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beta keratin. Unless the hair is greatly distended or -altered
by chemical agents, it will return immediately to its normal
alpha configuration. When you wash your hair, it can be
elongated to one and one-half its normal length due to the
weight of the water on the hair. This is possible because the
protein keratin can be readily stretched in the direction of
the long axis of the molec-ular chains of amino acids.
Permanent waves act on this principle and people can
change the texture of their hair by going to a beauty salon
for treatment. The hair stylist will stretch and mold the hair
into the desired new wave: big rollers for straighter hair,
small tight rollers for curlier hair. Then a chemical reducing
agent is placed on the hair to rupture the old disulfide
bonds of the alpha keratin chain. Next a new chemical
oxidizing agent is placed on the hair to reestablish new
stabilizing cross-links in the new posi-tion of the beta
chain. Remember, the chemicals only -affected the visible
portion of the hair or the shaft. The new cells growing from
the hair bulb will not have the new texture, and the
permanent wave or new style will eventually “grow out.”
Another visit to the beauty salon must occur in a few
months to redo the process.
Hair Color
Hair color is also determined by complex genetic factors.
For example, some people turn gray in their youth, yet
others turn gray in their 40s, 50s, or even as late as their
60s. We do know that gray hair occurs when pigment is
absent in the cortex of the hair. White hair results from both
the absence of pigment in the cortex plus the for-mation of
air bubbles in the shaft.
Hereditary and other unknown factors determine the
graying of hair. An interesting research project was done
with black cats and gray hair. The hair of a black cat turned
gray when its diet was deficient in pantothenic acid (an
amino acid). Restoring this substance to its diet caused the
gray hair to return to black. Unfortunately, this only works
with cats. So the hair coloring industry is still secure.
Great frights, like being in a serious plane or car ac-
cident, can cause people’s hair to change color and go gray
or white. This occurs quite rarely. We do not know what
physiologic processes are triggered that cause this to occur,
other than the trauma of such an experience.
Nails
At the ends of fingers and toes, we have nails (Figure 6-5).
Other animals have claws (birds, reptiles, cats, and dogs) or
hooves (horses, cows, deer, and elk). The nail is a modi-
fication of horny (leathery) epidermal cells composed of
Chapter 6
(A)^ Lunula^Free edge Nail body Eponychium
of nail (cuticle)(B)(^)
®^
Nail bed
Le ar ni ng^
Cengage^
Bone
©^
Figure 6- 5 The anatomy of a nail. (A) Posterior
view of finger- and (B) fingernail and underlying
structures.
very hard keratin. Air mixed in the keratin matrix forms the
white crescent at the proximal end of each nail called the
lunula (LOO-noo-lah) and the white as the free edge of
the nail. Again the size of the lunula will vary from person
to person and sometimes from nail to nail due to genetic
factors. The nail body is the visible part of the nail. The
nail root is the part of the nail body attached to the nail
bed from which the nail grows approximately 1 mm per
week unless inhibited by disease. The cuticle or
eponychium is stratum corneum that extends out over the
proximal end of the nail body.
Our fingernails grow faster than our toenails. Regen-
eration of a lost fingernail occurs in 3½ to 5½ months.
Regeneration of a lost toenail occurs in 6 to 8 months as
long as the nail bed remains intact. As we age, the rate of
growth of nails slows.
Sebaceous Glands
Sebaceous (see-BAY-shus) glands (see Figure 6-1)
develop- along the walls of hair follicles and produce
sebum (SEE-bum). This is an oily substance that is-
responsible- for lubricating the surface of our skin, giv-ing
it a glossy appearance. Sebaceous secretions consist of
entire cells containing the sebum. As the cells disin-tegrate,
the sebum moves along the hair shaft to the surface of the
skin where it produces a cosmetic gloss. Brushing hair
causes the sebum to cover the shaft of our hair, making hair
shiny. Remember how good your dog