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.

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 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.