=Adaptation of the scalp to a tight warm covering= is accomplished through the shedding of the hair rendered useless by the covering. It is impossible to stop the growth of superfluous hair unless the hair papillæ are destroyed with an electric needle, such is the vitality of hair; yet many men, by overheating the head and cutting off the circulation with tight hats, destroy much of the hair before reaching middle age. The health of the hair can be restored and its loss be stopped by going bareheaded except in the hot sun or in extremely cold weather. This frees the circulation; cold air and light stimulate the cells of the scalp. Some men wear hats, even at night in summer. The brain needs the protection of the hair. Beard protects the larynx or voice box, which is large and exposed in man. It was also a protection in hunting wild beasts and in war. Compare mane of lion, not possessed by lioness. “Goose-flesh” after a cold bath is caused by the contraction of small muscles (colored Fig. 1), raising the now tiny hairs in an absurdly useless effort to keep the body warm. =The nails= are dense, thick plates of epidermis growing from a number of papillæ situated in a groove, or fold, of the skin; there are many fine papillæ along the bed from which the nail grows. Since it grows from its under side as well as from the little fold of skin at its root, the nail is thicker at the end than near the root. [Illustration: FIG. 23.--_A_, DEVELOPMENT OF SWEAT GLAND; _B_, SWEAT TUBE DEVELOPED.] The =oil glands= empty into the hair follicles (colored Fig. 1). They form an oil from the blood that _keeps the hair glossy and the surface of the skin soft and flexible_ by preventing excessive drying. Hair oil should never be used upon the hair, as the oil soon becomes rancid, and besides causes dust and dirt to stick to the hair. =The sweat glands= (Fig. 23), like the hair bulbs, are deep in the lowest part of the dermis. _A sweat gland has the form, of a tube coiled into a ball_ (colored Fig. 1). This tube continues as a duct through the two layers of skin, and its opening at the surface is called a _pore_ (Fig. 24). The perspiration evaporates as fast as it flows out through the pores, if the secretion is slow; but if poured out rapidly, it gathers into drops (Exp. 3). The perspiration is chiefly water, containing in solution several salts, including common salt and a trace of a white, crystalline substance called _urea_. The material for the perspiration is furnished by the blood flowing around the gland in a network of fine tubes. The amount of the perspiration is controlled in two ways: by _nerves that regulate the activity of the epithelial_ cells lining the gland, and by nerves that _regulate the size of the blood vessels_ supplying the gland (Fig. 25). [Illustration: FIG. 24.--PORES on ridges in palm of hand.]