NAILS AST) HAIR. 119 



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 -j-J-g- of an inch ; and, in the thickest por- 

 tion of the body, it usually measures from ^ 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, though they are much more distinct. 

 The Malpighian layer is applied directly to the ridges of the 

 bed of the nail, and presents upon its upper surface ridges 

 much less strongly marked than in the underlying true skin. 

 This layer is rather thinner than the horny layer, is whitish 

 in color, and is composed of numerous strata of elongated, 

 prismoidal, nucleated cells, arranged perpendicularly to the 

 matrix. These cells are from 3^0 to 17 1 00 of an inch in 

 length. 



The horny layer, which constitutes the true nail, is ap- 

 plied by its under surface directly to the ridges of the Mal- 

 pighian layer. It is dense and brittle, and composed of nu- 

 merous strata of flattened cells, which cannot be isolated 

 without the use of reagents. If the different strata of this 

 portion of the nail be studied after boiling in a dilute solu- 

 tion of soda or potash, it becomes evident that here, as in the 

 horny layer of the epidermis, the lower cells are somewhat 

 rouncjed, while those nearer the surface are flattened. These 

 cells are nearly all nucleated, and measure from 10 1 00 to -y^-g- 

 of an inch in diameter. The thickness of this layer varies 

 in different portions of the nail, while the Malpighian layer 

 is nearly uniform. This layer is constantly growing, and con- 

 stitutes the entire substance of the free borders of the nails. 



