THE SKIN
OUR WONDERFUL COAT
What the Skin Is. The skin is the most wonderful and one of the most
important structures in the body. We are prone to think lightly of it
because it lies on the surface, and to speak of it as a mere coating, or
covering--a sort of body husk; but it is very much more than this. Not
only is it waterproof against wet, a fur overcoat against cold, and a
water jacket against heat, all in one, but it is also a very important
member of the "look-out department," being the principal organ of one of
our senses, that of touch.
The eyes in the beginning were simply little colored patches of the
skin, sunk into the head for the purpose of specializing on the
light-rays. The smelling areas of the nose also were pieces of the skin,
as were also the ears. Not only so, but--although it is a little hard
for you to understand how this could have happened--the whole brain and
nervous system is made up of folds of the skin tucked in from the
surface of the back; so that we can say that the skin, with the organs
that belong to it and have grown from it--the eyes, nose, ears, brain,
and nerves--forms the most wonderful part of the body. Everything that
we know of the world outside of us is told us by the skin and the
look-out organs that have grown out of it. The skin is not only the
surface part and coating of the body, far superior to any six different
kinds of clothing which have yet been invented, but it is related to,
and assists in, the work of nearly half the organs in the body. Not only
all that we learn by touch and pressure, but everything that we know of
heat and cold, of moisture and dryness, and most of pain, comes to us
through our skin, through the little bulbs on the ends of the nerve
twigs in it. It also helps the lungs to breathe, the kidneys to purify
the blood, and the heart to control the flow of blood through the body.
A healthy skin is of very great importance; and part of this health we
can secure directly, by washing and bathing, scrubbing and kneading and
rubbing, because the skin lies right on the surface, where we can
readily get at it. But, on the other hand, no amount of attention from
the outside alone will keep it healthy. All the organs inside the body
must be kept healthy if the skin is to be kept in good condition.
Although the external washing and cleaning are very important, the
greater part of the work of developing a healthy skin and a good
complexion must be done from the inside.
The Two Layers which Make Up the Skin. Like our "internal skin," the
mucous membrane, which lines our stomach and bowels, the skin is made up
of two layers--a deeper, or basement, sheet, woven out of tough strands
of fibrous stuff (_derma_); and a surface layer (_epidermis_) composed
of cells lying side by side like the bricks in a pavement, or the tiles
on a floor, and hence called "pavement" (_epithelial_) cells. These
pavement cells are fastened on the basement membrane much as the kernels
of corn grow on a cob; only, instead of there being but one layer, as on
a cob of corn, there are a dozen or fifteen of them, one above the
other, each one dovetailing into the row below it, as the corn kernels
do into the surface of the cob. As they grow up toward the surface from
the bottom, they become flatter and flatter, and drier, until the outer
surface layer becomes thin, fine, dry, slightly greasy scales, like
fish-scales, of about the thickness of the very finest and driest bran.
We are continually Shedding our Skin. One way in which the skin keeps
itself so wonderfully clean and fresh is by continually shedding from
its surface showers of these fine, dry, scaly cells, which drop, or are
rubbed off, as they dry. This is the reason why no mark, not even a
stain or dye, upon the skin, will stay there long; for no matter how
deeply it may have soaked into the layers of the pavement-cells, every
cell touched by it will ultimately grow up to the surface, dry up, and
fall off, carrying the stain with it.
If you want to make a mark on the skin that will be permanent, you have
to prick the colors into it so deeply that they will go through the
basement layer and reach cells which will not grow toward the surface.
This "pricking-in" operation is known as _tattooing_; and it is as
foolish as it is painful, for blood-poisoning and other diseases may be
carried into the system in the process.
[Illustration: THE LAYERS OF THE SKIN
_E_, epidermis; _C_, capillaries; _D_, dermis; _F_, fat globules and
connecting fibres.]
Perhaps you will wonder why, if you are shedding these scales from all
over your surface every day, you don't see them. This is simply because
they are so exceedingly small, thin, and delicate, that you cannot see
them unless you get a large number of them together; and when you are
changing your clothing, bathing, etc., they are rubbed off and float
away. If a part of the body has been shut in--as when a broken arm, for
instance, is in a cast, which cannot be changed for several weeks--when
finally you take off the bandage, you will find inside it spoonfuls--I
had almost said handfuls--of fine scales, which have been shed from the
skin and held in by the wrappings.
THE GLANDS IN THE SKIN
Sweat Glands. Like all the pavement (epithelial) surfaces of the body,
inside and out, the skin has the power of making glands by dipping down
little pouches or pockets into the layers below. In the skin, these
little gland-pockets are of two kinds, the _sweat glands_ and the _hair
glands_.
The sweat glands are tiny tubes which go twisting down through the
different pavement layers, through the basement layer, and right into
the coat of fat, which lies just under the skin. The tube of the sweat
gland soaks, or picks, out of the blood some of the waste-stuff--just as
the kidney tube does in the kidney,--together with a good deal of water
and a small amount of delicate oil, and pours them out on the surface of
the body in the form of the "sweat," or _perspiration_.
As you will remember, when the muscles work hard and pour more waste
into the blood, then the heart pumps larger amounts of blood out into
the skin; and this causes it to redden. The sweat glands work harder to
purify this extra blood, and they pour out the waste and oil and water
on the surface. As soon as this water gets upon our hot skin, it begins
to evaporate and cool us off, as well as to carry off some of the waste
in the form of gas. The trace of oil in the perspiration helps to
lubricate the skin and keep it soft; but when too much of it is poured
out we have that greasy feeling, which we have all felt after perspiring
freely.
From all this cooling and breathing and blood-purifying work going on
upon the surface of our skin, you can easily see why it is so important
that all our clothing should be loose and porous and that next the skin
easily washed; else it will very soon become clogged up and greasy, and
shut off the breathing and blood-purifying work of the skin and make it
dirty and unhealthy. This continual mist of water, rising and bubbling
up through our skin like springs out of a hillside, is another of
nature's wonderful ways of cleansing the skin and of preventing any kind
of dirt from permanently sticking to or lodging in it. Remember, you do
not need to dig below the surface when you wash.
Hair Glands. The other kind of skin glands, the hair glands, are also
pouches growing out from the deepest part of the stem of the hair, known
as the root, or _hair bulb_.
[Illustration: THE GLANDS IN THE SKIN
_S_, sweat gland; _H_, hair bulb; _O_, oil gland; _T_, touch bulb at tip
of nerve.]
From the root of the hairs, two or three little bundles of muscle run up
toward the surface of the skin. When these contract, they pull the root
of the hair up toward the surface, causing the hair to stand erect, or
"bristle," as we say. This is what makes the hair on a dog's or a cat's
back stand up when he is angry; but the commonest use of the movement
is, when animals are cold, to make their coats stand out so as to hold
more air and retain the body-heat better. We have lost most of our hairy
coating, but whenever we get chilly, whether from cold or from fright,
these little muscles of our hair bulbs contract and pull the hair glands
of our skin up toward the surface, so that it looks all "pimply" or
"goose-skinned."
Each hair pouch has sprouted out from its sides a pair of tiny pouches,
which form _oil glands_ to lubricate the hair and keep it sleek and
flexible. It is hard to beat nature at her own game, and her method of
oiling the hair is far superior to any hair oil that can be put on from
the outside. Keep your hair well brushed and washed, and nature will oil
it for you much better than any hair oil or scalp reviver ever
invented.[19]
THE NAILS
How the Nails are Made. Another "trade," which our wonderful skin has
literally "at its fingers' ends," is that of making nails. Indeed, every
kind of scale, armor, fur, feather, and leather coating possessed by
bird, beast, or fish was made by, and out of, the skin. Nail-making,
however, is one of its simplest feats, as it is carried out merely by
turning a little patch, or area, of itself into a horn-like substance.
This, the skin of insects, of fishes, of crocodiles, etc., does all over
the surface of their bodies; but in animals and birds only a number of
little patches at the tips of the toes harden up in this way, to form
the claws or nails; and in birds, the beak; and in some animals, the
horns. So it is quite correct to call the substance of our nails
"horn-like."
In some animals and birds, these little horny patches at the ends of the
toes grow out into long, curved hooks, or broad, digging chisels and
scoops; but on our own fingers, they simply make a little mould over the
finger-tip. If, however, they are protected from being broken off, they
will grow four or five inches long; in fact, they are carefully trained
to do this by some of the upper classes in China, merely for the purpose
of showing that they have never been obliged to degrade themselves, as
they foolishly regard it, by working with their hands.
You can easily prove that the nails do grow constantly from the root or
base, out toward the tip, by watching, some time when you have pounded
one of your nails, how the black or discolored patch in it will grow
steadily outward toward the tip, where it will be broken off and shed.
You cannot see the softest and youngest row, or layer, of the nail cells
at the base, because a fold of skin, the _nail fold_, has been doubled,
or folded, over them to protect them while they are young and soft. It
is not best to push this fold of skin back too much, as, by so doing,
you may uncover the young nail cells while they are soft and tender, and
expose them to injury. The reason why there is a little whitish crescent
at the base of the nail is that the cells of the nail do not grow hard
and horn-like and transparent until they have grown out a quarter of an
inch or so from under the fold, but at first look whitish, or opaque,
like the rest of the skin.
Health Shown by the Color of the Nails. Your nails and your lips are
not really any redder, or pinker, than the rest of your skin; but the
cells forming them are clear and transparent and allow the red blood to
show through. This is why we often look at the nails and lips to see
what the color of the blood is like, and how well or badly it is
circulating. If the blood is _anemic_, or thin, then both lips and nails
are pale and dull. If the blood is healthy and the circulation good,
then the nails are pink, and the lips clear red. If, on the other hand,
the circulation is bad, as in some forms of lung disease and heart
disease, so that the blood is loaded with carbonic acid until it is
blue and dark, then the lips may become purplish or dark blue, and the
finger nails nearly the same color.
THE BLOOD-MESH OF THE SKIN
The Blood Vessels under the Skin. Not merely the nails and the lips,
but the whole surface of the skin is underlaid with a thick mat, or
network, of blood vessels. These vessels are all quite small, so that a
cut has to go down completely through the skin, and generally well down
into the muscles, before it will reach any blood vessel which will bleed
at a dangerous rate. But there are so many of them, and they cover such
a wide surface throughout the body, that they are actually capable of
holding, at one time, nearly one-tenth of all the blood in the body.
This "water-jacket" coat of tiny blood vessels all over our body has
some very important uses: It allows the heart to pump large amounts of
blood out to the surface to be purified by the sweat glands, and to
breathe out a little of its carbon dioxid and other gas-poisons.
The Skin as a Heat Regulator. Heat, as well as waste, is given off by
the blood when it is poured out to the surface; so another most
important use of the skin is as a heat regulator. As we have already
seen, every movement which we make with our muscles, whether of arms and
limbs, heart, or food tube, causes heat to be given off. We very well
know, when we work hard at anything, we are likely to "get warmed up."
Although a certain amount of this heat is necessary to our bodily
health, too much of it is very dangerous.
Just as it is best for the temperature, or heat, of a room to be at
about a certain level, somewhere from 60° to 70° F., so it is best for
the interior of our bodies to be kept at about a certain heat. This, as
we can show by putting a little glass thermometer under the tongue, or
in the armpit, and holding it there for a few minutes, is a little over
98° F. (98.4° to be exact); and this we call "body heat," or "blood
heat," or "normal temperature." Our body cells are, in one way, a very
delicate and sensitive sort of hot-house plants, though tough enough in
other respects. Whenever our body heat goes down more than five or six
degrees, or up more than two or three degrees, then trouble at once
begins. If our temperature goes down, as from cold or starvation, we
begin to be drowsy and weak, and finally die. If, on the other hand, our
temperature climbs up two, three, or four degrees, then we begin to be
dizzy and suffer from headache and say we have "a fever."
A fever, or rise of temperature, that can be noted with a thermometer,
is usually due to disease germs of some sort in the body; and most of
the discomfort that we suffer is really due more to the poisons (toxins)
of the germs than to the mere increase of heat, though this alone will
finally work serious damage. However, as we well know from repeated
experience, we need only to run or work hard in the sun for a
comparatively short time to make ourselves quite hot enough to be very
uncomfortable; and if we had no way to relieve ourselves by getting rid
of some of this heat, we should either have to stop work at once, or
become seriously ill. This relief, however, is just what nature has
provided for in this thick coat of blood vessels in our skin; it enables
us to throw great quantities of blood out to the surface where it can
get rid of, or, as the scientists say, "radiate," its heat. This cooling
process is hastened by the evaporation of the perspiration poured out at
the same time, as we have seen.
One of the chief things in training for athletics is teaching our skin
and heart together to get rid of the heat made by our muscles, as fast,
or nearly as fast, as we make it, thus enabling us to keep on running,
or working, without discomfort. As soon as we stop running, or working,
the heart begins to slow down, the blood vessels in the skin contract
and diminish in size, the flush fades, and we begin to cool off. We are
not making either as much heat or as much waste as we were, and hence do
not need to get rid of so much through our skins.
When we feel cold, just the opposite kinds of change occur in the skin.
The blood vessels in the skin contract so as to keep as much of our warm
blood as possible in the deeper parts of our body, and prevent its
losing heat. As blood showing through the pavement-layer of the skin is
what gives us our color, or complexion, our skin becomes pale and
pasty-looking; and if all the blood is driven in from the surface, our
lips and finger nails will become blue with cold. Here again, by changes
in the skin, nature is simply trying to protect herself from the loss
of too much heat.
If we exercise briskly, or eat a good warm meal, and thus make more heat
inside of our body, then there is no longer any need to save its surface
loss in this way; and the blood vessels in our skin fill up, the heart
pumps harder, and the warm, rich color comes back to our faces and lips
and finger nails.
So perfectly and wonderfully does this skin mesh of ours work, by
increasing or preventing the loss of heat, that it is almost impossible
to put a healthy man under conditions that will raise or lower his
temperature more than about a degree, that is to say, about one per cent
above, or below, its healthful level. Men studying this power of the
skin have shut themselves into chambers, or little rooms, built like
ovens, with a fire in the wall or under the floor, and found that if
they had plenty of water to drink and perspired freely, they could stand
a temperature of over 150° F. without great discomfort and without
raising the temperature of their own bodies more than about one degree.
If, however, the air in the chamber was moistened with the vapor of
water, or steam, so that the perspiration could no longer evaporate
freely from the surface of their bodies, then they could not stand a
temperature much above 108° or 110° without discomfort.
Other men, who were trained athletes, have been put to work in a closed
chamber, at very vigorous muscular exercise, so as to make them perspire
freely. But while a thermometer placed in that chamber showed that the
men were giving off enormous amounts of heat to the air around them,
another thermometer placed under their tongues showed that they were
raising the temperature of their own bodies only about half a degree.
One man, however, happened to try this test one morning when he was not
feeling very well, and didn't perspire properly, and the thermometer
under his tongue went up nearly four degrees.
THE NERVES IN THE SKIN
How We Tell Things from Touch, and Feel Heat and Cold and Pain. Last
of all, the skin is the principal organ of the sense of touch, and also
of the "temperature sense"--the sense of heat and cold--and of the sense
that feels pain. All these feelings are attended to by little bulbs
lying in the deeper part of the skin and forming the tips of tiny nerve
twigs,[20] which run inward to join larger nerve branches and finally
reach the spinal cord. There are millions of these little bulbs
scattered all over the surface of the skin, but they are very much
thicker and more numerous in some parts than in others; and that is why,
as you have often noticed, certain parts of the skin are more sensitive
than others. They are thickest, for instance, on the tips of our fingers
and on our lips, and fewest over the back of the neck and shoulders,
and across the lower part of the hips.[21]
For a long time, it was supposed that all these little nerve-bulbs in
the skin did the same kind of work, because they looked, under the
microscope, exactly alike; but it was found that they divide the work up
among them, so that some of them give their entire attention to heat,
and others to cold, others to touch, and others again to pain. So
carefully has the work been mapped out among them that they report to
different centres in the brain and spinal cord, so that we now
understand why, in diseases which happen to attack one or other of these
centres, we may lose our sense of heat and cold, as in that terrible
disease, _leprosy_; or our sense of touch, as in _paralysis_; or we may
even, in some very rare cases, lose our sense of pain, and yet have all
our other senses perfect.
FOOTNOTES:
[19] Hairs are of value chiefly as protection against cold and wet,
although we have got rid of them and substituted clothing for this
purpose, except on the top of our heads; but their roots also are very
richly supplied with nerves so that they form almost a sort of feelers,
or organs of sense. Many animals that move about much in the dark, like
cats and bats, for instance, have their lips or faces studded with long,
delicate, stiff hairs called whiskers, which act in this way and prevent
their bumping into objects in the dark. And it is probable that the
bristling of the hair on a dog's back, when he is angry or frightened,
is in part for this purpose--to enable him to slip aside and dodge a
blow, even after it has touched the ends of the hairs. This great
sensitiveness of the hair roots is what makes it hurt so when any one
pulls your hair.
[20] See the diagram of the skin on page 171.
[21] You can easily test this by a very simple experiment. Take a pair
of dividers; or, if you haven't these, a couple of long pins or needles
will do. Set them with their points a quarter of an inch apart. Then
touch these points, first closing your eyes, so that you will not be
able to see them, to the tip of one of your fingers, and you will
readily feel that _two_ points touch the skin. Turn your hand over and
touch the back of it with the two points, and they will feel like one
point. Carry the test further, over other parts of the body, and you
will find that they are much less sensitive; thus you will find that at
the back of the neck, or over the shoulder-blades, you will have to put
the points nearly an inch apart before you can tell that there are two
of them. This simply means that you have to touch two separate touch
bulbs before you can get the idea of "two-ness." As these bulbs are an
inch or more apart in the skin of the back, you have to spread the
points of the dividers that distance. You can also prove that the
touching of two nerve-buds gives the idea of "two-ness" by crossing two
of your fingers and placing a pea, or small round piece of chalk,
between their crossed tips. If you close your eyes and roll the pea on
the table, or desk, you will think you have two peas between your
fingers.