SKIM. Considered in its general physiological and histological (or textural) relations, the skin is merely a part of the great. MUCOUS. system to which the mucous membrane and secreting glands also belong, and which consists of two eSsential elements—a base ment tissue, composed of simple cutaneous membrane, and an epithelium of nucleated resting on it—while beneath' the hasementwembrane are vessels, nerves, and connective tissue. See EPITHELIUM and MUCOUS MEMBRANES. In the skin, the hard and thick epithelium is termed cuticle or epidermis,. and the true skin below it is termed the depita or Mix sera, and is chiefly formed of modified and very dense connective (or areolwr cellular) tissue.
'he external surface of the skin formed by the cuticle is marked by furrows of dif ferent kinds. Some (termed furrows of motion) occur transversely in the neighborho'ed of joints, on the side of flexion; others correspond to the insertion of cutaneous muscles; while others, of quite another kind, arc seen in aged and emaciated persons, and after Vie subsidence of any great distention of the integument; and besides these coarse lines, ghost parts of the skin are grooved with very minute furrows, which assume various courses in relation to one another. These minute furrows are most distinctly seen on the palmar aspect of the hand and fingers. and on the sole of the foot. The outer sur face of the skin also presents innumerable pores for the discharge c f the contents of the sudorirrrous and sebaceous follicles, or the sweat and fat glands; and the modifications of epidermis known as hair and nails occur on the same surface.
The deep layer of the skin consists of connective tissue. in which both the white and yellow fibrous elements are considerably modified as to the proportions in which they occur. and smooth muscles are present in no inconsiderable quantity in some parts of the skin, Where great exiensibilitv, widi elasticity, is required. the yellow (elastic) element predominates; and where strength and resistance are specially required, as in the sole of the toot, the ruffs is chiel:y composed of a dense interweaving of the white (inelastic) clement. The thickness and strength of this layer differ greatly in different
parts, according to the amount of resistance required against pressure. The skin in thicker on the hinder surface of the body than in front, and on the outer than on the inner sides of the limbs. "It is unusnally thin over the flexures of the joints. It is par ticularly delicate in the eyelids, and proportionably so in some other situations where great mobility is demanded. In regions which are most subject to external pressure, .as the soles of the feet, it is firmly united by very dense lamina: to the sub-cutaneous fascia; and the intervals between these are provided with pellets of fat, forming a cushion, as an additional means of protection to the delicate organs it incloses and covers. Among the lower animals we may notice numberless examples of an analogous kind."—Todd and Bowman's Physiokviral Anatomy and Physiology (gr vol_ i. p. 407. The blubber of the whale merely represents, iu a very exaggerated form, the layer of fat which gen erally occurs in the sub-cutaneous areolar tissue of man and most animals, serving as it soft bed on which the skin may rest, and gives the appearance of plumpness and sym metry to the outline of the body. It is on the external surface of the cut's that the tactile papilla, or true organs of touch, are developed. Kfilliker divides the trite cutiC into the " reticular" and " papillary" portions, the hitter hieing the reddish-gray external superficial layer which contains the upper portion of the hair follicles and cutaucous glands, and whose most important element is these tactile Tney are most abundant and largest in the pain of the hand* and the,sole of the foot, while in the back and ill the outer sides of the limbs they arc almost entirely absent. They occur as small, semi-transparent, flexible elevations, which are usually conical or club-shaped in form; but in certain parts, as the palm of the hand, present numerous points (mu which case they are termed compound papilla[).