hair

Fine filament growing from mammalian skin. Each hair grows from a pit-shaped follicle embedded in the second layer of the skin, the dermis. It consists of dead cells impregnated with the protein keratin.

The average number of hairs on a human head varies from 98,000 (red-heads) to 120,000 (blondes). Each grows at the rate of 5–10 mm/0.2–0.4 in per month, lengthening for about three years before being replaced by a new one. A coat of hair helps to insulate land mammals by trapping air next to the body. The thickness of this layer can be varied at will by raising or flattening the coat. In some mammals a really heavy coat may be so effective that it must be shed in summer and a thinner one grown. Hair also aids camouflage, as in the zebra and the white winter coats of Arctic animals; protection, as in the porcupine and hedgehog; bluffing enemies by apparently increasing the size, as in the cat; sexual display, as in humans and the male lion; and its colouring or erection may be used for communication.

Filamentous outgrowths in other animals, for example insects and the hairy frog, and even in plants, are also called hair, though they are not actually similar structures.

Physiology of human hair A hair, like a fingernail, is built up from the corneous cells of the epidermis. Its base is enclosed in a bag, called a hair sac or follicle, at the bottom of which is a papilla. The superficial epidermis cells surrounding the papilla harden and form a shaft, which is finally thrust out above the surface skin by new growths from below. When it has reached its natural height it dies, but not before a fresh papilla and sac have been formed so as to send up a new hair to replace it.

Each hair shaft has an elaborate structure. In the centre is medullary matter, which may contain air. This is wrapped round with a cortical substance, composed of elongated horn cells. Enveloping the latter is an outer cuticle made of flat corneous plates fitting transversely round the shaft. The cuticle in its turn is enclosed in the epidermis of the hair sac which corresponds to that of the integument (skin), just as the dermis of the hair sac, which is the last coating, corresponds to the dermis of the integument. In these superficial layers of the follicle are the root sheaths, which contain the root of the hair. A fatty liquid, sebum, which lubricates the hair, is secreted from the sebaceous glands, whose ducts open into the hair follicle.

Goosepimples, the tiny bumps that form on the skin when the hairs stand erect through fear or cold, are produced by the involuntary contraction of tiny muscles known as the arrectores pilorum.

Growth of human hair Each head hair lasts 2–4 years. Hair loss is normally not noticed as new hairs replace the old ones. Baldness results when renewal is unequal to loss. It is a natural process for hair to turn grey with age. Since any change in the colour of hair must happen within the follicle and will only show when the changed hairs have grown out, hair cannot become grey overnight. In some families white patches, or premature blanching of the hair, are hereditary.

Hair and anthropology Hair type is one of the many hereditary factors that distinguish geographically isolated populations. The difference in colour varies from almost white to black, depending on how much of the pigment melanin is present in the hair shafts, and on how the structure of the hair reflects light. Straight hair of relatively broad diameter is common in Asian peoples, while European hair is often finer. The shaft of curly hair is alternately round and oval while tightly curled hair is flat in cross-section.

Diseases Various diseases that affect the hair are intimately related to skin diseases, such as eczema, and in general the condition of the hair reflects general health. Ringworm, which is a highly contagious and troublesome fungal disease, causes the hair to break off or fall out, leaving bald circular patches. Many diseases cause splitting or excessive shedding of the hair. General diseases such as fever, or a chronic disease such as tuberculosis, may cause baldness. Alopecia areata, baldness in patches, sometimes occurs in otherwise normal hair. The cause is unknown, but it is not serious or contagious, and the hair grows again in due course. There is no cure for grey hair or the baldness of old age.