HAIR. The characteristic covering of the mammiferous class of animals. It consists of slender, more or less elon gated, horny filaments, secreted by a matrix, consisting of a conical gland or bulb, and a capsule, which is situated in the mesh-work of the corium, or true skin. The hairs pass out through ca nals in the corium, which are lined by a thin layer of cuticle adherent to the base of the hair : the straightness or curl of the hair depends on the form of the ca nal through which it passes. Spines, bristles, fur, and wool, are all modifica tions of hair, having the same chemical composition, mode of formation, and general structure.
In the spines of the porcupine, the bulb secretes a fluted pith, and the cap sule invests it with a horny sheath, the transparency of which allows the ridges of the central part to be seen. In thg spines of the hedgehog, the spine-like whiskers of the walrus, and the bristles of the hog, the twofold structure of the hair is very conspicuous : but in the finer kind of hair, as in the human head and beard, the central pith can only be de monstrated in fine transverse sections viewed with the microscope. Some kinds of hair, as of the human head, the mane and tail of the horse, are perennial, and grow continuously by a persistent se tivity of the formative capsule and pulp ; other kinds, as the ordinary hair of the horse, cow, and deer, are annual, and the coat is shed at particular seasons. In the deer the horns are shed contempora neously with the deciduous hair.
Many quadrupeds, especially those of cold climates, have two kinds of hair a long and coarse kind, forming their visible external covering; and a shorter, finer, and more abundant kind, which lies close to the skin, and called "fur" It is one of the processes in the arts to remove the close hairs, and leave the fur attached to the dried skin, as in the pre paration of seal-skin, &c. The peculiar characteristic of wool, and that on which its valuable qualities chiefly depend, is the serrated character of its surface, aris ing from its structure, which consists of a series or succession of inverted cones, the base of each being directed from the root of the woolly fibre, and receiving the apex of the succeeding cone. It re
sults from this structure that the pres sure to which the workman subjects the wool in moving it backwards and for wards brings tile fibres together, and multiplies their points of contact. The agitation gives to each hair a progressive motion towards the root, and the serra tions of one hair fix themselves on thoso of another hair which happens to have its root turned in the opposite direction, and the mass at length assumes the compact form which is termed "felted" wool. The microscope has likewise de monstrated various other remarkable modifications in the form of the hair in different quadrupeds. In the mole, each hair is alternately constricted and ex panded from its root to its apex, where by it readily assumes any position, and lies flat ana smooth, either towards the head when the little burrower is retro grading in his subterranean galleries, or in the contrary direction when moving forwards. The organization of the hair is such as to allow of its undergoing certain changes when once formed, ac cording to the state of health and gene ral condition of the rest of the frame, and even to be affected by loss of color in consequence of violent mental emo tions in the human subject. Some of the lower animals, as the Alpine hare, are subject to periodical change of color of their fur, by which it is made to harmonize with the prevailing hue of the ground which they habitually tra verse.
The chemical properties of hair were first pointed out by Mr. Hatchett. It chiefly consists of an indurated albu men, and when boiled with water, it yields a portion of gelatine. Soft flexi ble hair, which easily loses its earl, is that which is most gelatinous. Vauquelin discovered two kinds of oil in .hair the one colorless, and in all hair ; the other colored, and imparting according to its color the peculiar tint of hair in the indi vidual.