HAIR (AS. htir, Icel., OHG. her, Ger. Haar; probably connected with ()Church Slay. kosma, Lith. kasa, hair, and perhaps with Lat. carere, to card wool). The characteristic covering of mammals, an epidermal outgrowth, the forma tion and structure of which are quite different from either scales or feathers. Hairs are not en tirely wanting in any mammals, though they vary greatly in abundance, some animals being completely and densely clothed in them, while others possess them only on limited parts of the surface, as in man, and others, as adult whales, have only a few bristles near the mouth. Hairs of two distinct kinds occur in many mammals, one of is stiff, straight, and smooth, the other curly, soft, and often minutely rough. It is this roughness of the outer surface which makes the processes of felting and skinning pos sible. When both kinds of hairs are present, the soft ones, usually called 'wool,' are much more abundant than the straight ones; but the latter give the smooth outer coat to the body and are therefore sometimes called 'contour' hairs. Un like feathers, hairs are uniformly distributed over the body, except in special cases, where par ticular regions are left bare. Hairs vary great ly in size and color; the longest hairs occur among the ungulates, especially in the tails of such species as the horse, while the shortest hairs of any mammal are usually on the face. The color of hairs depends upon the amount of pig ment they contain, the amount of air in the inter cellular spaces, and the character of the external surface.
Hairs arise as outgrowths of the epidermis and contain no dermal tissue. The original epi dermal thickening pushes down into the dermis as a solid hair-germ, around which the dermis forms a hair-follicle. The hair-germ soon dif ferentiates into two distinct parts, a peripheral and a central. The former gives rise to the ex ternal root-sheath and becomes the inner, epi dermal coat of the hair-follicle, from the outer dermal portion of which it is separated by the so-called hyaline layer. The central part of the hair-germ gives rise to the hair-shaft itself and to the inner root-sheath, which closely invests the base of the shaft. The latter consists of a cen
tral medulla or pith, a cortex, and an outer cuticle; the pith is made up of a loose, dry tissue with large intercellular spaces filled with air; the cortex is a dense layer containing the pig ment; the cuticle is the covering of the hair, and may he smooth, or more or less rough, and orna mented with scales and other projections. At the base of the shaft and inclosed by the swollen or bulbous base is the hair-papilla, which is well supplied with blood-vessels, and thus furnishes the nourishment necessary for the growth of the hair. When a hair is shed, a new hair may arise in the same follicle from a new hair-papilla. Each hair-follicle is provided with nerves, smooth muscles, and sebaceous glands, the latter keeping the hair oiled. By means of the muscles (arrec tores pili) the hairs can he moved independently, but the movement of large numbers of hairs in unison is accomplished by the movements of the striped muscles of the skin. Sensations of cold, or of fear, cause an involuntary contraction of these muscles in man, causing the feeling of an erection of the hair on the head in terror, or `gooseflesh' in cold. In the lower animals this effect is produced by ill health. Horsemen have a very graphic expression for it, saying of the coat of an animal out of condition, that it `stares.' An infant's hair is shed soon after birth, and replaced by a new covering. The short downy hairs on the human body are rudimentary re mains of complete hairy covering in ancestors of the race. Loss and repair appear to go on con tinuously up to a certain time, when baldness occurs with the majority. Each hair is said to live from two to four years. The grayness of hair in advanced life results from a deficient secretion of pigment and perhaps an increase of air in the medulla of the hair. Well-authenti cated cases are on record in which the hair has grown gray or white in a single night, from the influence of fear, distress, or any variety of strong mental excitement.