PHYSIOLOGICAL ANATOMY OF THE NAILS. 345 



phatics. Its external surface is marked by shallow grooves, which correspond 

 to the deep furrows between the papillae of the derma. Its internal surface 

 is applied directly to the papillary layer of the true skin and follows closely 

 all its inequalities. This portion of the skin is subdivided into two tolerably 

 distinct layers. The internal layer is called the rete mucosum, or the Mal- 

 pighian layer, and the external is called the horny layer. These two layers 

 present certain important distinctive characters. 



The Malpighian layer is composed of a single stratum of prismoidal, nu- 

 cleated cells, containing pigmentary matter, which are applied directly to all 

 the inequalities of the derma, and of a number of layers of rounded cells 

 containing no pigment. The upper layers of cells, with the scales of the 

 horny layer, are semi-transparent and nearly colorless ; and it is the pigment- 

 ary layer chiefly which gives to the skin its characteristic color and the pe- 

 culiarities in the complexion of different races and of different individuals. 

 All the epidermic cells are somewhat colored in the dark races, but the upper 

 layers contain no pigmentary granules. The thickness of the rete mucosum 

 is -J-J00- to T^J of an inch (15 to 333 ft). 



The horny layer is composed of a number of strata of hard, flattened cells, 

 irregularly polygonal in shape and generally without nuclei. The deeper 

 colls are thicker and more rounded than those of the superficial layers. 



The epidermis serves as a protection to the more delicate structure of the 

 true skin, and its thickness is in proportion to the exposure of the different 

 parts. It is consequently much thicker upon the soles of the feet and the 

 palms of the hands than in other portions of the general surface, and its 

 thickness is very much increased in those who are habitually engaged in 

 manual labor. Upon the face and eyelids, and in the external auditory pas- 

 sages, the epidermis is most delicate. The variations in thickness depend 

 entirely upon the development of the horny layer. The thickness of the 

 rete mucosum, although it varies in different parts, is rather more uniform. 



There is constantly more or less desquamation of the epidermis, particu- 

 larly of the horny layer, and the cells are regenerated from the subjacent 

 parts. It is probable that there is a constant formation of cells in the deeper 

 strata of the horny layer, which become flattened as they near the surface ; 

 but there is no direct evidence that the cells of the rete mucosum undergo 

 transformation into the hard, flattened scales of the horny layer. 



Physiological Anatomy of the Nails. The nails are situated on the dor- 

 sal surfaces of the distal phalanges of the fingers and toes. They serve to 

 protect these parts, and in the fingers, they are quite important in prehen- 

 sion. The general appearance of the nails is sufficiently familiar. In their 

 description, anatomists have distinguished a root, a body and a free border. 



The root of the nail is thin and soft, terminating in rather a jagged edge, 

 which is turned slightly upward and is received into a fold of the skin, ex- 

 tending around the nail to its free edge. The length of the root varies with 

 the size of the nail, but it is generally one-fourth to one-third of the length 

 of the body. 



The body of the nail extends from the fold of skin which covers the root 



