THE SKIN AND WHAT IT DOES.
~1. The Skin.~--The skin is the covering of the body. It fits so exactly
that it has the precise shape of the body, like a closely fitting
garment. If you will take up a little fold of the skin you will see that
it can be stretched like a piece of india-rubber. Like rubber, when it
is released it quickly contracts and appears as before.
~2. The Bark of Trees.~--Did you ever peel the bark off of a young tree?
If so, you have noticed that there were really two barks, an outer bark,
as thin as paper, through which you could almost see, and an inner and
much thicker bark, which lay next to the wood of the tree. You can peel
the outer bark off without doing the tree much harm. Indeed, if you will
notice some of the fruit or shade trees in the yard, at home, you will
see that the outer bark of the tree peels itself off, a little at a
time, and that new bark grows in its place. If you tear off the inner
bark, however, it will injure the tree. It will make it bleed, or cause
the sap to run. The sap is the blood of the tree. The bark is the skin
of the tree. When the bare place heals over, an ugly scar will be left.
~3. The Cuticle.~--Our bodies, like trees, have two skins, or really one
skin with an outer and an inner layer. When a person burns himself so as
to make a blister, the outer skin, called the _cuticle_, is separated
from the inner by a quantity of water or serum poured out from the
blood. This causes the blister to rise above the surrounding skin. If
you puncture the blister the water runs out. Now we may easily remove
the cuticle and examine it. The cuticle, we shall find, looks very much
like the skin which lines the inside of an egg-shell, and it is almost
as thin.
~4.~ The cuticle is very thin in most parts of the body, but in some
places, as the palms of the hands and the soles of the feet, it is quite
thick. This is because these parts of the skin come in contact with
objects in such a way as to be liable to injury if not thus protected.
The cuticle has no blood-vessels and very few nerves. With a fine needle
and thread you can easily take a stitch in it without making it bleed or
causing any pain.
~5. The Pigment.~--The under side of the cuticle is colored by little
particles of pigment or coloring matter. The color of this pigment
differs in different races. In the negro, the color of the pigment is
black. In some races the pigment is brown. In white persons there is
very little pigment, and in some persons, called albinos, there is none
at all.
~6. The Inner or True Skin.~--The inner skin, like the inner bark of a
tree, is much thicker than the outer skin. It is much more important,
and for this reason is sometimes called the _true skin_. It contains
nerves and blood-vessels.
[Illustration: SKIN OF PALM OF HAND MAGNIFIED.]
~7. The Sweat Glands.~--If you look at the palm of the hand you will see
many coarse lines, and by looking much closer you will see that the palm
is completely covered with very fine ridges and furrows. Now, if you
examine these ridges with a magnifying-glass, you will find arranged
along each ridge a number of little dark spots. Each of these points is
the mouth of a very small tube. This is called a _sweat duct_. These
ducts run down through both the outer and inner layers of the skin. At
the under side of the true skin the end of the tube is rolled up in a
coil, as you can see by looking at the illustration on the following
page. The coiled parts of the tubes are called _sweat glands_, because
they separate from the blood the fluid which we call sweat or
perspiration.
~8. The Oil Glands.~--There are other little glands in the skin which
make fat or oil. The oil is poured out upon the skin to keep it soft and
smooth.
[Illustration: THE STRUCTURE OF THE SKIN.]
~9. The Hair.~--There are some curious little pockets in the skin. Out
of each of these pockets grows a hair. On some parts of the body the
hairs are coarse and long; on other parts they are fine and short.
~10.~ Many of the ducts leading from the oil glands open into the
pockets or pouches from which the hairs grow. The oil makes the hair
soft and glossy. Nature has thus provided an excellent means for oiling
the hair.
~11.~ The hair is chiefly useful as a protection. It is also an
ornament.
~12. The Nails.~--The nails of the fingers and the toes grow out of
little pockets in the skin just as the hairs do. Both the hair and the
nails are really parts of the outer skin, which is curiously changed and
hardened. The nails lie upon the surface of the true skin and grow from
the under side as well as from the little fold of skin at the root of
the nail. They are made to give firmness and protection to the ends of
the fingers and toes. The nails of the fingers are also useful in
picking up small objects and in many other ways.
~13. Uses of the Skin.~--The skin is useful in several ways:
(1) _It Removes Waste._--The sweat glands and ducts are constantly at
work removing from the blood particles which have been worn out and can
be of no further use. If we get very warm, or if we run or work very
hard, the skin becomes wet with sweat. In a little while, if we stop to
rest, the sweat is all gone. What becomes of it? You say it dries up,
which means that it has passed off into the air. Sweating is going on
all the time, but we do not sweat so much when we are quiet and are not
too warm, and so the sweat dries up as fast as it is produced, and we do
not see it. Nearly a quart of sweat escapes from the skin daily.
(2) _Breathing through the Skin._--We breathe to a slight extent through
the skin. There are some lower animals which breathe with their skins
altogether. A frog can breathe with its skin so well that it can live
for some time after its lungs have been removed. Breathing is an
important part of the work of the skin, and we should be careful, by
keeping it clean and healthy, to give it a good chance to breathe all
that it can.
(3) _The Skin Absorbs._--The skin absorbs many substances which come in
contact with it, and hence should be kept clean. If the foul substances
which are removed in the sweat are allowed to remain upon the skin, they
may be taken back into the system and thus do much harm.
(4) _The Skin has Feeling._--When anything touches the skin we know it
by the feeling. We can tell a great many things about objects by feeling
of them. If we happen to stick a pin into the skin we feel pain. We are
also able to tell the difference between things which are hot and those
which are cold. Thus the sense of feeling which the skin has is very
useful to us.
(5) _The Skin Protects the Body._--The skin is a natural clothing which
protects us much better than any other kind of clothing could. It is so
soft and pliable that it cannot hurt the most delicate part which it
covers, yet it is very strong and tough.
SUMMARY.
1. The skin is the covering of the body. It has two layers, the outer,
called the cuticle, and the inner, called the true skin.
2. A substance called pigment is found between the two skins. This gives
the skin its color.
3. The true skin has blood-vessels and nerves, but the cuticle has no
blood-vessels and very few nerves.
4. In the true skin are glands which produce sweat, and others which
make fat, or oil.
5. The nails are really a part of the skin. They are firm and hard, and
protect the ends of the fingers and the toes.
6. The hair grows from the true skin. The hair is made soft and glossy
by oil from the oil glands of the skin.
7. The skin is a very useful organ. It removes waste matters, it
breathes, it absorbs, it has feeling, and it protects the body.