les have taken what they want of it for food to burn,
and warm you up, the "ashes" and the "smoke" go back into the blood
and dirty its color from red to purple. Then the blood is carried to
the lungs, where the fresh air you breathe in blows away the "smoke"
and makes the blood red again.

The blood is pumped all over the body through tubes or pipes, called
_blood vessels_. Those that carry the red blood out from the heart, we
call _arteries_. They are deep down under the skin, and we cannot see
them. The pipes that carry the purple blood from the muscles and other
parts back to the heart again, we call _veins_; and some of these are
so close to the surface that we can easily see them through the skin.
Let your hand hang down a minute or two, then you can see the veins on
the inside of your wrist, or on the back of your hand, if it is not
too fat.

[Illustration: IT IS GOOD TO PLAY OUT OF DOORS TILL THE BELL
RINGS--EVEN IN WINTER]

The muscles, the brain, the skin, and other parts of the body get
liquid food from the blood by "sucking" it through the walls of the
smallest of the blood vessels, for these walls are very thin. In the
same way, when waste passes from the muscles or the skin into the
blood, it, too, soaks through the thin walls of the tiniest blood
tubes, called _capillaries_.

Your heart beats or throbs about seventy-five times in a minute when
you are well. Look at the second hand of a watch, while you count the
beats in your wrist or in your neck.

Does your heart ever become tired? Not while you keep well, unless you
over-drive it by running or wrestling too hard. It can rest between
the beats. But the heart muscle, like any other muscle, must have
plenty of good red blood to feed on. You put food into the blood by
eating good breakfasts and dinners. The more you run and jump and
play, the more work the heart has to do and the stronger it grows; and
a good morning romp before school will send the blood flowing so
merrily round from top to toe that you will feel fresher and brighter
all the day.

III. FRESH AIR--WHY WE NEED IT

The heart is not the only thing that goes faster and harder when you
run about in the morning and play hard. You are breathing faster and
deeper as well, as if there were something in the air outside that you
needed in your body as much as food.

But, of course, you know that air is not good to eat. It has no
strength in it, as food has; it isn't even a liquid