Epidermis (Gr. epi, 'upon,' and derma, 'the skin'), scarf-skin or cuticle, forming an external covering of a protective nature for the true skin or corium. Its under or attached surface is accurately moulded upon the true skin, and when the two are separated the cuticle presents impressions which exactly correspond to the elevations and depressions in the corium. The thickness of the epidermis varies from to of an inch, according to the amount of protection which the delicate and sensitive corium requires in different localities.
Thus, it is thickest and hardest in the palms of the hands and soles of the feet, but this is not altogether the result of an increased growth due to intermittent pressure, for the difference is well marked even in the feet. The epidermis consists of many layers of stratified cells superimposed on each other; and since they cover a free surface exposed to the air, the cells are classed in the group of stratified epithelia. The cells vary in shape, those placed next the corium being elongated in form, while those most superficial are flat and scale-like. On this account it is usual to describe the epidermis as consisting of two strata—an outer or superficial horny stratum, and a deeper or mucous stratum (or rete Malpighii). The horny stratum consists of cells, many of which have lost their nuclei and assumed the form of hard flattened scales of polygonal outline. The deepest layer of this stratum consists of closely packed cells with indistinct outlines and a clear, almost homogeneous, appearance, which has given them the name of the stratum lucidum. The superficial cells of the horny layer are regularly cast off by desquamation, and replaced by those beneath them. In reptiles and amphibians this layer is periodically cast off in a more or less entire state, a new one being previously formed beneath it; and in man desquamation in large patches occurs after certain diseases, especially scarlet fever.
The mucous stratum lies next the corium. It consists of nucleated cells of various shapes. Those next the corium are elongated and placed perpendicularly to its surface. Above these are cells more rounded or polyhedral in outline, and possessing many spines or prickle-like processes, by the points of which the cells adhere to each other, and thus leave fine lymph channels through which the nutrient plasma is transmitted to the cells. The colouring matter of the epidermis is found in the cells of the mucous stratum. A more or less dark pigment is often deposited in the face, neck, and hands of the fair races of men during exposure to the sun, forming isolated colour-spots called freckles; but in the dark races the pigment-granules are distributed throughout the cells of the mucous stratum, the deepest or perpendicular cells being the darkest. Instances of white negroes are on record, not as a consequence of change of climate, but as an abnormal condition of the epidermis. Fine nerve-fibrils penetrate between the cells of the mucous stratum, and undergo a certain amount of ramification, but do not form a network. In some reptiles, and in the Edentata among mammals, the epidermis forms large plates or scales, while epidermal appendages assume various forms—e.g. hair, nail, horns, spines, bristles, feathers, claws, hoof, horns, &c.
In plants the epidermis is formed of flattened cells, usually only one layer deep, frequently bearing hairs and stomata. The exposed cell-walls upon the outer surface are frequently thickened as the cuticle.