The EPIDERMIS is the outer skin layer. It
is made up of tough, flat, scalelike epithelial cells.
Four different sublayers of epidermal cells have
been identified. The uppermost is called the horny
layer (stratum corneum). It is composed of scaly
dead cells that form a protective surface and are
gradually sloughed off naturally or by irritation
(e.g., sunburn) or abrasion. This scaly layer, if
unbroken, can block the passage of almost every
known type of germ; however, its protective
powers are reduced if the skin is not cleansed
regularly. Two middle layers of cells may be pres-
ent in a particular area of skin, depending on its
thickness (the soles of the feet are the thickest
skin, the eyelids the thinnest). In the innermost
sublayer, the stratum germinativum, new epider-
mal cells are constantly being produced to replace
the sloughed off cells. These newly formed cells
push the older cells outward. As they approach
the surface, they become drier or more scalelike.
Because of this constant activity of the deeper cells
of the epidermis, any injury of the outer layer of
the skin is repaired in a few days without leaving
a scar. Skin pigment, called melanin, which is
responsible for skin color, is found here in this
deepest sublayer. The color and quantity of the
melanin are the chief factors in determining ones
complexion. The pigment can be darkened by ex-
posure to the ultraviolet rays of the sun (tanning).
Freckles are patches of melanin.
The DERMIS, or true skin, lies below the
epidermis and gradually blends into the deeper
tissues. It is a wide area of connective tissue that
contains blood vessels, hair follicles, nerve end-
ings, smooth muscles, and sweat and oil glands.
The blood vessels of the dermis can dilate to
contain a significant portion of the bodys blood
supply. This ability, along with the actions of the
sweat glands, forms the bodys primary tempera-
ture regulating mechanism. The constriction or
dilation of these blood vessels also affects blood
pressure and the volume of blood available to the
The skin contains nerve endings that carry im-
pulses to and from the central nervous system.
The nerves are distributed to the smooth muscles
in the walls of the arteries in the dermis and to
the smooth muscles around the sweat glands and
hair roots. Through these nerves, messages about
the external environment are carried to the brain.
Smooth involuntary muscles are found in the
dermis. They are responsible for controlling the
skin surface area. When dilated, these muscles
allow for maximum skin surface exposure to aid
heat loss. When constricted, the skin surface ex-
posure is decreased, thus impeding heat radiation.
Repeated muscle contractions (shivering) are also
a rapid means of generating body heat.
The appendages of the skin are the nails, hairs,
sebaceous glands, sweat glands, and ceruminous
The NAILS are composed of horny epidermal
scales and are found on the dorsal surfaces of the
fingers and toes. They protect the many sensitive
nerve endings at the ends of these digits. New for-
mation of nail will occur in the epitheliums of the
nail bed. As new nail is formed, the whole nail
moves forward, becoming longer.
HAIR is an epithelial structure found on al-
most every part of the surface of the body. Its
color depends on the type of melanin present. The
hair has two components: the root below the sur-
face and the shaft projecting above the skin. The
root is embedded in a pitlike depression called the
hair follicle. Hair grows as a result of the divi-
sion of the cells of the root. A small muscle, the
arrector, fastens to the side of the follicle and is
responsible for the gooseflesh appearance of the
skin as a reaction to cold or fear. Each hair folli-
cle is associated with two or more sebaceous (oil)
SEBACEOUS GLANDS are found in most
parts of the skin except in the soles of the feet
and the palms of the hand. Their ducts open most
frequently into the hair follicles and secrete an oily
substance that lubricates the skin and hair, keep-
ing them soft and pliable and preventing bacterial
SWEAT GLANDS are found in almost every
part of the skin. They are control mechanisms to
reduce the bodys heat by evaporation of water
from its surface. The perspiration secreted is a
combination of water, salts, fatty acids, and urea.
Normally, about one liter of this fluid is excreted
daily. However, the amount varies with atmos-
pheric temperature and humidity and the amount
of exercise taken. When the outside temperature
is high, or upon exercise, the glands secrete ex-
cessive amounts to cool the body through evap-
oration. When evaporation cannot handle all the
sweat that has been excreted, the sweat collects
in beads on the surface of the skin.
CERUMINOUS GLANDS are modified
sweat glands found only in the auditory canal.