Hair , an elongated, more or less cylindrical epidermic appendage, analogous to the feathers of birds and the scales of reptiles. Its essential structure consists of an assemblage of epidermic cells at the bottom of a flask-shaped follicle in the substance of the skin, supplied with blood by vessels distributed to its walls; it is made up of a root, from which the hair is developed, and a stem or shaft continuous with it. The root exhibits a bulbous enlargement, which, with the lower part" of the stem, is enclosed in an inversion of the epidermis, having an outer or cellular and an inner or fibrous layer, formed of granular cells; each hair follicle is implanted in a depression in the dermis, between whose epidermic lining and the stem is a space into which the canals of sebaceous follicles frequently open, and in which entozoa are often developed; the inspissated sebaceous secretion forms the scurf at the roots of the hair; the follicle penetrates sometimes 1/12 of an inch, reaching on the head, face, and pubis the subcutaneous areolar tissue, but generally is imbedded in the substance of the true skin. The bottom of the follicle is occupied by a papilla upon which the hair rests, a compound cellular vesicle, the true germ of the hair.
The stem is composed of a cortical investing horny layer of scales, arranged in an imbricated manner, a softer medullary or pith-like substance in the centre, and a fibrous intermediate portion constituting two thirds of the bulk of the hair; the last two are by Carpenter considered as forming together the medullary substance. The growth of hair takes place at the root by the development of new cells at the bulb, the old being pressed forward by the new or becoming elongated in the stem. Hairs are very rarely cylindrical, but generally elliptical and flattened in proportion to the curl or crispness; the size is greatest toward the lower third, the root being smaller and the end terminating in a point. The hairs of the head are the longest, those of the heard the thickest, and those of the general surface the finest; among women the hair of the head has been known to fall below the feet, and the beard of man occasionally reaches to the waist; frequent cutting and shaving of hairs increase their thickness, but not necessarily their number. Hairs are observed in the fetus as early as the third or fourth month, in the order of follicle, bulb, and hair.
From the resemblance of the mucous membranes to the skin, it is not surprising that hairs are sometimes developed on the conjunctiva of the eye, in the intestines, ovaries, etc.; they are frequently found in encysted tumors and in other inversions of epidermic structure. Hairs may be transplanted, and will contract organic adhesion in the new tissues; according to Eble, a hair which has reached its full development becomes contracted just above the bulb and falls off. In vigorous health the hairs are thick and firmly set in the skin; in debilitated persons they fall out spontaneously or with very slight force; in the latter case the bulb generally alone comes away, the sheath and germ remaining behind, and capable of reproducing the hairs under proper treatment or favorable circumstances; even when the entire follicles are removed, it is possible that new ones with their germs may be formed; new shafts are constantly in process of formation, as is shown by the short and pointed hairs on the scalp of old persons.
The nutrition of hairs is effected through vessels in close contact with their tissue, without entering into their structure; so that causes affecting the general health, and especially the condition of the skin, act powerfully upon the nutrition of the hair; the premature baldness and grayness of the Americans as a people is in great measure a, shaft of hair above the skin; b, cortical substance of the shaft, the medulla not being visible; c, newest portion of hair growing on the papilla (i): d, cuticle of hair; e. cavity of hair sac; f, epidermis (and root sheaths) of the hair sac corresponding to that of the integument (m); g, division between dermis and epidermis; h, dermis of hair sac corresponding to dermis of integument (l); k mouths of sebaceous glands; n, horny epidermis of integument.
A Hair in its Hair Sac.
' owing to the non-observance of hygienic rules, and to excess of mental and physical labor in a climate foreign to the race. Hairs are distributed over the entire surface of the human body except the palms, soles, and terminal joints of the fingers and toes; but for special purposes most abundantly on the scalp, brows, edge of the lids, pubis, chin, cheeks, armpits, chest, and entrance of the nose and ears. In these situations the number varies according to temperament, age, health, and sex. According to Withof, the quarter of a square inch contained 293 hairs on the head, 39 on the chin, 23 on the forearm, 19 on the back of the hand, and 13 on the front of the thigh; in the same extent he counted 147 black, 162 brown, and 182 flaxen hairs, showing the comparative fineness. Long and strong hairs are often found growing from moles and naevi in various parts of the body. The hair generally grows in an oblique direction on account of the way in which the follicles are placed; these are sometimes placed wrongly on the scalp, causing much trouble to anxious mothers; perseverance will generally bring the refractory locks into the desired direction.
From contraction or corrugation of the skin from cold, fear, or other causes, the hair, especially on the head, becomes partially erect, though it can never stand on end "like quills upon the fretful porcupine." - The color of the hair depends partly on the presence of pigment granules, and partly on the existence of numerous minute air spaces which cause it to appear dark by transmitted light; its intensity generally bears a close relation to the color of the iris and the skin; in albinos and in gray-haired persons the coloring matter is deficient or absent. Long contact with chlorine decolorizes hair; and the undoubted fact that hair may turn white in a short time under the influence of strong emotions is doubtless to be explained by some chemical action upon the oily coloring matter, as suggested by Dr. D. F. Weinland, and more fully explained in the article Feathers. The turning gray of the hair is no sign of its loss of vitality, as hair of this color often grows for years as vigorously as the darker lined. Hair is remarkable for strength, elasticity, and durability, the first depending on its fibrous structure and the last two on its horny nature; a single hair will bear a strain of 1,150 grains.
Hairs will endure not only during a long life, but will grow after death, and last for centuries. It is well known that hairs, especially of cats and other animals, be-come electrical by rubbing; the hygroscopic property of hair has been painfully manifested to many a beau and belle whose rebellious locks have refused to retain their artistic curl on the sudden occurrence of a moist atmosphere. Nitrate of silver blackens hair, forming a sulphuret, and this substance and sulphur form the bases of most of the popular hair dyes. When burned, hair emits a disagreeable odor as of burning horn. - The uses of hair are manifest. On the head of man it is one of his chief ornaments, as well as a protection from injury; on the face it gives character and dignity; on the brows and lids, and at the entrance of the nose and ears, it prevents the contact and entrance of foreign bodies; and, even in man, the general covering of hairs doubtless contributes to the warmth and proper electric condition of the skin; the object of the hair in animals is obvious to every one. - The two essential parts of cortex and medulla are found in the hairs of all animals, however much they may differ in appearance.
In the cats, seals, and other animals, the whiskers are supplied with large nerves, and become exquisite organs of touch; in the soft hair of the sable there is very little fibrous portion; in the musk and reindeer the entire substance seems to be composed of delicate polygonal cells; in the smaller rodents the cortical tubular portion is crossed by transverse partitions, partial or complete; in the bats the projections of the cortical scales are often arranged in whorls around the stem; in the peccary the cortical substance sends inward radiating processes whose interspaces are filled with the medullary portion, and this is essentially the structure of the quills of the porcupine, which, as Shakespeare has hinted, are only modified hairs; even the horn of the rhinoceros is only an assemblage of compact hairs, and does not differ in its essential structure from the finest wool. In proportion to the prominence of the imbricated scales will the hair of animals have the property of becoming felted. - In most nations the adornment of the hair has always formed one of the principal duties of the toilet, and the caprices of different races and epochs in this respect are very remarkable.
While the Hebrews and Greeks considered long hair a beautiful object, the Egyptians, regarding it as an incumbrance, removed it, and substituted light wigs. The Roman ladies used artificial hair, dyed their own, sprinkled it with gold dust, and represented in it various fanciful devices. So desirable was a fine head of hair considered, that it became sacred, and was often dedicated to the gods on important occasions of marriage, victory, escape from danger and death, and burial of friends; plucking it out or neglecting it was a token of affliction. In the time of Francis I. short hair became the fashion, and under Louis XI11, long hair, curls, and wigs; then came hair powders, periwigs, and perukes of monstrous size, which were banished by the French revolution, since which civilized nations have been in the main content with natural heads of hair. - Some persons are born without hair on any part of the body; on the head it falls off after many febrile diseases, especially typhoid fever, and after erythematous affections of the scalp and irritating applications. Tightly fitting and unyielding hats no doubt contribute largely toward the premature fall and grayness of the hair.
The bulbs are often diseased, and in plica Polonica the hair, generally insensible, becomes exceedingly sensitive at the roots and liable to bleed. For the various diseases of the hair tonic and stimulating applications are sometimes beneficial; when the hair is thin and falls out easily, shaving the scalp will generally produce a thicker, firmer, and darker-colored growth. For diseases of the hair depending on parasites, see Epiphytes, and Epizoa. - Microscopic examination shows that the hair of the negro is not wool, though differing considerably in form from that of Europeans; the form is not connected with the color; the differences in the form of the hair, being permanent, are considered by some as of the same specific value as those of the fur, feathers, and scales of lower animals. Straight hair approximates to the cylindrical form, but the curled or crisp varieties are flattened; the negro hair has the deepest longitudinal groove, and a transverse section like that of a bean, and its peculiar twist is said to be due to a greater tension of the fibres along this groove; the closely matted hair of the Bushman is very flat and ribbon-like, four or five times broader than thick. - Many insects are provided with hairs, both in the larval and perfect states, which afford beautiful microscopic objects, from their branches, tufts, spines, and protuberances.
The cuticle of plants is often beset with hairs, made up of a linear series of elongated cells, attached end to end; they sometimes have glandular bodies connected with them, as those which secrete the viscidity on the leaf of the sundew (drosera), or the irritating liquid of the nettle. In the invertebrates and in plants there are many evident connecting links between hairs and scales; vegetable hairs generally exhibit the phenomena of rotating fluids, or circulation of currents.