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Part Two Get to Know Your Hair The amount of hair and where it grows vary with different mammals. The entire body of the dog, the sheep, the cow, and the horse is covered with a hairy coat. The whale and the hippopotamus have only a few hairs. In humans, hair is not found on palms of the hands or the soles of the feet. The coloration and pattern of coats in animals serve both as a camouflage for protection against enemies and as an allurement to mates. Fine and transparent, human hair is a vestige of our hairier animal forbears, that probably evolved from the scales of reptiles. The adult human body averages five million hairs, of which 100,000 to 150,000 are on the scalp. Scalp hair varies tremendously between races, between individuals of same race, and even within an individual. Mongolians have straight hair simply because their scalp hair has the greatest thickness and the roundest cross-section. In Caucasians the hair is more elliptical and slender; in Negroes it is flattened, resulting in kinky curls. Mongolians, both male and female, have much less public, axillary, facial, and body hair than Caucasians. In Caucasians, true blonds typically have more hair (about 140,000 hair) than brunette (about 105,000) or redhead (about 90,000). Below your skin is the hair root which is enclosed by a sack-like structure called the hair follicle. Tiny blood vessels at the base of the follicle provide nourishment. A nearby gland secretes a mixture of fats (called sebum) which keep the hair shiny and waterproof to some extent. Secretions from some sweat glands also produce a characteristic odor. A dog can differentiate a human being by the typical scent secreted by these glands. Two sets of glands discharge secretions through the skin. while sebaceous, or oil, glands arise from the walls of hair follicles and produce an oil called sebum that lubricates the skin and hair, Sweat glands, embedded in the subcutaneous layer, are scattered over the body, particularly in the palms and soles. Sweat glands produce moisture called perspiration that reaches the skin's surface through the pores and evaporates to cool the body. At the base of the follicle is the papilla, which is the "hair manufacturing plant." The papilla is fed by the blood-stream which carries nourishment to produce new hair. Male hormones or androgens regulate hair growth. Pubic and axillary (armpit) hair are particularly androgen-sensitive and grow at lower androgen levels than hair on the chest or legs. In boys, most pubic hair is grown by age 15, followed by the development of armpit hair two to three years later. In girls, too, an increase in androgens at puberty triggers growth of pubic and armpit hair. Scalp hair, not directly androgen-responsive, is influenced by local amounts of a testosterone derivative, dihydrotestosterone. Hair follicles initially form in utero. No new follicles are created after birth, and none are lost in adult life. The first hair to be produced by the fetal hair follicles is Lanugo hair, which is fine, soft, and unpigmented. This is usually shed in about the eighth month of gestation.
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