Hair

Hair, collective term for slender, thread-like outgrowths of the epidermis of mammals, forming the characteristic covering of those mammals. No animals other than mammals have true hair, and all mammals have hair. Even such apparently hairless mammals as the rhinoceros, elephant, and armadillo have hairs around the snout, at the tip of the tail, and behind each scale, respectively. Whale and manatee have hair only in the embryonic state. When the individual hairs are fine and closely spaced, the coat of hair is called fur; when soft, kinked, and matted together, the coat is called wool. Coarse, stiff hairs are called bristles; when they are also pointed, as in the hedgehog and porcupine, they are called spines (or, popularly, quills).

Individual hairs are composed chiefly of the horny scleroprotein known as keratin and contain neither blood vessels nor nerves. They usually contain pigment (except in the case of albinos) but sometimes also contain interstitial air bubbles that give the hair a silvery colour. The shaft of the hair consists of modified epithelial cells arranged in columns surrounding a central medulla (or core) and covered with thin, flat scales. The root of each hair is contained in a tubular pit of the epidermis called the hair follicle. The hair grows from the bottom of the follicle; it is nourished by the blood vessels in a papilla that extends into the follicle and, for a short distance, into the root of the hair. A minute muscle, the arrector pili, is attached to each hair follicle; under the control of the autonomic nervous system the muscle contracts to make the hair “stand on end”. Most mammals possess tactile hairs, usually growing from the upper lip and eyebrows, with their roots set in erectile tissue richly supplied with sensory nerves.

In humans the development of the hair begins in the embryo, and, by the sixth month, the foetus is covered by a growth of fine hair (lanugo). In the first few months of infancy the lanugo is shed and is replaced by hair which is characteristically coarse over the cranium and the eyebrows and fine and downy over the rest of the body. At puberty coarse hair develops in the armpits and over the pubic region in both sexes; in males the hair over the upper lip and the lower jaw begins to grow coarse to form the beard. The rate of growth of the hair varies with the age of the person and with the length of the hair. When a hair is short, its rate of growth averages about 2 cm (just under 1 in) per month; by the time the hair is a foot long, the rate of growth is reduced by one-half. The fastest growth is found in women from 16 to 24 years of age.

The form of the hair is one of the most important and reliable hereditary characteristics. The nearly black hair of the Papuan, Melanesian, and African peoples grows from a curved follicle, which imparts a spiral twist, and is flat or tape-like in cross section. The hair of Chinese, Japanese, and Native Americans is straight, coarse, long, and almost always black. It grows from a straight follicle, is round in cross section, and has an easily distinguished medulla. The hair of the Ainu, European, Hindu, and Semitic peoples is wavy and intermediate between the straight and curly types. It grows from a straight follicle but has a slight tendency to curl; it is oval in cross section and among individuals exhibits a wide range of colour, from light blond to black.

Disorders of the hair shaft or hair follicle cause abnormal growth or abnormal or premature falling of the hair. Dull or dry hair is caused by physical or chemical agents. Too frequent use, for instance, of permanent-waving chemicals or of shampoos or lotions, especially those containing alcohol or free alkalis, can cause this. The cause of excessive hairiness is obscure, but in several cases it has been traced to a tumour of the adrenal cortex or to disorders of the pituitary gland, the thyroid gland, or the ovary. Premature greying of the hair is associated with anxiety, shock, deficiency diseases, and hereditary elements. Alopecia, or baldness, is also due principally to hereditary elements. Certain forms of baldness may, however, be due to other causes: alopecia prematura, in which the hair of a young person falls out without first turning grey, may also be caused by seborrhoea; alopecia areata, in which the hair falls out in irregular patches, is believed by doctors to be caused by inflammation, nerve disorders, or local infections. Diffuse falling of the hair, ordinarily a normal phenomenon, may reach abnormal proportions after a fever higher than 39.4° C (103° F), during a debilitating disease, or as a result of surgical shock.

No nonprescription hair “restorers” actually prevent hair loss or grow hair. A research drug called minoxidil, however, has shown limited success in tests run on men with hereditary baldness who had experienced restricted hair loss within ten years preceding the tests.

Infections of the hair follicle also cause a variety of hair diseases. Tinea favosa, or favus, is caused by the fungus Achorion schoenleinii; it is characterized by the formation around the mouths of the follicles of small crusts, which frequently resemble a honeycomb. Tinea trichophytina, or ringworm, is caused by fungi of the genus Trichophyton. These diseases have been treated successfully by epilation (removal of the hair from the affected follicles), cleansing with soaps or oils to remove encrustation, and application of fungicides.

Hairy parts, particularly of the head and pubis, are subject to troublesome infestations by minute insects and mites, such as pubic lice.