The stratum lucidum, so-called by Oehl, is a thin, ribbon-like, translucent or shining layer, situated immediately above the stratum granulosum and below the stratum corneum of which it is considered a part by most authorities. There is no particular reason why these two or three rows of cells which only represent a stage in cornification should be designated as a separate layer, except that the subject can be more clearly presented by so doing. Kaposi believes this layer to be due to some chemicobiologic change that must take place in the cells of the granular and mucous layers before they become horny cells.
The stratum corneum (horny layer; corneus layer) (C, Fig. 1) is the external layer of the epidermis and hence of the entire skin and extends from the stratum lucidum below to the outside atmosphere. It is essentially the dead layer or surface division of the epidermis and consists of several layers of flattened imbricated cells whose exact shape depends upon the degree of keratinization which they have undergone. By rapid changes, the upper cells of the granular layer are transformed by the apparent melting of their granules into the surrounding cell protoplasm, and clear transparent epithelia are produced. The lower cells of the corneous layer are arranged in polygonal plates with shrivelled nuclei and rudimentary threads, which still serve to connect the frame-work of cells with each other. Still nearer the surface, the cells become thin, flattened, dry scales, lying parallel to the cutaneous surface, the outer rows wrinkled and curled up, preparatory to being shed. According to Unna, cornification of the epidermis is not a complete process but consists of an alteration of the periphery and connecting threads of the cells into horny tissue which, after digestion of the central part of the cells, presents a honeycomb-like structure. No traces of the intercellular canals are found in the corneous layer. The hard, dry character of the cells of this layer is due to the presence of a very hard and resistant substance, known as keratin.
The epidermis has no vascular supply, but the corium and the subcutaneous tissue are liberally supplied with truncal and capillary vessels. Tomsa has shown that the arterial supply is derived from subcutaneous branches which pass through the fascia and, by division and subdivision, form three separate vascular districts. The deepest variety supply the subcutaneous fat with numerous capillaries in a net-like arrangement inside and between the fat lobules (A, Fig. 1). The middle district sends off arterioles to form a capillary plexus for the sweat glands (B, Fig. 1). These capillaries end in small veins, some of which pass upward with the ducts of the sweat-glands and anastomoses with the veins of the papillary region. The third or upper district is supplied from an ascending artery (F, Fig. 1), which sends off branches to form capillaries for the hair-follicles, the sebaceous glands and the papillae. Each papilla is furnished with one or two capillary loops. The papilla of the hair has its own arteriole and capillary similar to those of the papillae of the corium. The capillaries of the papillary layer anastomose freely with those of the upper part of the hair-follicles from which loops pass to supply the sebaceous glands.
Venous plexuses accompany the arterial in all portions of the skin and in general, it may be said that all the venous vessels are somewhat larger than the arterial. The most superficial veins of the skin are derived from the capillaries of the papillae (V, Fig. 1) and form narrow meshes which, together with the deeper and circular veins, form venous branches which anastomose with branches from the hair-follicles and sebaceous glands. These unite into larger vessels and, with the veins from the sweat-glands and fat lobules, merge into the venous sinuses which end in the subcutaneous veins.