Hair is a threadlike structure that grows from the skin of mammals. Other living things, including bees and some plants, have hairlike coverings on their bodies. But these coverings are not true hair. Most kinds of mammals have a thick coat of hair that serves chiefly to provide warmth. Many species also have certain hairs for special uses, such as for protection or for sense functions. Among human beings, however, hair has primarily a cosmetic value.
Most of the human body is covered by tiny, light-colored hairs that are barely visible. Thick hair grows from the scalp and some other parts of the body. However, certain areas, such as the palms of the hands and the soles of the feet, have no hair at all. In human beings, as in other mammals, hairs around the eyes and ears, and in the nose, serve a protective function. They prevent dust, insects, and other matter from entering these organs. In addition, the eyebrows decrease the amount of light reflected into the eyes.
Among mammals other than human beings, a number of species have special hairs that respond to touch. Many nerves lie around these tactile hairs, which are commonly called whiskers. Whiskers grow on the lips and cheeks of most mammals, but they also occur on other parts of the body. These hairs help the animals feel their way through narrow or dark places.
Hair also provides various kinds of protection for animals. The hair color of many species of mammals blends with their surroundings and helps them hide from their enemies and prey. The quills of porcupines, which also furnish protection against enemies, are a special type of hair. Hair acts as padding against blows and falling objects as well.
Manufacturers use animal hair in making a variety of products. The thick, soft fur that covers some mammals is used for coats and other warm clothing. The woolly fleece of sheep is spun into thread and into cloth for such products as blankets, clothing, and rugs. Felt is manufactured by pressing and matting animal hair. Bristles, the short, stiff hair of hogs, are used in making various kinds of brushes.
The structure and growth of hair
The root and shaft.
The part of a hair below the surface of the skin rests in a baglike structure called the follicle. The root, which is the lowest section of a hair, enlarges at the end into a soft, light-colored structure called the hair bulb. Hair develops from the cells of the bulb, which divide rapidly. A structure called the papilla projects into the hair bulb at the base of the follicle. It contains connective tissue and blood vessels that supply the blood necessary for the growing cells.
The cells of the hair bulb move upward as new cells begin to form beneath them. As these cells move higher, they are cut off from their supply of nourishment and start to form a hard protein called keratin. This protein is found in the nails, claws, and hoofs of mammals; the scales of reptiles; and the feathers of birds. The formation of keratin is called keratinization, and the hair cells die as it occurs.
The keratinization process is completed by the time the hair has risen about a third of the way to the surface of the skin. The part of the hair above where keratinization has occurred is called the shaft.
Three layers of dead cells compose the shaft of a hair. The outer layer, called the cuticle, consists of flattened cells that are known as cuticular scales. Beneath the cuticle lies the cortex, a layer of tightly packed, cigar-shaped cells. The cortex contains the pigment (coloring matter) that determines the color of hair. The core of the shaft, called the medulla, is made up of loosely packed, boxlike cells.
Glands and muscles.
Most hair follicles contain an oil gland called the sebaceous gland. This gland secretes oil into the follicle. The oil flows over the hair, lubricating it and keeping it soft.
A muscle known as the arrector pili is attached to most hair follicles. When this muscle contracts, it causes a hair to “stand on end.” In mammals with heavy coats, hair in this position traps air close to the animal’s body, providing extra insulation against the cold. Hair standing on end also makes an animal look larger and more dangerous. When a human being is cold or frightened, the muscles contract and produce little bumps around the hair. These bumps are commonly known as “goosebumps.”
Color and texture.
The color of hair is determined largely by the amount and distribution of a brown-black pigment called melanin. Hair also contains a yellow-red pigment that is most visible in people whose hair has little melanin. Most people’s hair gradually becomes gray or white as they grow older, because pigment no longer forms.
The texture of hair depends largely on the shape of the hair, which can be seen in cross section under a microscope. Straight hairs have a round shape, and wavy and curly hairs are flat. The flattest hairs are the waviest or curliest.
Cycles of hair growth.
The growth of hair in the follicles occurs in cycles. In each cycle, a follicle goes through a growth phase, called anagen, and a resting phase, called telogen. A hair stops growing during every telogen, when it is known as a club hair. The club hair stays in the follicle until the next growing phase. In the growing phase, the club hair is shed as a new hair grows and pushes it out of the follicle.
The length of hair depends on the length of the growing phase of the follicle. The follicles in the human scalp are active for two to six years, and then they rest for about three months. Scalp hairs grow less than half an inch (13 millimeters) per month. Shorter hairs, such as those in the eyebrows and eyelashes, grow for about 10 weeks and then rest for 9 months.
The human scalp contains an average of about 100,000 hairs. From 5 to 15 percent of the hairs in the scalp are in the resting stage at any time. A person sheds from 70 to 100 hairs a day from follicles that are in the resting stage.
Many factors affect the growth of hair. They include age, diet, general health, and changes in the seasons. For example, hair grows faster in children than in adults, and it grows faster in summer than during any other season. Cutting or shaving hair on any part of the body does not have any effect on the future growth or texture of the hair.
Disorders of the hair and scalp
Baldness occurs if the follicles on the scalp die and no longer produce new hair. Heredity is the most common cause of baldness. However, other factors, including scalp infections and reactions to drugs or radiation, can also cause it. Baldness caused by heredity is more common in men than in women. Such baldness cannot be cured.
Various factors, including illness and pregnancy, can affect the growth cycles of the hair follicles. The growing phases of the follicles are shorter in such cases, and a large number of club hairs occur. These club hairs may be shed at one time, causing thinning of the hair or even baldness. This type of hair loss is not permanent because new hairs grow after the growing cycles start again.
Excess hair on the face or elsewhere can also be caused by heredity or by certain medical problems. Such hair can be removed by a process called electrolysis. This process, in which the papillae are destroyed by an electric needle, prevents new hair from growing. Unwanted hair above the surface of the skin can be removed by the application of a liquid or paste that is called a depilatory. However, the root of the hair stays alive, and so the hair grows out again.
Other scalp disorders include dandruff, ringworm, and head lice. Dandruff consists of scales of skin that fall from the scalp. It does not cause hair loss unless accompanied by an infection severe enough to damage the follicles. Ringworm is an infection caused by tiny organisms called fungi, which feed on the keratin. The hairs break off, leaving bare areas on the scalp. Head lice are insects that suck blood from the scalp and attach their eggs to hairs.
