HAIR

a characteristic of all mammals} even if in some it is reduced to a few bristles on the lips, as in whales. Itisa product of the superficial skin or epider= mis, is nourished by blood-ve els, and consists of a spongy centre and a harder outer layer. In mammals the temp. of the body must be kept at a constant high pitch; hair conserves the heat gained at the expense of energy and checks exce ive di ipation of heat by radiation. It sometimes undergoes profound modifica tions. Thus in the hedgehog and porcupine, belonging to different orders, it it develops into long, hard, and sharp spines, which are clearly specialized defensive structures. Only one kind of hair may be present, as in the cat, but often the long, coarse, apparent Over= hair is supplemented by a fine, soft, thick under-fur, as in seals. The double coat is developed in animals exposed to cold temperatures—e.g., martens of N. American and Europe, Arctic and Antarctic foxes, the ermine from Asia and Europe, etc. Hair, like feathers, is seasonably moulted, and the winter coat, which is thickest and warmest, often changes in northern animals to a colour matching or approaching that of the snow amongst which they move. Histologically, hairs are outgrowths of epidermis; develop in little pits (follicles) epidermis (root-sheath) forms inner layer of follicle; corium rises up at base of follicle as a vascular papilla; hair itself aay be divided into body or shaft, and root; shaft has externally a cuticle of overlapping scales; beneath is pigmented fibrous layer, while sometimes in centre is dark-coloured medulla composed of angular cells. Root of similar structure, but enlarged; and consists chiefly of young growing cells. Each hair follicle has attached to it little bundle of muscujar fibres by which it can be erected; when they contract under influence of emotion (e.g., fear) produces ‘goose skin.’ Colouring of hair depends on mature and amount of pigment.; k Human hair and beards are po ibly sexual ornaments. Racial varieties are: curly or smooth, generally fair hair of N. Europe ans; crisp and short, woolly, very black hair of most negroes; black coarse straight hair of Mongols, Chinese, Amer. Indians; crinkly black hair of Australian Blacks. Chairs are stuffed with short horsehair; cloth*is woven from long horsehair and hair of goats. especially the Angora variety. Felt, for roofing and packing for pipes, is manufactured from cowhair. Artists’ brushes are made from hair of camel and sable, clothes brushes from hog bristles