(5) Hair growth and replacement. There is a cyclic pattern-- alternating
growing and resting periods--to hair growth and replacement. The matrix cells are
active during the growing phase when these cells divide thus increasing their number,
push upward, and eventually die. A hair, dead protein tissue, is the product. Hair grows
about 1 mm (0.04 inch) every three days. During the resting phase, the matrix is
inactive, and these cells shrink causing the hair root to detach itself from the matrix and
the hair to move up the hair follicle slowly. The hair may stay in this position for a while
until it is pulled out, shed, or pushed up by a replacing hair. The cycle of hair growth is
different in different parts of the body. Scalp hair grows steadily and continuously for
two to six years. Growth stops, and after three months, the hair is shed. Three months
later a new hair starts to grow from the same follicle. On the other hand, eyebrows
have a growing phase of only about ten weeks, which is why eyebrow hair is not very
long.
b. Nails.
(1) Parts of the nail. Nails are hardened, horny-like epidermal cells. These
cells form a clear, solid covering over the back surfaces of the fingers and toes. Each
nail has three parts: nail body, free edge, and nail root. The part of the nail you can
see is the nail body. The free edge is the part of the nail that extends beyond the finger
or toe, and the nail root is the part of the nail that is hidden in the nail groove. Tissues
under the nails have many blood vessels, which accounts for the fact that the nail body
appears to be pink. The whitish, moon-shaped area near the nail root is the lunula. It is
whitish in color due to tissues with blood vessels that do not show through. The nail fold
is the fold of skin around the borders of the nail. The skin beneath the nail is the nail
bed.
(2) Growth of the nail. Nail growth takes place in the part of the nail under
the body and nail root. Superficial cells in the matrix change into nail cells, and the
outer, harder nail layer is pushed forward making the nail longer. Fingernails grow at a
rate of approximately 1 mm (0.04) per week while toenails grow a little more slowly. It
takes approximately three and a half to five and a half months for a lost fingernail to
regrow. A lost toenail takes a little longer--six to eight months.
c. Skin Glands. These are the three kinds of skin glands: sebaceous glands;
sweat glands (sudoriferous glands); and modified sweat glands. See figure 1-4.
(1) Sebaceous glands. These glands, not found in the palms of the hands
or the soles of the feet, are oil glands that are connected to hair follicles. The glands
are in many parts of the body and exist in a variety of shapes and sizes. Sebaceous
glands secrete a substance called sebum, an oily substance that is a mixture of fats,
cholesterol, proteins, and inorganic salts. Sebum acts as a skin lubricant keeping hair
from drying and becoming too brittle. This substance also forms a protective film that
prevents too much water from evaporating from the skin; this protective film is
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