160
UNIT 2
Covering, Support, and Movement of the Body
5
Check Your Understanding
15.
Why is the lunule of a nail white instead of pink like the rest
of the nail?
16.
Why are nails so hard?
For answers, see Appendix H.
Sweat (Sudoriferous) Glands
Sweat glands
, also called
sudoriferous glands
(su
0
do-rif
9
er-
us;
sudor
5
sweat), are distributed over the entire skin surface
except the nipples and parts of the external genitalia. Teir
number is staggering—up to 3 million per person.
We have two types of sweat glands: eccrine and apocrine.
In both types, the secretory cells are associated with myoepi-
thelial cells, specialized cells that contract when stimulated by
the nervous system. Teir contraction forces the sweat into and
through the gland’s duct system to the skin surface.
Eccrine (Merocrine) Sweat Glands
Eccrine sweat glands
(ek
9
rin; “secreting”), also called
mero-
crine sweat glands
, are far more numerous and are particu-
larly abundant on the palms, soles of the feet, and forehead.
Each is a simple, coiled, tubular gland. Te secretory part lies
coiled in the dermis, and the duct extends to open in a funnel-
shaped
pore
(
por
5
channel) at the skin surface
(Figure 5.7b)
.
Homeostatic Imbalance
5.5
Hair thinning can be induced by a number of factors that upset
the normal balance between hair loss and replacement. Out-
standing examples are acutely high fever, surgery, severe emo-
tional trauma, and certain drugs (excessive vitamin A, some
antidepressants and blood thinners, anabolic steroids, and most
chemotherapy drugs). Protein-deficient diets and lactation lead
to hair thinning because new hair growth stops when protein
needed for keratin synthesis is not available or is being used for
milk production. In all of these cases, hair regrows if the cause
is removed or corrected.
In the rare condition
alopecia areata
, the immune system at-
tacks the follicles and the hair falls out in patches. But again, the
follicles survive. Hair loss due to severe burns, excessive radia-
tion, or other factors that eliminate the follicles is permanent.
Check Your Understanding
11.
What are the concentric regions of a hair shaft, from the
outside in?
12.
Why is having your hair cut painless?
13.
What is the role of an arrector pili muscle?
14.
What is the function of the hair papilla?
For answers, see Appendix H.
Nails
A
nail
is a scalelike modification of the epidermis that forms a
clear protective covering on the dorsal surface of the distal part
of a finger or toe
(Figure 5.6)
. Nails, which correspond to the
hooves or claws of other animals, are useful as “tools” to help
pick up small objects or scratch an itch.
In contrast to
sof keratin
of the epidermis, nails (like hairs)
contain
hard keratin
. Each nail has a
Free edge
, a
nail plate
or
body
(visible attached portion), and a proximal
root
(embedded
in the skin). Te deeper layers of the epidermis extend beneath
the nail as the
nail bed
, and the nail itself corresponds to the
superficial keratinized layers. Te thickened proximal portion
of the nail bed, called the
nail matrix
, is responsible for nail
growth. As the nail cells produced by the matrix become heavily
keratinized, the nail body slides distally over the nail bed.
Nails normally appear pink because of the rich bed of capillar-
ies in the underlying dermis. However, the region that lies over
the thick nail matrix appears as a white crescent called the
lunule
(lu
9
nool; “little moon”). Te proximal and lateral borders of the
nail are overlapped by skin folds, called
nail folds
. Te proximal
nail fold projects onto the nail body as the
cuticle
or
eponychium
(ep
0
o-nik
9
e-um; “on the nail”). Te thickened region beneath the
free edge of the nail where dirt and debris tend to accumulate is
the
hyponychium
(“below nail”), informally called the quick. It
secures the free edge of the nail plate at the fingertip.
Changes in nail appearance can help diagnose certain con-
ditions. For example, yellow-tinged nails may indicate a respi-
ratory or thyroid gland disorder, while thickened yellow nails
may signal a fungus infection. An outward concavity of the nail
(spoon nail) may signal an iron deficiency, and horizontal lines
(Beau’s lines) across the nails may hint of malnutrition.
Lateral
nail fold
Lunule
(a)
Nail
matrix
Root of nail
Proximal
nail fold
Hyponychium
Nail bed
Phalanx (bone of fingertip)
Eponychium
(cuticle)
Body
of nail
Free edge
of nail
(b)
Figure 5.6
Skin appendages: Structure of a nail.
(a)
Surface
view of the distal part of a finger.
(b)
Sagittal section of the
fingertip. The nail matrix that forms the nail lies beneath the lunule.
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