Human hairSeveral kinds of hair occur on the human body. The appearance depends on age and body location. The so-called lanugo is hair that develops on an unborn child. Usually, it is shed before birth, or within the first few months after birth. The lanugo is immediately replaced by secondary hair which is fine and soft and is often called "baby hair." The coarser hair of later life is called tertiary hair. Hairs are continually lost from all parts of the body through life, and up to a certain age, those which replace them often are coarser tan their predecessors.
The scalp of the average person has about 125,000 hairs. Darker people have fewer scalp hairs than blonds. Scalp hair usually grows from 3 to 5 inches per year and, if not cut, can becomes as long as 2 to 3 feet or more.
The hairs of the body originate from hair follicles embedded in the skin. The lower part of the follicle extends into the dermis where it is supplied with blood vessels. Generally, only one hair grows from a single follicle. That part of the hair below the surface of the skin is called the root, while that extending out from the surface of the skin is called the shaft. The sebaceous glands of the skin have their openings in the hair follicles. These glands secrete a substance (sebum) which is responsible for the oily appearance of the skin or scalp. People with oily skin have overactive sebaceous glands. When the hair follicle becomes plugged, the sebum collects within it, turns dark at the surface, and becomes a "black head."
Minute muscles (erectors pilorum) are connected to the hair follicle. When these muscles contract, they temporarily displace the entire follicle, causing the hair to "stand on end." The skin surrounding the hair is also elevated by the contractions of these muscles, giving the skin a prickled appearance, sometimes called "goose pimples." Contraction of the muscles also exerts pressure on the sebaceous glands, causing the emission of extra amounts of sebum. Thus, this set of reactions aids in protecting the body from sudden cold, he hairs forming better insulation when standing erect, and the sebum coats the skin with a further barrier against the cold.
Some skin diseases affect the condition (and existence) of the hair. See also alopecia.
Related category• ANATOMY AND PHYSIOLOGY
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