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Hair

Originally evolved as a means of regulating body temperature, hair is not strictly necessary for human health, but can be very important - particularly head and facial hair - in the way we present ourselves to the world.

There is hair of some sort on every part of our bodies except the palms of our hands and the soles of our feet. Sometimes it is fine and downy and barely noticeable, and in other places strong and vigorous, such as the long hairs that grow on the scalp and the short, stiff ones that form our eyebrows.

HAIR STRUCTURE
Every hair we have, wherever it appears on our bodies, has the same basic structure. It has a visible part, the shaft, which is dead, and a hidden, living root. The shaft is made of a hard protein called keratin, the same material as our finger- and toenails.

The root from which the hair develops is known as the dermal papilla. It lies at the bottom of a tube-like depression in the skin, a follicle, and is nourished by the bloodstream. Keratin-producing cells in the root divide more rapidly than most; scalp hair grows 1.25cm (1.2in) a month on average. If the root is damaged, growth stops, and may never start again.

Each follicle also contains a sebaceous gland, which secretes a greasy substance, sebum. This lubricates the hair shaft and surrounding skin, keeping it supple and protecting it, but may give hair a greasy  appearance.

Every follicle also contains an arrector pili muscle. This muscle contracts when you are cold or afraid, making your hair stand on end and bunching the skin around the shaft to form goose pimples.

COLOUR AND LINE
The type of hair shaft determines whether hair is straight or curly. A cylindrical shaft produces straight hair, and an oval one gives wavy or curly hair, depending on the degree to which it is flattened. The tightly-curled hair characteristic of black people is the result of a flattened or kidney-shaped shaft.

Lining the follicles and mixed in with the cells making keratin are other cells laden with a pigment, melanin, which stains the keratin and gives hair its colour; the more melanin there is, the darker the hair (red hair has an extra pigment). The amount of melanin produced is determined by heredity.

As we get older, the melanin-producing cells gradually shut down, and as a result our hair eventually turns grey and ultimately white. When this change occurs is also a question of heredity.

RESTING
Hair growth is not continuous. Each hair has a short resting phase, twice a year on average, during which no growth takes place. The roots of resting hair become club-shaped - hence their name, club hairs - and lose their normal pigmentation. It is the club hairs that seem to come out in profusion when we wash our hair. No damage is done to the follicles, and when the root has finished its rest, normal hair growth begins again.

     
     

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Hair

 

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