of the greenhouse tightly, set fire to it.
The smoke rises and fills the whole house. In less than an hour it has
killed many of the bugs and beetles which were destroying the plants.

A person not used to tobacco will sometimes be made sick by sitting
only an hour in a room where persons are smoking. It is wrong for
smokers to poison the air which others must breathe. For this reason a
smoking room should be well ventilated.

CHAPTER XVI

THE BLOOD AND HOW IT FLOWS THROUGH THE BODY

[Illustration: FIG. 68.--The cells in the blood. The two white ones
were drawn while crawling. Much enlarged.]

=The Blood keeps the Body Clean within and gives it Food.=--Every tiny
particle of the body, whether in the legs, arms, or head, must have
food to keep it alive and help it do its work. It must also have
oxygen, and it must be washed clean of its waste matter. All this is
done by the streams of blood, which bathe every cell to bring it food
and oxygen and to wash away its waste.

=Parts of the Blood.=--Blood consists of a clear, watery part called
_plasma_ and many little bodies named _cells_. The liquid found in a
blister is the clear part of the blood. The cells which float in the
watery part are so little and so close together that more than a
million are in each drop of blood.

A few of the cells are white, but most of them are red, and it is
their color that makes the blood look red. Your body contains about
one gallon of blood. It is carried through the body in branching tubes
called _blood vessels_ (Fig. 70).

[Illustration: FIG. 69.--Photograph of the heart from in front with
the lungs pinned aside. One fourth natural size.]

=The Blood Vessels.=--There are four kinds of blood vessels. They are
the _heart_, the _arteries_, the _capillaries_, and the _veins_. The
heart lies in the chest between the lungs. It squeezes the blood into
the arteries. These carry the blood to all parts of the body. It then
runs into the capillaries, which are tiny tubes connecting the
arteries with the veins. The veins return the blood to the heart.

The blood flows so fast that it goes from the heart down to the toes
and back again in a half minute.

=The Heart or Pump of Life.=--When the heart stops we die, because the
blood can no longer flow to carry food and oxygen to the hungry
tissues. The heart is a sac with thick walls of muscle. It is shaped
like a strawberry and is about as large as your fist. Its cavity is
divid