Skiev, the external covering of the animal body, protecting the internal parts from external violence, and adapting itself by its elasticity to the various movements and changes of position; it also acts as the organ of touch, and as an excretory and absorbing surface. In the human skin, which may be taken as the type of that of the higher animals, the deepest portion is the corium, dermis, or cutis vera, as distinguished from the deciduous cuticle which overlies it, described under Epidermis. This true skin is dense and tough, somewhat elastic, composed of fibres interlaced in all directions, in whose interstices are masses of fat; the whole rests upon a layer of subcutaneous areolar tissue; within and below it are the sudoriparous or sweat glands (see Perspiration), the hair follicles (see Hair), and the sebaceous glands. From its upper surface rise the sensitive papillae, which are minute conical elevations, most numerous on the palmar surface of the hands and fingers, feet and toes, where they are arranged in double rows in parallel curved lines; the average length of the papillae, including the height of the ridge upon which they are placed, is about 1/100 of an inch, and the diameter at the base 1/250 they are abundantly supplied with blood, which explains their erectile turgescence under stimuli; they adhere more or less firmly to the cuticle.
The sebaceous or oil glands of the skin are formed on the same plan as the sudoriparous, and can often be distinguished from them only by the nature of their oily secreted product; they are distributed over the whole surface of the body, being least abundant where the perspiratory glandular are most numerous, and vice versa; they are absent on the palms and soles, but abundant on the face and scalp; they vary considerably in size, but the tubes are generally wider and straighter than those of the sweat glands; the structure is sometimes complicated. In the parts of the skin covered with hair, there is usually a pair of sebaceous ducts opening into the follicle of each hair. The object of their secretion is doubtless to prevent drying and cracking of the hair by the sun and air; this secretion is most abundant in tropical nations, and in some dark races has a characteristic odor, as in the case of the negro; its protective action in the tropics is often assisted by vegetable oils applied externally. The Meibomian glands on the edges of the eyelids are a double row of sebaceous follicles set along a straight duct; they secrete an oily matter for the lubrication of these parts, which in diseased conditions frequently sticks them together.
Another modification of sebaceous glands is to be found in the external ear passage, where is secreted the cerumen or waxy matter; they consist here of long, highly contorted tubes, well supplied with blood vessels. The color of the skin depends on pigment cells mixed with the inferior epidermic ones, in what is called the rete mucosum, or mucous layer, and considered by Flourens and other authors as a distinct membrane; all the hues of the races of man depend on the relative abundance of these cells and the tint of the contained pigment. The folds of the skin are for the most part produced by the contractions of the superficial muscles. The skin is pierced at the eves, ears, nostrils, mouth, rectum, and genitourinary opening; it is continuous internally with the mucous membrane, consisting of the same elements modified according to the variety of functions to be performed; it is very vascular, and freely supplied with nerves and lymphatics. The skin is the seat of the sense of touch in man, though in most animals hairs, scales, bony or horny plates and envelopes, and shells, render it nearly insensible to external influences, this sense in them being confined to particular portions or projecting organs; even in man the sensibility varies much in different parts, being most acute at the ends of the fingers and on the lips, and dullest on the back and limbs.
Aeration of the blood takes place to a certain extent through the skin, and in some naked-skinned fishes and batrachians this is a very important part of the respiratory process. It has been shown by experiment that in a frog, after the removal of the lungs, one fourth of a cubic inch of carbonic acid is exhaled from the skin in eight hours; in the human subject the amount of this gas given off by the skin varies from 1/60 to 1/30 of that exhaled from the lungs during the same time; where the lungs perform their office imperfectly, the temperature of the skin is often elevated; in all febrile diseases the skin should be kept moist. The absorbent powers of the skin are noticed under Absorption.
Compound Papillae of the Skin from the surface of the Hand, showing double, triple, and quadruple divisions. a. Base of a compound papilla, b, b, b. Its upper extremities, c. c, c. Points of other papillae, the base of which is not visible.
Vertical Section of the Skin, magnified.
a. Epidermis, b. Inferior layer of epidermis, or rete Malpi-ghianum. c. Papillae of the skin. d. Corium, or dermis. e ,f. Lobules of adipose tissue, g. Perspiratory glands. h. Ducts of the perspiratory glands, i. Their external orifices, k. Hair follicle. l. Hairs projecting from the skin. m. Hair papilla, n. Hair bulb. o. Shaft of hair in the hair follicle, p. Openings of the sebaceous glands.