CHAPTER XVI - THE SKIN
Protective coverings are found at all the exposed surfaces of the body. These vary considerably at different places, each being adapted to the conditions under which it serves. The most important ones are the skin, which covers the entire external surface of the body; the mucous membrane, which lines all the cavities that communicate by openings with the external surface; and the serous membrane, which, including the synovial membranes, lines all the closed cavities of the body. In addition to the protection which it affords, the skin is one of the means by which the body is brought into proper relations with its surroundings. It is because of this function that we take up the study of the skin at this time.
The Skin is one of the most complex structures of the body, and serves several distinct purposes. It is estimated to have an area of from 14 to 16 square feet, and to have a thickness which varies from less than one eighth to more than one fourth of an inch. It is thickest on the palms of the hands and the soles of the feet, the places where it is most subject to wear. It is made up of two distinct layers—an outer layer called the epidermis, or cuticle, and an inner layer called the dermis, or cutis vera (Fig. 121).
The Dermis.—This is the thicker and heavier of the two layers, and is made up chiefly of connective tissue. The network of tough fibers which this tissue supplies, [pg 265] forms the essential body of the dermis and gives to it its power of resistance. It is on account of the connective tissue that the skins of animals can be converted into leather by tanning. A variety of structures, including blood and lymph vessels, oil and perspiratory glands, hair follicles, and nerves, are found embedded in the connective tissue (Fig. 122). These aid in different ways in the work of the skin.
Fig. 121—Section of skin magnified, a, b. Epidermis, b. Pigment layer. c. Papillæ, d. Dermis. e. Fatty tissue. f, g, h. Sweat gland and duct. i, k. Hair and follicle. l. Oil gland.
On the outer surface of the dermis are numerous [pg 266]elevations, called papillæ. These average about one one-hundredth of an inch in height, and one two hundred and fiftieth of an inch in diameter. They are most numerous on the palms of the hands, the soles of the feet, and the under surfaces of the fingers and toes. At these places they are larger than in other parts of the body, and are closely grouped, forming the parallel curved ridges which cover the surfaces. Each papilla contains a loop of capillaries and a small nerve, and many of them are crowned with touch corpuscles (page 342).
Fig. 122—Diagram of section of skin showing its different structures.
The Epidermis is much thinner than the dermis. It is made up of several layers of cells which are flat and scale-like at the surface, but are rounded in form where the epidermis joins the dermis. The epidermis has the appearance of being moulded onto the dermis, filling up the depressions between the papillæ and having corresponding irregularities (Fig. 121). No blood vessels are found in the epidermis, its nourishment being derived from the lymph which reaches it from the dermis. Only the part next to the dermis is made up of living cells. These are active, however, in the formation of new cells, which take the place of those that are worn off at the surface. Some of the cells belonging to the inner layer of epidermis contain pigment granules, which give the skin its color (Fig.[pg 267] 121). The epidermis contains no nerves and is therefore non-sensitive. The hair and the nails are important modifications of the epidermis.