SKIN. 315 



scrotum, and in the nipple. Striated muscle fibers in the skin of the face 

 represent the insertions of the muscles of expression. The vessels and 

 nerves of the corium are described on page 327. 



Epidermis. The epidermis is stratified epithelium, the many layers 

 of which are divisible into a stratum germinativum and a stratum corneum. 

 The former includes a basal row of columnar cells without membranes, 

 which rest on the papillae of the corium. Although mitoses are seldom 

 seen, these cells multiply and produce the several layers of polygonal cells 

 which overlie them. The latter are connected by numerous slender inter- 

 cellular bridges, as seen in Fig. 31, p. 30. Because of this striking feature 

 the stratum germinativum was formerly called the stratum spinosum 

 [and rete Malpighii]. The transition to the stratum corneum or outer 

 layer of horny flat cells is quite abrupt, except in the thick skin of the 

 palms and soles. An incomplete layer of coarsely granular cells may 

 intervene. In the corneum the cells acquire a horny exoplasmic mem- 

 brane; the bridges become short stiff spines; the protoplasm and nucleus 

 are dried and shrunken and in the outermost cells the nucleus may 

 wholly disappear. The cells become flatter toward the surface, from 

 which they are constantly being desquamated. 



The process of cornification presents a more elaborate picture in 

 sections of the palms and soles. Passing outward from the stratum ger- 

 minativum there is a darkly staining, coarsely granular layer, one or two 

 cells thick, which is followed by a clear somewhat refractive band in 

 which the cell outlines are indistinct. This layer seems saturated with 

 a dense fluid formed by dissolution of the underlying granules. In haema- 

 toxyline and cosine specimens the granular layer or stratum granulosum is 

 followed by a pink and then by a bluish band, which are subdivisions of 

 the clear stratum lucidum. They are followed by a thick stratum corneum. 

 Except in the palms and soles the granulosum is thin and the lucidum is 

 absent. Chemically the coarse granules of the stratum granulosum resemble 

 keratin (from which they differ by dissolving in caustic potash); they 

 are therefore called kerato-hyalin granules. Their diffuse product in the 

 stratum lucidum is named eleidin. In the corneum it becomes pareleidin, 

 which, like fat, blackens with osmic acid, but the reaction occurs more 

 slowly. The pareleidin is not due to fat entering the skin from oily secre- 

 tions on its outer surface. 



The color of the skin is due to fine pigment granules in and between 

 the lowest layers of epidermal cells; a few smaller granules occur in the 

 corium. Pigmented connective tissue cells are found near the anus, 

 but they are generally infrequent and are absent from the palms and 

 soles. The possibility of the mesenchymal origin of epithelial pigment 



