EPIDERMIS (Gr. epi, upon, and derma, the skin), a semi-transparent membrane, containing neither vessels nor nerves, and everywhere forming an external covering to the corium or true skin. See SKLN. The E. is called in ordinary language the scarf-skin. It consists of two layers, chemically and morphologically distinct—viz., the mucous layer, which lies immediately upon the corium, and the horny layer, which forms the outermost surface of the body.
The mucous layer ( known formerly as the rete MUCOMM s. malpighianum) is of a whitish or slightly brown tint (in the negro, dark gray or black), and is composed of small soft cells. The innermost of these cells, resting on the surface of the corium, are elongated and arranged perpendicularly; upon these follow elongated or roundish cells in many layers, which, in proportion to their distance from the corium, acquire, from their mutual pressure, a polygonal form, which may even be recognized in individual cells.
All the cells in the mucous layer are nucleated vesicles distended with fluid, and likewise containing minute granules, which diminish in number in the more external cells.
The horny layer forms the external semi - transparent part of the E., which in the white races is colorless, and is composed almost wholly of uniform cells, metamor phosed into plates or scales. The deepest plates in some degree resemble the upper most cells of the mucous layer; but in the second or third layer we find the flattening commence; till at length, after a gradual series of modifications, we have the hard, horny scales which occur on the surface, where they are regularly cast off with more or less rapidity, and replaced by those beneath them.* The color of the E. 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 E. which is colored, while the corium completely resembles that of Europeans. The perpendicular cells are the darkest, and form a sharply marked fringe at the edge of the clear corium. To these succeed brown cells, which accumulate in the depressions between the papillae, and as we approach the horny layer, we have yellowish cells. The horny layer of the negro also inclines to a yellow
or brownish tint.
Morbid coloration of the E. (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 E. varies extremely. While upon the cheeks, brow, and eyelids, it varies from th to of a line, on the palm of the hand it ranges from id to k 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 E. 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.
In plants, as in animals, the E. is formed of flattened cells, of which also new layers are continually produced from the bark below, whilst the outer ones dry up, lose their vitality, and peel off, crack and split off, or otherwise become separated from the living organism. The cells of the E. are often enlarged outwards, so as to form projections, sometimes very slight, sometimes elongated into hairs (q.v.). Glands (q.v). are also connected with the E., sometimes by the intervention of hairs, some times without, and in this way it contributes to the secretion of substances formed in plants by the wonderful chemistry of nature, and on which their value to man often greatly depends. The cells of the E. are usually filled with a colorless fluid, but resinous and waxy substances are sometimes found in them, and sometimes silica (as in grasses and equisetacem), sometimes carbonate of lime (as in the charm). The epider mis is pierced by stomata (q.v.). When the E of plants is subjected to prolonged maceration, it can often be made to separate into two parts; one, which is more strictly called the E., being the inner, lower, and thicker membrane; the other, which is called the pellicle or cuticle, being very thin, and extending continuously over every part of the plant except where it is pierced by the stomata. Thus, this superficial pellicle invests even the finest hairs. In some of the alga, it seems to constitute the whole integument. In the greater number of plants, the E. is thin and soft, but some times it is thick, and sometimes hard.