116 EXCRETION. 



The Epidermis and its Appendages. The epidermis, 

 or external layer of the skin, is a membrane composed ex- 

 clusively of cells, containing neither blood-vessels, nerves, 

 nor lymphatics. Its external surface is marked by exceed- 

 ingly 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 in- 

 ternal layer is called the rete mucosum, or the Malpighian 

 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, nucleated cells, containing a greater or less 

 amount of pigmentary matter, applied directly to all the 

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

 cells containing no pigment. The tipper layers of cells, 

 with the scales of the horny layer, are semitransparent and 

 nearly colorless ; and it is the pigmentary layer chiefly which 

 gives to the skin its characteristic color and the peculiarities 

 in the complexion of different races and of different individu- 

 als. In the negro, this layer is nearly black ; and when the epi- 

 dermis is removed, the true skin does not present any marked 

 difference from the skin of the white race. All the epider- 

 mic cells are somewhat colored in the dark races, but the 

 upper layers contain no pigmentary granules. The cells 

 of the pigmentary layer are *from 4 Q \ 6 to 3^0 of an 

 inch in length, and from go 1 o0 to 4 ^ d of an inch in their 

 short diameter. The rounded cells in the upper layers are 

 from 40 1 00 to -g-oVu" of an inch in diameter. The absolute 

 thickness of the rete mucosum is from 17 1 OC) to -$ of an inch. 



The horny layer is composed of numerous strata of hard, 

 flattened cells, irregularly polygonal in shape, generally with- 

 out nuclei, and measuring from -g-gVs- to ^^ of an inch in 

 diameter. The deeper cells are thicker and more rounded 

 than those of the superficial layers. 



