HAIR, including bristles, wool, fur, etc., is a modification of the epidermis (q.v.), and consists essentially of nucleated particles. An ordinary hair consists of a shaft and a bulb. The shaft is that part which is fully formed, and projects beyond the surface. If we trace it to the skin, we find it rooted in a follicle in the cutis or true skin, or even in the connective or cellular tissue beneath it. This follicle is bulbous at its deepest part, like the hair which it contains, and its sides are lined with a layer of cells continu ous with the epidermis. The layer of epidermic cells, according to Todd and Bowman, (Physiological Anatomy, vol. p. 417), "resembles the cuticle in the rounded form of its deep cells, and the scaly character of the more superficial ones, which are here in contact with the outside of the hair. The hair grows from the bottom of the follicle, and the cells of the deepest stratum gradually enlarge as they mount in the soft bulb of the hair, which owes its size to this circumstance. If the hair is to he colored, the pig ment cells are also here developed. It frequently happens that the cells tu the axis of the bulb become loaded with pigment at one period, and not at another, so that, as they pass upwards in the shaft, a dark central tract is produced, of greater or less length, and the hair appears here and there to be tubular. The shaft is much narrower than the bulb, and is produced by the rather abrupt condensation and elongation into the hard fibers of the cells, both of those which contain pigments and those which do not." If the tissue is softened by acetic acid, these fibers may be readily seen under the micro scope; they seem to he united into a solid rod by a material similar to that which cements the scales of the cuticle. The central cells, when filled with pigment, have les.; tendency to become fibrous than those lying more externally; and hence some writers have described the center as a medal* in distinction from the more fibrous part of the shaft, which they term the cortex. (This tubular character is constant in the hair of many animals, but is very variable in human hair, and even in the same hair at differ ent parts of its length.) The term cortex or bark is more correctly applied to the single outermost layer of cells which overlap one another, and cause the sinuous transverse lines which are seen on examining a hair under the microscope.
In some hairs, especially those which act as tactile organs in some of the lower ani mals (as, for instance, in the whiskers of the various eats), a true papilla, furnished with nerves and capillaries, projects into the hair-bulb, and an approach to this papil lary projection may often he seen in human hairs.
The hairs, like epidermis, are thus seen to he organized. and to maintain a vital, although not usually a vascular connection with the body. The color of hair seems to depend on the presence of a peculiar oil, which is of a sepia tint in dark hair, blood-red in red hair, and yellowish in fair hair. This oil may be extracted by alcohol or ether, and the hair is then left of a grayish yellow tint. The chemical composition of hair closely resembles that of horn, and will be described in the article Honky TISSUES.
Hair is extremely strong and elastic, and hence its uses for the construction of fish the stuflid4.6f Cushions,' !JAS. etc. AmOngst its other phirsical properties, we may mention that, when dry and warm, it is easily rendered electrical, and that it is extremely hygroscopical; readily attracting moisture from the atmosphere, and no doubt from the body also, and yielding it again by evaporation when the air is dry. Hairs elongate very considerably when moist—a property of which Sansone availed himself in the construction of his hygrometer, in which a human hair, by its elongation and contraction, according as the atmosphere is moist or dry, is made to turn a delicate index.
Hairs are found on all parts of the surface of the human body, except the palms of the hands and the soles of the feet; they differ, however, extremely in length, thickness, shape, and color, according to situation, age, sex, or race. The differences dependent on situation, age, and sex, are so obvious that we shall pass them over without notice, and proceed to the most important differences dependent on race. With respect to the quantity of hair that grows on the human body, there are great differences in different races. The Mongols, and other northern Asiatics who are similar to them. aro noted for the deficiency of their hair and for scanty beards, and the same character is ascribed to all the American nations; while, on the other hand. among the Ainos, or in the Kurilian race, there are individuals who have the hair growing down the back, and covering nearly the whole body. The northern Asiatics and the Americans have gen rally straight lank hair, while Europeans have it sometimes straight and flowing, and occasionally curled and crisped. Negroes present every possible gradatinn, from a completely crisp, or what is termed woolly hair, to merely curled, and even flowing hair; and a similar observation holds regarding the natives of the islands in the great Southern ocean. As there is a generally diffused opinion that the head of the African is covered with a species of wool instead of with true hair, we may mention that all true wools which have been examined microscopically (as merino wool, the wool of the 'tiger, rabbit, bear, seal, and wolf-dog, which were investigated by the late Mr. Yount t), 'present a more or less sharply serrated' or jagged surface, while hairs present merely an imbricated appearance. "Hairs of a negro, of a mulatto, of Europeans, and of seine Abyssinians, sent to me (says Dr. Prichard) by M. d'Abbadie, the celebrated traveler, were, together with the wool of a soutlidown sheep, viewed both as transparent and opaque bodies. The filament of wool had a very rough and irregular surface; the filament of negro's hair, which was extremely unlike that of wool and of 1111 the other varieties mentioned, had the appearance of a cylinder, and the coloring matter was apparently much more abundant than in the others." It is in consequence of the above named difference between hair and wool that although the former will entangle to a certain degree, it will not felt into a compact mass, which is the characteristic property of good wool.