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_. [Illustration: FIG. 13.--Development of young hair.] [Illustration: FIG. 14.--Connective-tissue fibres. (Ranvier.)]