The _hairs_ also, which occur all over the body, except on the palms of the hands and the soles of the feet, are a modification of the epithelium. Each hair has a bulbous root springing from an involution in the epidermis and derma called the _hair follicle_, into which one or two sebaceous glands empty. It is raised by involuntary muscle fibers and grows by constant additions to the surface by which it is attached. This growth seems, however, to be limited, and when its term is reached the hair falls out and is replaced by another. The horny epithelial cells that go to form the hair contain the pigment that gives it its color. [Illustration: FIG. 12.--Skin and longitudinal section of hair: _a_, Epidermis; _b_, corium; _c_, sebaceous gland; _d_, fibrous root-sheath; _e_, glassy membrane; _f_, outer root-sheath; _g_, inner root-sheath; _h_, expanded bulbous end of hair; _i_, papilla of hair; _j_, arrector pili; _k_, adipose tissue. (Leroy)] Like the hairs, the _sebaceous glands_ are situated in all parts of the body except the palms of the hands and the soles of the feet. They lie in the papillary layer and empty into the hair follicles, except occasionally, when they empty directly upon the surface of the skin. They secrete an oily substance, _sebum_, the débris resulting from the degeneration of the epithelial cells of the gland itself, which serves to keep the hair glossy and the skin soft and flexible. The _sweat glands_, on the other hand, are more frequent on the palms and soles and though sometimes found in the derma are usually situated lower down in the subcutaneous cellular tissue. They are least numerous on the back and neck. Coiled up in the lower layers of the skin, they discharge the sweat through a spiral excretory duct upon its free surface. The _sweat_ is a clear, colorless, watery fluid with a salty taste, an alkaline reaction, and a characteristic odor that varies with the individual. If very scanty, it may be acid in reaction. Besides water it contains a small percentage of solids, as inorganic salts, especially sodium chloride, fatty acids, neutral fats, and at times, especially in some diseases of the kidneys, urea, that is, the end-products of the metabolism of starches and fats chiefly. There is usually also some carbon dioxide, whence the expression cutaneous respiration. The sweat serves to keep the skin moist and in good condition, to remove outworn and poisonous or irritating matters, and to regulate the temperature. As a rule it evaporates upon reaching the surface, in which case it is known as invisible or insensible perspiration, but if conditions of the atmosphere are not favorable to prompt evaporation, as when the air is damp, the skin becomes damp and there is visible perspiration.