THE STRUCTURE OF THE SKIN.
The skin consists of a deep layer called the dermis, corium, or true skin (cutis vera), and of a superficial layer—the epider mis (Greek, epi, upon, and derma, the skin), cuticle, or scarf skin (a, Fig. 164). The true skin consists of fibrous tissue, the bundles of which form a felted interlacement. It lies upon a bed of fatty tissue (c' d, Fig. 164) which fills up the inequalities of the surface on which the skin rests. Groups of the fat cells of this tissue also abound in the deepest layers of the true skin. Pervading the fibrous tissue is also an abundance of fibres of the elastic sort 4 (p. 56) which confer elasticity on the skin. The true skin (b–e) is very vascular, that is, is richly supplied with blood-vessels, so that when cut it bleeds ; and nerve fibres are likewise disposed in it, conferring sensibility. The surface of the true skin is thrown into a series of elevations, papillae, or finger-like prominences (b, Fig. 164) which are specially rich in capillary blood vessels and nerve-endings, and which are thus particularly vascular and sensitive. Above the true skin is the scarf skin, the projections of the former fitting into excavations in the latter. The epidermis, however, is composed entirely of cells, and is quite devoid of blood-vessels or nerves, so that it may be cut without bleeding or pain. _mere are several layers En cells, anu the shape of the cells alters from the deep parts upwards. The cells directly lining the surface of the true skin and the papillae are columnar and nucleated. They are soft and active cells, and clothing the papillae are several layers of them. In the layers nearer the surface the cells lose their columnar shape and become more flattened. They also gradually become less soft and more horny, until towards the surface they are flattened and scale-like. The surface scales are continually being thrown or rubbed off, and their places are supplied by deeper cells which reach the surface by growth from below. New cells are continually being produced in the deep layer in contact with the true skin; and as they are formed they push upwards the already existing cells. So that cells originally active and columnar gradually pass upwards, becoming horny, till they are finally cast off. The fine white dust that one may scrape off the skin consists of these horny scales.
It is in the deep and active layers, called the rete mucosum, of the epidermis that colour ing matters are present, which give the hue to 12 the skin. For example, in dark races black
pigment is present in these cells. The epi dermis is thickest over the parts exposed to greatest pressure or friction, securing protec tion to the sensitive true skin below.
At the openings of the body the skin passes into mucous membrane, the structure of the two being practically identical, the difference being merely in the thinness of the epidermal covering of the mucous membrane and the in creased supply of blood to the membrane.
Glands of the Skin.--The special glands of the skin are the sudoriparous (e, Fig. 164) or sweat glands (Latin, sudor, sweat, andpario, to bring forth). They are tubular glands. Deep in the substance of the true skin, or in the fatty tissue beneath it, the tube is coiled up into a sort of ball. From the coil the tube passes upwards through the true skin, follow ing a wavy course, till it reaches the epidermis, which it penetrates in a spiral manner till it opens on the surface. Two of such glands are shown in Fig. 164 (e e'). The tubes consist of delicate membranous walls lined within by cells. The coiled part of the gland is sur rounded by a dense net-work of fine blood vessels, and thus the cells of the gland are separated from the blood by only a very line membranous partition, and can draw from it what supplies they need for their particular work.
It is estimated that the total number of sweat glands in the human skin is over two millions. They are not, however, equally dis tributed over the body. They are fewest in the back and neck, where it is estimated there are on an average 400 to the square inch. They are in greatest number in the skin of the palm of the hand, where they amount to nearly 3000 in each square inch ; according to Erasmus Wilson, 3528. Their openings occur on the ridges into which the skin is there thrown, and may be made out by a hand lens. Next to the palm of the hand they occur in greatest number in the sole of the foot, next on the back of the hand and foot, and the smallest number is that already noted in the skin of the back. The length of a tube, when fully straightened out, is about inch ; so that, according to Sir E. Wilson, in 1 square inch of skin from the palm of the hand there is a length of sweat tube equal to 73-i feet. If we estimate the number of glands in the body to be between two and three millions, the total length of tube devoted to the secretion of sweat would be about 10 miles. According to Erasmus Wilson's estimate it amounts to even 28 miles.