STRUCTURE OF NAILS. 43 1 



lined by epidermal cells continuous with those covering the 

 general surface of the skin ; as if indeed the follicle had 

 been formed by a simple thrusting in of the surface of the 

 integument (figs. 117, 1 1 8). This epidermal lining of the 

 hair-follicle, or root-sheath of the hair, is composed of two 

 layers, the inner one of which is so moulded on the 

 imbricated scaly cuticle of the hair, that its inner surface 

 becomes imbricated also, but of course in the opposite 

 direction. When a hair is pulled out, the inner layer of 

 the root-sheath and part of the outer layer also are com- 

 monly pulled out with it. 



Nails. A nail, like a hair, is a peculiar arrangement 

 of epidermal cells, the undermost of which, like those of 

 the general surface of the integument, are rounded or 

 elongated, while the superficial are flattened, and of more 

 horny consistence. That specially modified portion of the 

 corium, or true skin, by which the nail is secreted, is called 

 the matrix. 



The back edge of the nail, or the root as it is termed, is 

 received into a shallow crescentic groove in the matrix, 

 while the fron* part is free, and projects beyond the ex- 

 tremity of the digit. The intermediate portion of the nail 

 rests by its broad under surface on the front part of the 

 matrix, which is here called the led of the nail. This part 

 of the matrix is not uniformly smooth on the surface, but 

 is raised in the form of longitudinal and nearly parallel 

 ridges or laminae, on which are moulded the epidermal 

 cells of which the nail is made up (fig. 119). 



The growth of the nail, like that of a hair, or of the 

 epidermis generally, is effected by a constant production of 

 cells from beneath and behind, to take the place of those 

 which are worn or cut away. Inasmuch, however, as the 



e, imbricated scales about to form a cortical layer on the surface of the 

 hair. The adjacent cuticle of the root-sheath is not represented, and 

 the papilla is hidden in the lower part of the knob where that is repre- 

 sented lighter. 



