Hair
Hair, which is part of the skin, varies
considerably in texture from the soft, almost invisible type on the forehead to
the long scalp hairs and the shorter, stiff hairs on the eyelids. The root of
each hair is anchored at the bottom of a follicle, or tiny shaft. The follicle
passes through the epidermis, deep into the dermal layer, and the follicle of a
long hair may penetrate into the subcutaneous layer. Emptying into the
follicles is sebaceous, or oil, glands. Each hair has a shaft which extends
past the top of the follicle and is covered with microscopic overlapping
scales. Cells in an inner layer of the shaft contain pigment that determines
hair color. In white hair, these cells contain air rather than pigment.
Attached to each hair follicle, too, is a
small bundle of muscle fibers which, under influence of cold or emotions,
causes the hair to become erect and produces "goose flesh." The
average scalp has about 100,000 hairs. Blonds generally have about 20,000 more
and redheads about 20,000 less. Hereditary influences determine color, texture,
and distribution of hair over head and body. Hairs can grow only so long as
follicles are alive. Sometimes follicles wear out or are destroyed by illness.
Baldness indicates that the follicles have stopped functioning. If the baldness
is caused by disease, the hair may grow back. Most often, however, baldness is
a hereditary condition and there is no known way to renew the hair follicles.
Like baldness, the tendency for hair to turn gray is inherited. There is no
evidence that graying is caused by a vitamin deficiency or a sudden emotional
crisis. Hair follicles follow a cycle: they are active for a period, then
wither, then rest, then become active again. Withering lasts several weeks and
during this period hair shafts fall out. About 30 to 60 hairs are lost daily
from the scalp but the follicles are not lost. After a resting period of several
months, the hairs begin to grow again and continue to grow for several years.
Hair growth during the active period varies
according to body location. Scalp hair growth has been measured at about .012
inch daily, beard growth at .15 inch. Elsewhere on the body, growth is slower.
Nails Like hair, the fingernails and toenails are a specialized form of skin, as
fingers and toes develop, a thin layer, known as the stratum lucidum, which
separates dermis and epidermis, thickens and hardens. The fully developed nail
overlays a modified part of the dermis, that is the nail bed. At its base, the
nail is covered by epidermis.
