ing the blood back within the body’s core. This
same mechanism also responds to strong emotions
such as fear or embarrassment, similarly sending
blood rushing to the skin. The blood supply of the
dermis also nourishes the epidermis, supporting
the perpetual production of new cells in the stra-
tum germinativum.
About 5 million sweat glands aid both tempera-
ture and fluid regulation, dripping out about 500
milliliters of somewhat salty water every hour.
Most of this moisture evaporates without con-
scious awareness of its presence. Sweating
becomes an obvious event only when it becomes
profuse, as with intense exercise or heat, or in
conditions such as HYPERHIDROSIS. The eccrine
sweat glands secrete their fluid directly to the sur-
face of the skin through sweat pores. The apocrine
sweat glands, found primarily in the axillae
(underarms) and groin areas, secrete their fluids
into the hair follicles. Sweat from the eccrine
glands is water and electrolytes (salts), while that
from the apocrine glands contains lipids and pro-
teins to help it mix with the sebum in the hair fol-
licles. These extra substances account for the
familiar odor of sweat as well as the yellowish
stains sweating leaves in the underarm areas of
clothing.
THE SUNLIGHT VITAMIN
Though the negative consequences of ultraviolet light
get the most press, sunlight is a valuable resource for
strong bones and TEETH. The dermis contains ergosterol,
a chemical that ultraviolet light converts into vitamin
D, which the body needs to use calcium to build BONE
and tooth enamel.Millions of specialized nerve endings reside
within the dermis, gathering data about tempera-
ture, humidity, motion, and contact, which they
transmit to the NERVOUS SYSTEM. The skin is the
organ of tactile perception, the sense of touch. The
highest concentrations of tactile receptors are in
the fingertips, the lowest on the soles of the feet.
The key structures that detect touch are the discs
of Merckel (light touch) and the corpuscles of
Meissner (moderate touch), specialized nerve end-
ings that transmit impulses to specially dedicated
regions of the cerebral cortex that interpret the
nerve messages and initiate the appropriate
responses. The structures that detect heavier pres-
sure, the corpuscles of Ruffini and the Pacinian
corpuscles, reside deeper in the dermis, near or
extending beyond the subcutaneous border.
Cushion and connection: the subcutaneous
layer The subcutaneous layer, also called the
hypodermis, contains mostly adipose tissue (body
fat) and some connective tissue. The adipose tissue
warehouses the excess calories the body converts
to fat, varying in thickness to accommodate this
stored energy source. A rich network of blood ves-
sels and nerves permeates the subcutaneous layer,
supplying nourishment to all layers of the skin
and conducting nerve impulses from sensory
receptors and other nerve structures. The subcuta-
neous layer gives shape and context to the skin,
softening the protrusions and angles of the under-
lying musculoskeletal structures. It also cushions
those underlying structures from the multitude of
minor traumas the body’s surface experiences
every day.
The connective tissue of this deepest layer of
the skin binds the upper skin layers to the internal
structures of the body, overlaying the muscles.
Fibers of connective tissue lace from the subcuta-
neous layer upward into the epidermis and down-
ward to the musculoskeletal structures, holding
the skin in place. These connections allow the skin
to respond to the movement of muscles beneath
it. In locations such as the knuckles, the connec-
tions are loose (appearing deeply creased) to
accommodate substantial motion. In the face, by
contrast, the skin tethers tightly to the underlying
structures. When the facial muscles move the skin
on the face moves, too, forming facial expressions.
The hair Hair has different characteristics and
functions, depending on its location on the body.
Though in many other mammals hair may serve
for protection and heat regulation, in humans hair
has mostly lost these purposes. The exceptions are
the hair on the head and a man’s unshaven beard,
which help preserve heat and shelter the skin
from sunburn. Hair in other locations has specific
functions. Hair in the auditory canal and in the
nasal passages helps move debris to the outside of
the EARand NOSE, respectively. The eyebrows keep
sweat from running into the eyes, and the eye-
lashes help prevent environmental debris such as
dust or pollen particles from entering the eyes.128 The Integumentary System