320 



NORMAL HISTOLOGY. 



in 



The stratum lucidum, usually wanting in other localities, in the palm 

 and sole appears as a thin, almost homogeneous layer, separating the cor- 

 neous from the granular layer. With the latter it constitutes the stratum 

 intermedium. As indicated by its name, the stratum lucidum appears clear 

 and without distinct cell boundaries, although suggestions of these, as well 

 as of the nuclei of the component elements, are usually distinguishable. 

 The cells of the stratum lucidum contain a substance, eleidin, derived from 

 the keratohyaline particles, which soften and coalesce into a homogeneous 

 semifluid material that fills the cells. 



The stratum corneum includes the remainder of the epidermis and 

 consists of many layers of horny epithelial cells, that contain pareleidin and 

 form the exterior of the skin. Where no stratum lucidum exists, as is usually 

 the case, the corneous layer rests upon the stratum granulosum, from which 

 its horny elements are being continually recruited. During their migration 

 towards the free surface, the cells lose their vitality and moisture and become 

 more flattened, until the most superficial ones are converted into the dead 

 horny scales that are being constantly displaced by abrasion. 



The pigmentation of the skin, which even in white races is conspicuous 

 certain regions, as on the external genital organs and around the anus, 



depends upon the pres- 

 ence of colored parti- 

 cles. These lie chiefly 

 within the epidermis, 

 although, when the 

 dark hue is decided, 

 a few small branched 

 pigmented connective 

 tissue cells may appear 

 within the subjacent co- 

 rium. The distribution 

 of the pigment particles 

 varies with the intensity 

 of color, in skins of 

 lighter tints being prin- 

 cipally limited to the 

 columnar cells next the 

 corium. With increas- 

 ing color the pigment 

 particles invade the 

 neighboring layers of 



Pigmental 

 'epidermis 



Duct of 

 sweat gland 



FIG. 365. Section of skin surrounding anus, showing pigmentation of 

 deep layers of the epidermis. X 5. 



epithelium until, in the dark skin of the negro, they are found within the cells 

 of the stratum corneum, in diminishing numbers towards the free surface. 

 Even when the cells are dark and densely packed, the colored particles never 

 encroach upon the nuclei, which appear as pigment-free areas. The source 

 of the pigment within the epidermis is disputed, some assuming a transference 

 of the colored particles by means of wandering cells or of the processes of 

 pigmented connective tissue cells that penetrate the cuticle, and others 

 accepting an independent origin in situ within the epithelial elements. While 

 it is established that at times the connective tissue cells are capable of modify- 

 ing pigmentation, it is equally certain that the earliest, and probably also 

 later, intracellular pigmentation of the epidermis appears without the assist- 

 ance of the connective tissue or migratory cells, minute colored particles 

 first becoming evident within the epithelial cytoplasm. 



