384 



EXCRETION". 



transverse lines. It is usually reddish in color, from the great vascularity of the subja- 

 cent structure. At the posterior part, is a whitish portion of a semilunar shape, called 

 the lunula, which has this appearance simply from the fact that the corium in this part is 

 less vascular and the papillae are not so regular as in the rest of the body. That portion 

 of the skin situated beneath the root and the body of the nail is called the matrix. It 

 presents highly vascular papillae, arranged in regular, longitudinal rows, and it receives 

 into its grooves corresponding ridges on the under surface of the nail. 



The free border of the nail begins at the point where the nail becomes detached from 

 the skin. This is generally cut or worn away and is constantly growing ; but, if left to 

 itself, it attains in time a definite length, which may be stated, in general terms, to be 

 from an inch and a half to two inches. 



FIG. 106. Section of the, nail, etc. (Sappey.) 



A, section of the nail: 1, 1, superficial layer ; 2, deep layer ; 3, 3, 4, 4, section of the grooves on the attached sur- 

 lace ; 5, 5 union of the superficial with the deep layer ; 6, 6, dark line between the two layers. B, cells of the 

 superficial layer, lateral view. C, cells of the superficial layer, flat view. D, cells of the deep layer. 



Examining the nail in a longitudinal section, the horny layer, which is usually 

 regarded as the true nail, is found to increase progressively in thickness from the root to 

 near the free border. If the nail be examined in a transverse section, it will also be 

 found much thicker in the central portion than near the edge, and that part which is 

 received into the lateral portions of the fold becomes excessively thin like the rest of the 

 root. The thickness of the true nail at the root is from ^ to -^ of an inch ; and, in 

 the thickest portion of the body, it usually measures from ~V to ^ of an inch. The nail 

 becomes somewhat thinner at and near the free border. 



Sections of the nails show that they are composed of two layers, which correspond 

 to the Malpighian and the horny layer of the epidermis, although they are much more 



