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All mammals have hair. Some, such as the red panda, have a thick coat of beautiful fur. Others, such as the sea lion, have hair that is so short and fine, it’s difficult to see. A sea lion’s whiskers, however, are front and center.

People are mammals, so we, too, have hair, though not as much as our primate cousins (chimpanzees, gorillas, orangutans and others). Many of the hairs on your body are so small, you would need a magnifying glass to see them. The places where you won’t find hair include your lips, the palms of your hands, the soles of your feet, and the sides of your fingers and toes.

Hair grows from hair follicles, which are tubular structures located in the dermis, the layer of skin below the outermost layer, or epidermis. The shape of your hair follicles determines whether your hair will be straight, wavy, curly or coiled. Each hair follicle is paired with a sebaceous (oil) gland. The same oil that keeps your skin soft coats your hair to keep it from becoming dry.

Each hair follicle is also paired with a tiny muscle called the arrector pili. This muscle attaches to the base of the hair follicle on one end and to the upper layer of dermis on the other. When the muscle contracts, the hair stands up straight.

An animal’s arrector pili muscles have two roles. In cold weather, it’s easier to trap and preserve heat if hairs are straight. If an animal is threatened, hairs that are standing up make the animal look bigger and more intimidating. It’s unclear what purpose arrector pili muscles have in people, because we don’t have much body hair. However, the next time you get goose bumps because you’re cold or nervous, remember: It’s your arrector pili muscles in action.

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The human scalp contains an average of 100,000 hairs. Those hairs grow about six inches per year, but they’re not all growing at once. At any time, 15 percent of your hair follicles are in a resting state, which can last up to three months. When the resting hair follicles become active again, the old hairs are pushed out. That’s why people shed, or lose, about 50 to 100 hairs per day.

Pigment-producing cells called melanocytes are responsible for hair color. Older people develop gray or white hair because the melanocytes stop making pigment. There is actually no such thing as “gray” hair. Hair either has pigment or it doesn’t. Pigmentless hair can look gray because it reflects light from nearby darker hair.

Hair requires the same upkeep as the rest of you. Washing your hair removes dirt and excess oil and helps keep your scalp healthy. Contrary to what your little brother might think, haircuts do not hurt. That’s because, as with fingernails, there are no nerves in hair. However, if your little sister complains when your mom combs her tangled hair, she has a good reason. Pulling hair hurts because hair follicles are in the dermis, which is loaded with nerves.

Finally, I’d like to debunk the most common myth about hair. Shaving or cutting hair will not make it grow back thicker. Hair growth occurs beneath the skin, so cutting it has no effect on future growth.

Howard J. Bennett

Bennett is a Washington pediatrician. His Web site, www.howardjbennett.com, includes past KidsPost articles and other cool stuff.