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—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[pg 268] 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 (Chapter XX). To a slight extent also the skin[pg 269] may absorb liquid substances, these being taken up by the blood and lymph vessels, and perform a respiratory function, throwing off carbon dioxide. But the most important function of the skin, in addition to protection, is that of serving as