[Illustration: FIG. 12.--_a_, hair; _b_, hair cuticle; _c_, inner root-sheath; _d_, outer root-sheath; _e_, dermic coat of hair-sheath; _f_, origin of inner sheath; _g_, bulb; _h_, hair-papilla.] Near the opening of the hair-sheath to the surface of the skin the ducts of the sebaceous or fat-glands pass into the sheath and secrete a sort of oil to lubricate the hair. The glands themselves are formed of large nucleated cells arranged somewhat like a bunch of grapes; the upper and more central ones being highly charged with fatty matter. Their appearance is shown in Fig. 11. The base of the hair is a bulb, enclosing the hair papilla _h_ (Fig. 12), which is a projecting knob of the true skin and which by means of the blood-vessels contained in it supplies nourishment to the hair. The hair-bulb is composed of round soft cells, which multiply rapidly, and pressing upwards through the hair-sheath, become hardened, thus increasing the length of the hair. The cells outside the bulb, shown at _f_ in Fig. 12, pass upwards as they grow, and form a coating around the hair, known as the “inner root-sheath.” In embryonic development, a small knob of cells forms on the under side of the _epidermis_, over a knot of capillary blood-vessels in the _corium_, and enlarges and sinks deeper into the latter, while the root-bulb of the young hair is formed within it, surrounding the capillaries from which it derives nourishment, and which form the hair-papilla, Fig. 13. In the renewal of hair in the adult animal the process is very similar. The bulb of the old hair withers, and the hair falls out, and in the meantime a thickening takes place in the epidermal coating of the bottom of the sheath, and the young hair is formed below, and usually to one side of the old one, growing into the sheath, and taking the place of the old hair. This is one cause of the difficulty of removing ground-hairs in the process of unhairing, since they are not only short, but deeper seated than the old ones.