its being lodged in a 

 Fig. 1 12.* groove of the 



skin, cannot grow 

 backwards, on ad- 

 ditions being 

 made to it, so 

 easily as it can 

 pass in the op- 

 posite direction, 

 any growth at its 

 hinder part sim- 

 ply pushes the 

 whole forwards. 

 At the same time 

 fresh cells are 

 added to its un- 

 der surface, and 

 thus each portion 

 of the nail becomes gradually thicker as it moves to the 

 front, until, projecting beyond the surface of the matrix, 

 it can receive no fresh addition from beneath, and is 

 simply moved forwards by the growth at its root, to be 

 at last worn away or cut off. 



Excretion ly the Skin. 



The skin, as already stated, is the seat of a two-fold 

 excretion; of that formed by the sebaceous glands and 

 hair- follicles, and of the more watery fluid, the sweat or 

 perspiration, eliminated by the sudoriparous glands. 



The secretion of the sebaceous glands and hair-follicles 



* Fig. 112. Vertical transverse section through a small portion of the 

 nail and matrix largely magnified (after Kolliker). 



A, corium of the nail-bed, raised into ridges or laminae a, fitting in 

 between corresponding lamina? b, of the nail. , Malpighian, and 

 C, horny layer of nail : d, deepest and vertical cells ; e, upper flattened 

 cells of Malpighian layer. 



