Hair. Hair, collective term for slender, threadlike outgrowths of the epidermis

Hair. Hair, collective term for slender, threadlike outgrowths of the epidermis of mammals, forming a characteristic body covering. 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. (Whales and manatees 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 bristles are also pointed, as in the hedgehog and porcupine, they are called spines or 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), and sometimes they also contain interstitial air bubbles that give the hair a silvery color. 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 fetus is covered by a growth of fine hair, the lanugo. In the first few months of infancy the lanugo is shed and is replaced by hair, relatively coarse over the cranium and the eyebrows, but 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 facial hair 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 (about ? 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 African, Papuan, and Melanesian grows from a curved follicle, which imparts a spiral twist, and is flat or tapelike in cross section. The hair of the Chinese, Japanese, and Native American is straight, coarse, 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 European, Semite, and Ainu is wavy and intermediate between the straight and curly types; growing from a straight follicle, it nevertheless has a slight tendency to curl, is oval in cross section, and exhibits a range of color from light blond to black. Disorders of the hair shaft or hair follicle cause abnormal growth or abnormal or premature falling of the hair. Certain abnormal conditions such as dull or dry hair are 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 alkalies, often causes such conditions. The cause of excessive hairiness is obscure, but in several cases it has been traced to tumor of the adrenal cortex or to disorders of the pituitary gland, the thyroid gland, or the ovary. Premature graying of the hair is associated with anxiety, shock, deficiency diseases, and, in certain cases, 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 preliminary graying, may also be caused by seborrhea; while 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 invasive surgical procedures. No nonprescription hair "restorers" actually prevent hair loss or grow hair. A drug called minoxidil, however, has shown some effectiveness in treating men and women with hereditary baldness. 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 chiggers and lice. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.