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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).

Fig. 24.—Pores on ridges in palm of hand.

Thought Questions.—Freckles, Warts, Moles, Scars, Proud Flesh, Pimples, Blackheads. Use these names in the proper places below:—