Fig. 11.—a, sebaceous gland; b, hair; c, erector muscle. Mag. 200.
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.
The process of development of the sudoriferous or sweat-glands is very similar to that of the hairs. They consist of more or less convoluted tubes with walls formed of longitudinal fibres of connective tissue of the corium, lined with a single layer of large nucleated cells, which secrete the perspiration. The ducts, which are exceedingly narrow, and with walls of nucleated cells like those of the outer hair-sheaths, sometimes open directly through the epidermis, but more frequently into the orifice of a hair-sheath, just at the surface of the skin. Each hair is provided with a slanting muscle called the arrector or erector pili (see [Fig. 11]), which is contracted by cold or fear, and causes the hair to “bristle,” or stand on end; by forcing up the attached skin, it produces the effect known as “goose-skin.” The muscle, which is of the unstriped or involuntary kind, passes from near the hair-bulb to the epidermis, and just under the sebaceous glands, which it compresses when it contracts.
Beside the hair, and hair-sheaths, and the sebaceous and sudoriferous glands, the epidermis layer produces other structures of a horny character, including horns, hoofs, claws and finger-nails; which both chemically and anatomically are analogous to exaggerated hairs, such as the quills of the porcupine.
The whole of the epidermis, together with the hairs, is separated from the corium by an exceedingly fine membrane, called the hyaline or glassy layer. This forms the very thin buff-coloured “grain-” surface of tanned leather, which is evidently of different structure from the rest of the corium, since, if it gets scraped off before tanning, the exposed portion of the underlying skin remains nearly white, instead of colouring. The whole of the hair-sheath is enclosed in a coating of elastic and connective-tissue fibres, which are supplied with nerves and blood-vessels, and form part of the corium.