Chapter 5
The Integumentary System
155
5
and resiliency that prevent most jabs and scrapes from penetrating
the dermis. In addition, collagen binds water, helping to keep skin
hydrated. Elastic fibers provide the stretch-recoil properties of skin.
Flexure lines
, a third type of skin marking, are dermal folds
that occur at or near joints, where the dermis is tightly secured
to deeper structures. (Notice the deep creases on your palms.)
Since the skin cannot slide easily to accommodate joint move-
ment in such regions, the dermis folds and deep skin creases
form (Figure 5.4c). Flexure lines are also visible on the wrists,
fingers, soles, and toes.
Homeostatic Imbalance
5.1
Extreme stretching of the skin, such as during pregnancy, can
tear the dermis, leaving silvery white scars called
striae
(stri
9
e;
“streaks”), commonly called “stretch marks.” Short-term but
acute trauma (as from a burn or wielding a hoe) can cause a
blister
, a fluid-filled pocket that separates the epidermal and
dermal layers.
Friction ridge patterns are genetically determined and unique
to each of us. Because sweat pores open along their crests, our
fingertips leave identifying films of sweat called
fingerprints
on
almost anything we touch.
Reticular Layer
Te deeper
reticular layer
, accounting for
about 80% of the thickness of the dermis, is coarse, irregularly
arranged, dense fibrous connective tissue (Figure 5.3). Te net-
work of blood vessels that nourishes this layer, the
cutaneous
plexus
, lies between this layer and the hypodermis. Its extracel-
lular matrix contains pockets of adipose cells and thick bundles
of interlacing collagen fibers. Te collagen fibers run in various
planes, but most run parallel to the skin surface. Separations, or
less dense regions, between these bundles form
cleavage (ten-
sion) lines
in the skin. Tese externally invisible lines tend to
run longitudinally in the skin of the head and limbs and in cir-
cular patterns around the neck and trunk (Figure 5.4b).
Cleavage lines are important to surgeons because when an inci-
sion is made
parallel
to these lines, the skin gapes less and heals
more readily. Te collagen fibers of the dermis give skin strength
Friction ridges
(a) Friction ridges of fingertip (SEM 12
m
)
(b) Cleavage lines in the
reticular dermis
(c) Flexure lines of the hand
Openings of
sweat gland ducts
Flexure lines
on digit
Flexure lines
on the palm
Figure 5.4
Dermal modifications result
in characteristic skin markings.
(a)
Scanning electron micrograph of friction
ridges (epidermal ridges topping the deeper
dermal papillary ridges). Notice the sweat
duct openings along the crests of the ridges,
which are responsible for fingerprints.
(b)
Cleavage (tension) lines represent
separations between underlying collagen
fiber bundles in the reticular region of the
dermis. They tend to run circularly around the
trunk and longitudinally in the limbs.
(c)
Flexure lines form where the dermis is
closely attached to the underlying fascia.
previous page 189 Human Anatomy and Physiology (9th ed ) 2012 read online next page 191 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off