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, 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]

Fig. 121--*Section of skin* magnified, _a, b._ Epidermis, _b._ Pigment layer. _c._ Papillae, _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 elevations, called _papillae_. 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]

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 papillae 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. 121). The epidermis contains no nerves and is therefore non-sensitive. The hair and the nails are important modifications of the epidermis.

*A Hair* is a slender cylinder, formed by the union of epidermal cells, which grows from a kind of pit in the dermis, called the _hair follicle_. The oval and somewhat enlarged part of the hair within the follicle is called the _root_, or _bulb_, and the uniform cylinder beyond the follicle is called the _shaft_. Connected with the sides of the follicles are the _oil_, or _sebaceous, glands_ (Figs. 121 and 122). These secrete an oily liquid which keeps the hair and cuticle soft and pliable. Attached to the inner ends of the follicles are small, involuntary muscles whose contractions cause the roughened condition of the skin that occurs on exposure to cold.

*A Nail* is a tough and rather horny plate of epidermal tissue which grows from a depression in the dermis, called the _matrix_. The back part of the nail is known as the _root_, the middle convex portion as the _body_, and the front margin as _the free edge_ (Fig. 123). Material for the growth of the nail is derived from the matrix, which is lined with active epidermal cells and is richly supplied with blood vessels. Cells added to the root cause the nail to grow in length (forward) and cells added to the under surface cause it to grow in thickness. The cuticle adheres to the nail around its entire circumference so that the covering over the dermis is complete.

[Fig. 123]

Fig. 123--*Section of end of finger* showing nail in position.

*Functions of the Skin.*--The chief function of the skin is that of protection. It is able to protect the body on account of the tough connective tissue in the dermis, the non-sensitive cells of the epidermis, and also by the touch corpuscles and their connecting nerve fibers. This protection is of at least three kinds, as follows:

1. _From mechanical injuries_ such as might result from contact with hard, rough, or sharp objects. The main quality needed for resisting mechanical injuries is _toughness_, and this is supplied both by the epidermis and by the connective tissue of the dermis.

2. _From chemical injuries_ caused by contact with various chemical agents, as acids, alkalies, and the oxygen of the air. The epidermis, being of such a nature as to resist to a considerable extent the action of chemical agents, affords protection from these substances. (89)

3. _From disease germs_ which are everywhere present. The epidermis is the main protective agent against attacks of germs, but should the epidermis be broken, they meet with further resistance from the fluids of the dermis and the white corpuscles of the blood.

4. _From an excessive evaporation of liquid from the surface of the body_. In the performance of this function, the skin is an important means of keeping the tissues soft and the blood and lymph from becoming too concentrated.

*Other Functions of the Skin.*--Through the perspiratory glands the skin is an _organ of excretion_. While the secretion from a single gland is small, the waste that leaves the body through all of the perspiratory glands is considerable (90) (page 206). By means of the nerves terminating in the touch corpuscles, the skin serves as the _organ of touch_, or feeling (