EP'IDER'MIS (Lat., from Ok. irri4eppic, up per skin, from hri, epi, upon ± (Simla, derma, skin). The cuticle, or searf-skin, a semi-trans parent membrane. containing neither vessels nor nerves, and everywhere forming an external cov ering to the derma, or true skin. It consists of two layers, chemically and morphologically dis tinct—viz., the mucous layer, which lies imme diately upon the corium, and the horny layer, which forms the outermost surface of the body. The mucous layer (the rete inucositin or rele mat piyhi) is of a whitish or slightly brown tint (in the negro dark gray or black), and is composed of rounded or cuboidal cells, distended with fluid, and likewise containing minute granules, which diminish in number in the more external cells. The horny layer forums the external semi-trans parent part of the epidermis, which in the white races is colorless, and is composed almost wholly of uniform cells agglutinated and flattened. The color of the epidermis differs in different persons and in different parts of the body. It is deepest around the nipple, especially in women during pregnancy and after they have borne children. A more or less dark pigment is often deposited in persons who are exposed to the sun, in the face, neck, back of the hands, etc. These tints are not produced by special pigment-cells, but are seated in the common cells of the mucous layer, round whose nuclei granular pigment is deposited.
In the negro and the other colored races it is also only the epidermis which is colored. :Morbid col oration of the epidermis (freckles, mother's marks, etc.) is produced in the same way as the color of the negro's skin. Numerous instances of partially or entirely white negroes and of black Europeans, not as a consequence of change of climate, but as an abnormal condition of the skin, are on record.
The thickness of the epidermis varies extreme ly. While upon the cheeks, brow, and eyelids, it varies from to 1. of a line, on the palm of the hand it ranges from one-third to one-half a line, and on the sole of the foot sometimes even exceeds a line. In some parts of the body the horny layer is thicker than the mucous; in other, the mucous is the thicker of the two. As the chief use of the epidermis is that of affording protection to the soft and tender subjacent parts, it attains its greatest thickness on those portions of the body (the palm of the hand and the sole of the foot) which are most exposed to pressure and friction. The hair and nails belong to the integumentary system, as well as horns in lower animals, and are modifications of epidermis. See INTEGUMENT.