HAIR, strictly speaking, the peculiar epi dermal covering of the body in mammals, al though by analogy the term is loosely applied elsewhere, as to the setae of annelids, the slender modified spines of caterpillars, etc. Hair is present in every mammal, although the amount may be greatly reduced so that in certain whales it occurs only in the foetal stage, in others is limited to two bristles on the lips. The struc ture is best understood by following the develop not a secretion but is composed of cornified cells. It is also apparent that the hair is not hollow.
The differences between the different kinds of hair are largely those of shape and of the amount of the various parts present. Thus in many animals two kinds of hair occur, longer ment. In the earliest stage (Fig. 1) there is merely a thickening of the Malpighian layer of the epidermis (see SKIN) at the point where the hair is to be found. This thickening in creases in amount, and thus forms a solid plug (Fig. 2) which projects into the underlying derma. At the same time the cells, which are scanty in most parts of the derma, become abundant beneath the ingrowing plug, and form the basis of the future papilla. Next a ring shaped pit appears on the outer surface of the plug and gradually becomes deeper, cutting the epidermis into two parts, an outer root-sheath and an inner rod-like part, the hair itself, while the pit forms the follicle (see Fig. 3). The papilla grows into the base, bearing blood-ves sels, while the Malpighian layer at this point forms the tissue from which the hair grows. In the hair itself several parts are recognized—a central pithy axis, the medulla; next, a layer of cells, the cortex, and outside this, forming the outer surface of the hair, the cuticle. Farther down in the follicle is the inner root-sheath, formed of two layers of cells known respectively the names of the two anatomists, Henle and Huxley, who first described them. The Mal pighian cells, at the base of the follicle, divide continually, and the new cells thus formed are pushed outward and are transformed into the hair. From this it will be seen that the hair is and coarser hair on the outside, and beneath this a closer and softer under-fur. The coarser hairs may be enlarged into bristles, or still more enlarged to form spines, like those of the porcu pines and hedgehogs. Again the hairs may be
come united to each other, the result being the formation of scales like those of the pangolins or horns like those of the rhinoceros. In some cases the hair is perfectly straight, again it may be curly. The straight hairs are circular in section, the curly are flattened, the amount of curl being proportional to the amount of flatten ing. Certain hairs (wool of sheep, etc.) have the property of felting. This depends upon the scale-like projections of the cells of the cuticular layer. The color of the hair is due to the pres ence of pigments belonging to the group of melanins.
Several accessory structures (Fig. 5) are connected with the hair: sebaceous glands which empty an oily substance into the follicle to keep the hair in a moist, soft condition; nerves which are distributed to the wall of the follicle and thus render the hairs to a certain extent organs of touch, as in the whiskers (vibrisste) of cats; and muscles for the erection of the hair (erectorespilte). This erection may be to in crease the warmth of the body by entangling a layer of air among the hairs, or it may have the purpose of protection against injury, either by terrifying some enemy or by affording a loose envelope around the body some distance from the flesh. Usually the hair is shed (molted) at regular intervals, but there are exceptions, as in the mane and tail of horses, as well as in the case of man. The hair is not scattered irregu larly over the body but occurs with more or less regular arrangement. In the early embryos it is not uncommon to find it distributed in regu lar lines (Fig. 4). Later the lines become broken up into groups of hairs, the arrangement being characteristic of the species, but without any broad morphological significance. It should be noted that although hair and pinfeathers closely resemble each other in general appear ance they are very distinct structures, hair origi nating in a thickening of the epidermis, while feathers (q.v.) like scales are dermal in origin. Most of the literature relating to the hair is in German. Consult general works in physiology and zoology, and the writings of Maurer, Meig erle, Weber and Poulton, Quarterly Journal of Microscopical Science (Vol. XXXVI, 1894).