Epidermis

Epidermis (Gk. derma, skin), the outer covering of the skin. The skin consists of two parts, the corium or true skin, which is raised externally forming a number of prominences called papillae, and the epidermis or cuticle, which is superimposed upon the corium, and is accurately moulded over the papillee, between the depressions of which it dips. The epidermis is composed of layers of epithelial cells. [Epithelium.] These cells are usually described as being arranged in four layers; commencing from within, the first layer, that lying next the corium, is the stratum Malpiyhii, which is composed of spindle-shaped or rounded cells; it is this layer which fills up the depressions between the papillee; proceeding outwards the next layer is known as the stratum granulosum, and consists of a single layer of flattened cells, containing a number of granules.

External to this is the stratum lucidum, so called from its cells presenting a clear, bright appearance, while most external of all is the stratum corneum, which consists of a large number of layers of much-flattened cells. The nucleus, which is a prominent feature in the cells of the deeper layers, becomes gradually lost on proceeding towards the surface, and is completely absent in the most superficial cells of the stratum corneum; these last, in fact, consist of mere degenerate scales, and can scarcely be termed cells at all. The outer layers of the stratum corneum are continually being rubbed off, and are replaced by the cells from below; these as they approach the surface become more and more flattened and degenerate. The thickness of the epidermis varies much in different parts of the body; it is thickest in the palms of the hands and soles of the feet. In negroes pigment granules are dispersed throughout the stratum Malpiyhii, and these give rise to the coloured skin of the dark races of mankind. In Botany the term is applied to the outer system of cells in most plants serving as a protection and checking excessively rapid transpiration. In some algae and mosses a rudimentary or indistinctly differentiated epidermis occurs; but in higher plants this layer is formed from a primitive merismatic dermatogen. Some of the cells of the epidermis, both on roots and on aerial structures, are often elongated into hairs, which may become divided into several cells, and the walls of which are commonly unthickened. The outer cell-walls of the rest of the epidermis of aerial organs generally form the continuous structureless membrane known as cuticle (q.v.); but the epidermis of the root, requiring to admit moisture ratherthan to retain it, is generally not cuticularised. The cells of the epidermis commonly contain air or water, with little or no chlorophyll, and can be peeled off as a colourless membrane. In some evergreen leaves there are two or three layers of epidermis. On the stems of perennial plants the epidermis is generally split and thrown off at the end of the first season by the development of cork (q.v.) beneath it, so that smooth green shoots become brown and finely furrowed; but mistletoe (q.v.) retains its epidermis by its remaining merismatic, and in roses and willows the epidermis itself becomes thethe Uogen or cork-cambium, and gives rise to cork externally. Most surfaces of epidermis in contact with the air are interrupted by openings for transpiration known as stomates (q.v.), which are surrounded by two or more guardcells that retain both protoplasm and chlorophyll. Submerged structures and roots have no stomates, for which reason the epidermis of the latter was at one time distinguished as "epithelium."