the stratum corneum, stratum lucidum (not always
present), stratum granulosum, stratum spinosum,
and stratum basale.
The dermis, or true skin, lies below the epidermis
and gradually blends into the deeper tissues (fig. 1-30).
It is a wide area of connective tissue that contains blood
vessels, nerve fibers, smooth muscles, and skin
BLOOD VESSELS.The blood vessels of the
dermis can dilate to contain a significant portion of the
body's blood supply (fig. 1-30). This ability, along with
the actions of the sweat glands, forms the body's
primary temperature-regulating mechanism. The
constriction or dilation of these blood vessels also
affects blood pressure and the volume of blood
available to the internal organs.
NERVE FIBERS.The skin contains two types
of nerve fibers that carry impulses to and from the
central nervous system (fig. 1-30). The nerve fibers 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. The first type of nerve
fiber carries impulses to the dermal muscles and
glands, while the other type carries impulses from
sensory receptors (i.e., detecting touch). Both nerve
fibers send messages about the external environment
to the brain.
SMOOTH MUSCLES.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 exposure 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
NAILS.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
formation of nail will occur in the epithelium of the
nail bed. As a new nail is formed, the whole nail moves
forward, becoming longer.
HAIR.Hair is an epithelial structure found on
almost every part of the surface of the body (fig. 1-30).
Its color depends on the type of melanin present. The
hair has two components: the root below the surface
and the shaft projecting above the skin. The root is
embedded in a pit-like depression called the hair
follicle. Hair grows as a result of the division of the
cells of the root. A small muscle, known as the
arrector (fig. 1-30), 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 follicle is
associated with two or more sebaceous glands.
SEBACEOUS GLANDS.Sebaceous glands
are found in most parts of the skin except in the soles of
the feet and the palms of the hand (fig. 1-30). Their
ducts open most frequently into the hair follicles and
secrete an oily substance that lubricates the skin and
hair, keeping them soft and pliable and preventing
SWEAT GLANDS.Sweat glands are found in
almost every part of the skin (fig. 1-30). They are
control mechanisms to reduce the body's heat by
evaporating water from its surface. The perspiration
secreted is a combination of water, salts, amino acids,
and urea. Normally, about one liter of this fluid is
excreted daily. However, the amount varies with
atmospheric temperature and humidity and the amount
of exercise taken. When the outside temperature is
high, or upon exercise, the glands secrete large
amounts to cool the body through evaporation. When
Figure 1-30.Cross section of the skin.