Hair

HAIR, a word common to Teutonic languages ; the general term for the characteristic outgrowth of the epidermis forming the coat of mammals. The word is also applied by analogy to the filamentous outgrowths from the body of insects, etc., plants, and metaphorically to anything of like appearance.

For anatomy, etc., of animal hair see SKIN and EXOSKELETON ; FIBRES and allied articles; FUR; LEATHER.

Anthropology.

The human hair has an important place among the physical criteria of race. While its general structure and quantity vary comparatively little, its length in individuals and relatively in the two sexes, its form, its colour, its general consistency, and the appearance under the microscope of its transverse section show persistent differences, which give it its ethnological importance. The hair grows uniformly over the head in all races. The structure of the hair is threef old : (I) Short and crisp, generally termed "woolly"; elliptical or kidney-shaped in section, with no distinguishable medulla or pith. Its colour is almost always jet black, and it is characteristic of all the black races except the Australians and aborigines of India. This type of hair has two varieties. When the hairs are rela tively long and the spiral of the curls large, the head has the appearance of being completely covered, as with some of the Melanesian races and most of the negroes. This type is called ulotrichous or "woolly." Among the Hottentots and Bushmen the hair grows in very short curls with narrow spirals and forms little tufts separated by spaces which appear bare. The head looks as if it were dotted over with pepper-seed, and thus this hair has gained the name of "peppercorn-growth." Most negroes have this type of hair in childhood and, even when fully grown, signs of it around the temples. The space between each tuft is not bald, as was at one time generally assumed.

(2) Straight, lank, long and coarse, round or nearly so in sec tion, with the medulla or pith easily distinguishable, and almost without exception black. This is the hair of the yellow races, the Chinese, Mongols and Indians of the Americas. This is designated leiotrichous or lissotrichous.

(3) Wavy and curly, or smooth and silky, oval in section, with medullary tube but no pith. This is the hair of Europeans, and is mainly fair, with black, brown, red, or towy varieties. This is termed cymotriclious.

There is a fourth type of hair describable as "frizzy." It is easily distinguishable from the Asiatic and European types, but not from the negroid wool. It is always thick and black, and is characteristic of the Australians, Nubians, and certain of the Mulattos. Generally hair curls in proportion to its flatness. The rounder it is the stiffer and lanker. These extremes are respec tively represented by the Papuans and the Japanese. The woolly type is found to be the most persistent.

Wavy types of hair vary most in colour : almost the deepest hue of black being found side by side with the most flaxen and towy. Colour varies less in the lank type and scarcely at all in the woolly. The only important exception to the uniform blackness of the negroid wool is to be found among the African pygmies, whose hair is described as "of a dark, rusty brown hue." Fair hair in all its shades is frequent among the population of northern Europe, but much rarer in the south.

The percentage of brown hair is 75% among Spaniards, 39 among French, and 16 only in Scandinavia. Among the straight haired races fair hair is far rarer; it is, however, found among the western Finns. Among those races with frizzy hair red is almost as common as among those with wavy hair. Red hair, however, is an individual anomaly associated ordinarily with freckles. There are no red-haired races.

A certain correlation appears to exist between the nature of hair and its absolute or relative length in the two sexes. Thus straight hair is the longest (Chinese, Red Indians) , while woolly is shortest. Wavy hair holds an intermediate position. In the two extremes the difference of length in man and woman is scarcely noticeable. In some lank-haired races, men's tresses are as long as women's, as in the Chinese pigtail, and the hair of Redskins which grows to the length sometimes upwards of 9 ft. In the frizzy-haired peoples, men and women have equally short growths. It is only in the wavy, and now and again in the frizzy types, that the difference in the sexes is marked. The growth of hair on the body corresponds in general with that on the head. The hairiest races are the Australians and Tasmanians. The least hairy peoples are the yellow races, the men often scarcely having rudimentary beards, e.g. Indians of America and the Mongols. Negroid peoples may be said to be intermediate but usually incline to hairlessness.

The wavy-haired populations hold also an intermediate position, but somewhat inclined to hairiness. Among negroes especially no rule can be formulated. Bare types such as the Bushmen and western negroes are found contiguous to hairy types such as the inhabitants of Ashanti. Neither is there any rule as to baldness. The lanugo or downy hairs, with which the human foetus is cov ered for some time before birth and which is mostly shed in the womb, and the minute hairs which cover nearly every part of the adult human body, may be regarded as rudimentary remains of a complete hairy covering in the ancestors of mankind. Perhaps the primary divisions of mankind were distinguished by hair the same in texture and colour as that which characterizes to-day the great ethnical groups. The wavy type bridges the gulf between the lank and woolly types. No test has proved, on repeated examination, to be a safer one of racial purity than the hair, and Pruner-Bey goes so far as to suggest that "a single hair presenting the average form characteristic of the race might serve to define it." At any rate a hair of an individual bears the stamp of his origin.

See Dr. Pruner-Bey in Memoires de la societe d'anthropologie, ii.; P. A. Brown, Classification of Mankind by the Hair; P. Topinard, L'Homme dons la nature, chap. vi. (1891) ; A. C. Haddon, Races of Man (1924) • Characteristically, Mammalian hairs are developed in rela tively deep pits in the skin, the hair-follicles, which extend down wards into the thickness of the corium, or even into the sub cutaneous tissue. In man the hair-rudiments begin to appear about the third or fourth month of foetal life as small solid down growths from the Malpighian layer of the epidermis, their growth being completed about the fifth or sixth month, when they con stitute the very delicate hairy covering, the lanugo, which is entirely shed before birth. The hairs constituting this are fine, slender, faintly or not pigmented, with large cortical scales and no medulla, and possess some of the characteristics of wool. At birth the hairs of the eyelashes, eyebrows and scalp, though still soft and more or less retaining the characteristics of lanugo, already show a much more vigorous growth and may be pig mented. During the first few months of infancy this growth is shed, being replaced by the typical coarser hair of the eyebrows and head, while over the rest of the body grows the fine, short, generally unpigmented down-hair or vellus. Finally, at and fol lowing puberty, coarse, longer and more heavily pigmented hair (terminal hair) is developed in armpits (axilla), pubes, certain areas of the trunk and limbs, and in males on the upper lip and chin.

The amount of terminal hair varies according to race, sex and even individual, though generally more abundant in males, the greater part of the body in adult females still being covered by vellus. While there are little or no sexual differences distinguishing the auxiliary hairs, slight differences are observed in the pubic hairs, which appear rather longer and more abundant in men, and relatively coarser in women. One characteristic human trait is that most of the body hairs never develop beyond more or less rudimentary vellus, whereas in other mammals the coarser forms predominate, and, in addition, tactile hairs (sensory vibrissae) are present. The total area of really hairless skin in man is relatively small, being confined to the palms of the hands, soles of the feet, under-surface of the fingers and toes, the margin of the lips, areolae of the nipples, umbilicus and immediate vicinity of the urogenital and anal openings.

The Hair-follicle.

This structure, which is essentially a recess of the skin, is composed of two tissue elements, one, of epithelial origin, closely invests the hair-root, while the second is connective tissue. A cross-section of a hair-follicle shows that the epithelial layer consists of an outer layer of polyhedral cells forming the outer root-sheath, and an inner, horny stratum, the inner root-sheath, composed of three layers, known respectively as Henle's layer (the outermost) of horny, fibrous, oblong cells; Huxley's layer, consisting of polyhedral, nucleated cells contain ing pigment granules ; and the cuticle of the root-sheath, composed of a layer of downwardly imbricated scales that fit over the up wardly imbricate scales of the hair proper. The connective tissue element consists internally of a vascular layer separated from the root-sheath by a basement-membrane, the hyaline layer of the follicle, and externally has a more open texture corresponding to the deeper part of the cutis containing the larger branches of the arteries and veins. A small muscle, the arrector pili, is at tached to each hair-follicle. It passes from the superficial part of the corium, on the side to which the hair slopes, obliquely down wards, to be attached near the bottom of the follicle to a pro jection formed by localized hypertrophy of the outer root-sheath. If this muscle is contracted, the hair becomes more erect, and the follicle is dragged upwards to cause a prominence on the general surface of the skin, producing that temporarily roughened condition popularly called "goose-skin." Process of Growth.—The hair grows upwards from the bot tom of the follicle by multiplication of the soft cells which cover the papilla and these become elongated and pigmented to form the fibrous substance of the hair-shaft, and are otherwise modified to produce the central medulla and cuticle of the hair. The hair shaft is chiefly composed of a pigmented, horny, fibrous material, which consists, really, of long, tapering, fibrillated cells that have coalesced. Externally this fibrous substance is covered by a deli cate layer of imbricated scales forming the cuticle. In many hairs, the centre of the shaft is occupied by an axial substance, the medulla, formed of angular cells containing granules of eleidin, and frequently in addition, minute air-bubbles which give the cells a dark appearance. The medullary cells tend to be grouped along the central axis of the hair as a core; continuous or inter rupted in single, double or multiple columns. The variations in the medulla may be summarized as : (a) the continuous type, which may be homogeneous, as in the chimpanzee, or nodose as in the gelada baboon; (b) discontinuous medullas, which in simple forms may be ovate, elongated, or flattened (in Hylobates it is discontinuous and elongated) ; (c) a fragmental type, as in Semnopithecus.

Cuticular Scales.

The delicate cuticular scales are most varied in shape and size, and constitute the most important micro scopical structure of the mammalian hair, for they possess defi nite and constant specific characters. The dominant form is an imbricate scale, like a tile of irregular shape, having its edges rounded, minutely notched or flattened. There are many varieties of the imbricate scale, each typical of its species ; thus in man, chimpanzee, gorilla and orang-utan, the hairs have imbricate scales which are, however, quite distinctive in size, shape and structure of the edge (slightly oval in chimpanzee, slightly ovate and crenulate in man and gorilla, with strongly marked [crenu late] edges in orang). The second type of cuticular scale is the coronal in which each individual cuticular cell completely en circles the hair-shaf t, and may have a simple, serrated or den tated edge. While the imbricate scale is typical of the higher Primates, the coronal scale in its simpler form is present in the Lemuridae and Tarsius; becoming in the Insectivora more specialized, with serrate or dentate edges.

In some Indian bats the cuticular scales are developed as leaflet like processes arranged in whorls at regular intervals along the hair-shaft. In many deer (Cervus), the cortical substance is nearly indistinguishable, almost the entire hair appearing to be composed of thin-walled polygonal cells. In the peccary the corti cal envelope sends inwards radial prolongations, the interspaces of which are occupied by medullary substance ; and this, on a larger scale, is the structure of the porcupine's "quills." One of the most remarkable mammalian hairs is that of the Australian Ornithorlync1ius, in which the lower portion of the shaft is slender and wool-like, while the free-end terminates as a flattened, spear shaped, pigmented hair with broad imbricated scales. In the three-toed sloth (Bradypus tridactylus), a microscopic alga grows between the cuticular scales of the hairs, and would appear to be symbiotic, inasmuch that its presence, giving a curious greenish gray hue to the coat of the sloth, helps to disguise the animal among the trees, giving it when viewed from the ground almost the appearance of a mass of moss.

Tactile Hairs.

These occur in all mammals except man, and are large, stiff hairs of pre-eminently sensory character, having high specialized follicles, the root being embedded in a mass of true erectile tissue (corpus cavernosum, corpus spongiosuet pili), and having a rich sensory nerve supply, presumably controlled by the sympathetic nervous system. These specialized hairs are few in number, their distribution being chiefly confined to the lips, cheeks and supra-orbital regions, occasionally occurring elsewhere.

BIBLIOGRAPHY.-J. Hutchinson, "Notes on the Distribution of Hair Bibliography.-J. Hutchinson, "Notes on the Distribution of Hair on the Human Body," Arch. Surg. (1894) ; W. Kidd, The Direction of Hair in Animals and Man (19o3) ; L. A. Hausman, "Structure Characters of the Hair of Mammals," Amer. Naturalists (1920) ; C. F. Sonntag and F. M. Duncan, "Contributions to the Histology of the Three-toed Sloth," Journ. R. Micr. Soc. (1922); F. M. Duncan, "On the Hair of Hapale, Galago and Tarsius," Proc. Zool. Soc. Lond. (1927), pt. iii. p. 604. (F. M. Du.)