Through digestion and respiration, the blood is continually supplied with material for its renewal; and, while the nutritive constituents of the food are retained to promote the growth of the body, those which are useless or injurious are in various ways expelled. There are, perhaps, few parts of the body more actively concerned in this removal than the skin.

Illustration: Fig. 46: An ideal view of the papillÆ.
Fig. 46: An ideal view of the papillÆ. 1, 1. Cutis vera. 2, 2. Papillary layer. 3, 3. Arteries of the papillÆ. 4, 4. Nerves of the papillÆ. 5, 5. Veins of the papillÆ.

The skin is a membranous envelope covering the entire body. It consists of two layers, termed the Cutis Vera, or true skin, and the Epidermis, or cuticle. The Cutis Vera is composed of fibers similar to those of the cellular tissue. It consists of white and yellow fibers, which are more densely woven near the surface than deeper in the structure; the white give strength, the yellow strength and elasticity combined. The true skin may be divided into two layers, differing in their characteristics, and termed respectively the superficial or papillary layer, and the deep or fibrous layer. Upon the external surface, are little conical prominences, known as papillÆ. The papillÆ are irregularly distributed over the body, in some parts being smaller and more numerous than in others, as on the finger-ends, where their summits are so intimately connected as to form a tolerably smooth surface. It is owing to their perfect development, that the finger-tips are adapted to receive the most delicate impressions of touch. Although every part of the skin is sensitive, yet the papillÆ are extremely so, for they are the principal means through which the impressions of objects are communicated. Each papilla not only contains a minute vein and artery, but it also incloses a loop of sensitive nerves. When the body is exposed to cold, these papillÆ can be more distinctly seen in the form of prominences, commonly known as "goose-pimples."

Illustration: Fig. 47. A section of the skin, showing its arteries and veins.
Fig. 47. A section of the skin, showing its arteries and veins. A, A. Arterial branches. B, B. Capillaries in which the branches terminate. C. The venous trunk into which the blood from the capillaries flows.

The internal, or fibrous layer of the skin, contains numerous depressions, each of which furnishes a receptacle for fat. While the skin is supplied with a complete net-work of arteries, veins, and nerves, which make it sensitive to the slightest touch, it also contains numerous lymphatic vessels, so minute that they are invisible to the naked eye.

Among the agents adapted for expelling the excretions from the system, few surpass the Sudoriferous Glands. These are minute organs which wind in and out over the whole extent of the true skin, and secrete the perspiration. Though much of it passes off as insensible transpiration, yet it often accumulates in drops of sweat, during long-continued exercise or exposure to a high temperature. The office of the perspiration is two-fold. It removes noxious matter from the system, and diminishes animal heat, and thereby equalizes the temperature of the body. It also renders the skin soft and pliable, thus better adapting it to the movements of the muscles. The Sebaceous Glands, which are placed in the true skin, are less abundant where the sudoriferous glands are most numerous, and vice versa. Here, as elsewhere, nature acts with systematic and intelligent design. The perspiratory glands are distributed where they are most needed,—in the eyelids, serving as lubricators; in the ear passages, to produce the cerumen, or wax, which prevents the intrusion of small insects; and in the scalp, to supply the hair with its natural pomatum.

Illustration: Fig. 48. A perspiratory gland, highly magnified.
Fig. 48. A perspiratory gland, highly magnified. 1, 1. The gland. 2, 2. Excretory ducts uniting to form a tube which tortuously perforates the cuticle at 3, and opens obliquely on its surface at 4.

Illustration: Fig. 49. A representation of oil-tubes from the scalp and nose.
Fig. 49. A representation of oil-tubes from the scalp and nose.

Illustration: Fig. 50. Anatomy of the skin.
Fig. 50. Anatomy of the skin. 5, 5. Cutis vera (true skin). 4, 4. Nervous tissue. 3, 3. Sensitive layer in which are seen the nerves. 2, 2. The layer containing pigment cells. 1, 1. Epidermis (cuticle).

The Epidermis, or Cuticle, so called because it is placed upon the skin, is the outer layer of the skin. Since it is entirely destitute of nerves and blood-vessels, it is not sensitive. Like the cutis vera, it has two surfaces composed of layers. The internal, or Rete Mucosum, which is made up chiefly of pigment cells, is adapted to the irregularities of the cutis vera, and sends prolongations into all its glandular follicles. The external surface, or epidermis proper, is elastic, destitute of coloring matter, and consists of mere horny scales. As soon as dry, they are removed in the form of scurf, and replaced by new ones from the cutis vera. These scales may be removed by a wet-sheet pack, or by friction. The cuticle is constantly undergoing renewal. This layer serves to cover and protect the nervous tissue of the true skin beneath. We may here observe that the cuticle contains the pigment for coloring the skin. In dark races, as the negro, the cuticle is very thick and filled with black pigment. The radiation of animal heat is dependent upon the thickness and color of this cuticle. Thus, in the dark races, the pigment cells are most numerous, and in proportion as the skin is dark or fair do we find these cells in greater or lesser abundance. The skin of the Albino is of pearly whiteness, devoid even of the pink or brown tint which that of the European always possesses. This peculiarity must be attributed to the absence of pigment cells which, when present, always present a more or less dark color. The theory that climate alone is capable of producing all these diversities is simply absurd. The Esquimaux, who live in Greenland and the arctic regions of America, are remarkable for the darkness of their complexion. Humboldt remarks that the American tribes of the tropical regions have no darker skin than the mountaineers of the temperate zone. Climate may modify the complexion, but it cannot make it.

Illustration: Fig. 51. Structure of the human hair.
Fig. 51. Structure of the human hair. A. External surface of the shaft, showing the transverse striÆ and jagged boundary, caused by the imbrications of the scaly cortex. B. Longitudinal section of the shaft, showing the fibrous character of the medullary substance, and the arrangement of the pigmentary matter. C. Transverse sections, showing the distinction between the cortical and medullary substances, and the central collection of pigmentary matter, sometimes found in the latter. Magnified 310 diameters.

Hairs are horny appendages of the skin, and, with the exception of the hands, the soles of the feet, the backs of the fingers and toes, between the last joint and the nail, and the upper eyelids, are distributed more or less abundantly over every part of the surface of the body. Over the greater part of the surface the hairs are very minute, and in some places are not actually apparent above the level of the skin; but the hair of the head, when permitted to reach its full growth, attains a length of from twenty inches to a yard, and, in rare instances, even six feet. A hair may be divided into a middle portion, or shaft, and two extremities; a peripheral extremity, called the point; and a central extremity, inclosed within the hair sac, or follicle, termed the root. The root is somewhat greater in diameter than the shaft, and cylindrical in form, while its lower part expands into an oval mass, called the bulb. The shaft of the hair is not often perfectly cylindrical, but is more or less flattened, which circumstance gives rise to waving and curling hair; and, when the flattening is spiral in direction, the curling will be very great. A hair is composed of three different layers of cell-tissues: a loose, cellulated substance, which occupies its center, and constitutes the medulla, or pith; the fibrous tissue, which incloses the medulla, and forms the chief bulk of the hair; and a thin layer, which envelops this fibrous structure, and forms the smooth surface of the hair. The medulla is absent in the downy hairs, but in the coarser class it is always present, especially in white hair. The color of hair is due partly to the granules and partly to an inter-granular substance, which occupies the interstices of the granules and the fibers. The quantity of hair varies according to the proximity and condition of the follicles. The average number of hairs of the head may be stated at 1,000 in a superficial square inch; and, as the surface of the scalp has an area of about one hundred and twenty superficial square inches, the average number of hairs on the entire head is 120,000. The hair possesses great durability, as is evinced by its endurance of chemical processes, and by its discovery, in the tombs of mummies more than two thousand years old. The hair is remarkable for its elasticity and strength. Hair is found to differ materially from horn in its chemical composition. According to Vauquelin, its constituents are animal matter, a greenish-black oil, a white, concrete oil, phosphate of lime, a trace of carbonate of lime, oxide of manganese, iron, sulphur, and silex. Red hair contains a reddish oil, a large proportion of sulphur, and a small quantity of iron. White hair contains a white oil, and phosphate of magnesia. It has been supposed that hair grows after death, but this theory was probably due to the lengthening of the hair by the absorption of moisture from the body or atmosphere.

The nails constitute another class of appendages of the skin. They consist of thin plates of horny tissue, having a root, a body, and a free extremity. The root, as well as the lateral portion, is implanted in the skin, and has a thin margin which is received into a groove of the true skin. The under surface is furrowed, while the upper is comparatively smooth. The nails grow in the same manner as the cuticle.