hair

hair slender threadlike outgrowth from the skin of mammals. In some animals hair grows in dense profusion and is called fur or wool . Although all mammals show some indication of hair formation, dense hair is more common among species located in colder climates and has the obvious function of insulation against the cold. Other functions include camouflage and protection against dust and sand. The long, sensitive hairs, called tactile hairs, that are located around the mouth area of most mammals are extremely sensitive to touch. Each hair filament originates in a deep pouchlike depression of the epidermis, called a hair follicle, which penetrates into the dermis. The root of the hair extends down into the hair follicle and widens into an indented bulb at its base. Extending into the indentation is the papilla, the center of hair growth, which contains the capillaries and nerves that supply the hair. Newly dividing cells at the base of the hair multiply, forcing the cells above them upward. As the cells move upward, they gradually die and harden into the hair shaft. The hair shaft has two layers, the cuticle and the cortex. The cuticle (outer layer) consists of flat, colorless overlapping cells; below the cuticle is the cortex, containing pigment and a tough protein called keratin; it forms the bulk of the hair shaft. Coarse hair, such as that of the scalp, contains an additional inner core called the medulla. Hair is lubricated by sebaceous glands that are located in the hair follicle. Illness or stress may lessen the secretion of pigment, which normally gives color to hair, and cause the hair shaft to whiten. However, the normal process of whitening that comes with age is determined by heredity. In humans, scalp hairs are generally shed every two to four years, while body hairs are shed more frequently. Straight-textured hair, round in cross section, is common among Native Americans, Eskimos, and Mongolic peoples. Kinky or woolly hair, flat in cross section, prevails among the dark peoples of Africa, Australia, and elsewhere. Wavy or curly hair, common among Caucasians, is oval in cross section. The color of hair is determined by the amount of pigment and air spaces in the cortex and medulla. Hair color and texture are inherited characteristics.

hair
1. A multicellular threadlike structure, consisting of many dead keratinized cells, that is produced by the epidermis in mammalian skin. The section of a hair below the skin surface (the root) is contained within a hair follicle, the base of which produces the hair cells. Hair assists in maintaining body temperature by reducing heat loss from the skin. Bristles and whiskers are specialized types of hair.

2. Any of various threadlike structures on plants, such as a trichome.

hair Threadlike structure covering the skin of mammals. It has insulating, protective and sensory functions. Hair grows in a follicle, extending down through the epidermis to the dermis. New cells are added to the base of the hair; older hair cells become impregnated with keratin and die. Hair colour depends on the presence of melanin in the hair cells. A small muscle attached to the base of the hair allows it to be erected in response to nerve signals sent to the follicle. Erecting the hairs traps a thicker layer of air close to the skin, which acts as insulation. See also fur