h the front part cut away to
show the four chambers and valves. The arrows show the direction in
which the blood flows.]

The movement of the walls of the heart in and out is called the _heart
beat_. This can be plainly felt by placing the hand on the left side
of the chest. The heart beats about seventy times each minute in grown
persons, but much oftener in children. At each beat a wave of blood
flows along the arteries. This is known as the _pulse_. It may be felt
at the base of the thumb, where an artery runs just under the skin.

=Why the Heart sometimes beats Faster.=--When we run or do hard work,
the heart may beat twice as fast as when we are lying down. This is
because the muscles need more oxygen to help them act. Work makes them
get hungry, and they send word by the nerves to the heart to hurry
along the blood to bring more oxygen from the lungs.

When germs make the body sick, the heart often beats faster because it
is affected by the poison made by the germs. The doctor then feels the
pulse to tell how much the body is poisoned.

=Use of Blood Cells.=--The red cells act like boats. They load up with
oxygen in the lungs and carry it to all parts of the body. Here they
trade it off for carbon dioxide, a waste substance. This they carry
back to the lungs to be cast out of the body.

There is one white blood cell to every four hundred red ones. The
white cells are the body-guards. They change their shape and are able
to crawl through the walls of the capillaries. Wherever the body is
hurt, they collect in large numbers and eat the germs which are always
trying to get into the body through sores. The white matter called
_pus_ in a sore is largely made of white blood cells which came there
to fight the germs and were killed in the battle.

The germs of boils and fevers often get into the blood, but the white
cells usually kill them before they have a chance to grow into large
numbers and make the body sick.

=How to stop Bleeding.=--Most of the larger arteries are deep in the
flesh and seldom get cut. There are many veins just under the skin. If
the blood comes out in spurts, it is from an artery; but if it flows
steadily, it is from a vein. If the blood does not run out in a
stream, it will stop without any special care. As soon as the blood
gets to the air it forms a jellylike mass called a _clot_. This helps
stop the flow. All hurt places in the skin should be tied up in a
clean cloth.

[Illustration