Heart.--The heart may be called a live pump, which keeps
pumping away during our whole lives. If it should stop, even for a
minute or two, we would die. If you will place your hand over your heart
and count the beats for exactly one minute, you will find that it beats
about seventy-five or eighty times. When you are older, your heart will
beat a little more slowly. If you count the beats while you are lying
down, you will find that the heart beats more slowly than when you are
sitting or standing. When we run or jump, the heart beats much harder
and faster.

~3. Why the Heart Beats.~--We have learned in preceding lessons that the
digested food is taken into the blood. We have also learned that both
water and oxygen are taken into the blood. Thus the blood contains all
the materials that are needed by the various parts of the body to make
good the wastes that are constantly taking place. But if the blood were
all in one place it could do little good, as the new materials are
needed in every part of the body. There has been provided a wonderful
system of tubes running through every part of the body. By means of
these tubes the blood is carried into every part where it is required.
These tubes are connected with the heart. When the heart beats, it
forces the blood through the tubes just as water is forced through a
pipe by a pump or by a fire-engine.

~4. The Heart Chambers.~--The heart has four chambers, two upper and
two lower chambers. The blood is received into the upper chambers, and
is then passed down into the lower chambers. From the lower chambers it
is sent out to various parts of the body.

[Illustration: THE INSIDE OF THE HEART.]

~5. The Blood-Vessels.~--The tubes through which the blood is carried
are called _blood-vessels_. There are three kinds of blood-vessels. One
set carry the blood away from the heart, and are called _arteries_
(ar'-te-ries). Another set return the blood to the heart, and are called
_veins_. The arteries and veins are connected at the ends farthest from
the heart by many very small vessels. These minute, hairlike vessels are
called _capillaries_ (cap'-il-la-ries).

~6. The Arteries.~--An artery leads out from the lower chamber of each
side of the heart. The one from the right side of the heart carries the
blood only to the lungs. The one from the left side of the heart carries
blood to every part of the body. It is the largest artery in the body,
and is called the _aorta_. Soon af