Epidermis, Or Cuticle, the thin semi-transparent pellicle which covers the surface of the dermis or true skin. It is composed of layers of tessellated or pavement epithelium cells, of a flattened oval or polygonal shape, and about 1/1900 of an inch in diameter; each cell contains a nucleus with several distinct paler granules. The cells are developed from germs supplied by the basement membrane, nourished by the subjacent vessels, and cast off externally from time to time, to be succeeded by others; when first formed they are spherical, gradually becoming dry and flattened ; the deeper layers are more distinctly cellular, while the outer ones are scale-like. The epidermis has no vessels nor nerves, but is pierced by the ducts of the sebaceous and sweat glands, and by the shafts of the hair or feathers. The rete mucosum seems to be composed of the same microscopic elements as the overlying epidermis, being the principal seat of the pigment cells which give the color to the skin. The epidermis covers the whole exterior of the body, even the front of the eye, and is continuous with the epithelium of the internal mucous membrane; it is thickest in those parts most subjected to friction, as on the heel and the palms of the hands, where it becomes almost as hard as horn.
Its use is to protect the sensitive true skin from mechanical injury or the contact of air; in the living body, when abraded, it is speedily replaced; but when it is removed after death, the cutis underneath soon becomes brown and dry. The chemical composition of the thick epidermis of the heel has been found to be very nearly the same as that of the corneous matter of nails, hoofs, horns, and hair. The epidermis is familiarly seen in the occurrence of blisters, constituting the raised portion under which the fluid is effused. The epidermis prevents not only evaporation from the dermis, but also absorption of fluids from without; it is well known to the physician that in introducing medicinal agents into the system by the endermic method, the process is rendered very much more rapid and effectual by previously removing the epidermis by a blister.