is a substance that grows out of the skin of mammals. It may grow only on certain parts of the body, as in man, or it may cover most of the body, as in such animals as sheep. Hair provides warmth, protects the skin, and responds to touch.
All hair,
regardless of what kind of mammal it grows on, is made of
the same kind of material that forms the nails, claws,
and hoofs of mammals, the scales of reptiles, and the
feathers of birds. Each hair consists of a root and a
shaft. The root of the hair is a soft,
light-colored bulb. The rest of the hair is the shaft.
The root and a small section of the shaft lie below the
surface of the skin in a sac called a follicle.
At the bottom of the follicle is a projection called the papilla,
which contains an artery that nourishes the root of the
hair.
The outside portion of the hair shaft is called the cuticle. It consists of a layer of flattened cells called cuticular scales. In most mammals, these cells overlap like shingles on a roof. Beneath the cuticle lies the cortex, a layer of tightly packed cigar-shaped cells. The cortex contains the pigment (melanin) that determines the hair color. The hair of most mammals has a central core, the medulla, made up of loosely packed boxlike cells.
Hair grows by forming new cells at the base of
the root. As new cells form around the papilla, the old
ones are pushed away and die. The new cells gradually
force the rod of dead cells up out of the follicle, so
that all of the hair above the outermost skin layer is
composed of dead cells. Usually only one hair grows from
each follicle. But in some animals, such as the
chinchilla and other fur-bearing animals, a number of
hairs grow from a single follicle.
Hair continues to grow as long as the papilla provides nourishment for new cells. The papilla may remain active from weeks to years, depending on the species of mammal and the part of the body on which the hair grows. A hair on the human scalp, for example, usually grows about one-half inch each month for two to six years. The hair falls out when it stops growing, and a new hair replaces it. The shorter hairs of the body reach their greatest length and are replaced within a much shorter period. An eyelash, for example, grows for only about three to five months before a new eyelash replaces it. Baldness results when the hair on the scalp is no longer replaced after it falls out. It may be of interest to some to know that human males are not the only animals to experience baldness -- "chrome domes" are also relatively common amongst mature chimpanzees, gorillas, and other primates.