ed into four parts. The two upper ones are called _auricles_ and
the lower ones are named _ventricles_. The blood enters the auricles
and then pours through an opening into each ventricle, from which it
passes out into the arteries.

=The Arteries or Sending Tubes.=--The blood is sent out from the heart
through the arteries leading to all parts of the body. The chief
artery is the _aorta_. It is larger than your thumb and extends from
the heart down through the body in front of the backbone. It has more
than twenty branches. All of these branch again and again like the
limbs of a tree until they are finer than hairs.

A large tube, the _lung artery_, takes blood directly from the heart
to the lungs. Here it branches into more than a thousand divisions, so
that the blood can take in oxygen and give off to the lungs its waste.

[Illustration: FIG. 70.--Arteries, the tubes carrying the blood from
the heart through the body. Only the chief vessels are shown on one
side.]

=The Capillaries or Feeding Tubes.=--These are the tiny tubes, finer
than hairs, which join the smallest end branches of the arteries with
the beginnings of the little veins. They are so thickly scattered in
the flesh that you cannot stick it with a pin without piercing one.

They are called feeding tubes because they have such very thin walls
that the food in the blood and the oxygen brought from the lungs can
pass through to feed the muscles and other organs. The dead parts of
the body and also the ashes of the food used up, pass from the organs
into the capillaries.

=The Veins or Returning Tubes.=--The veins, beginning in fine branches
formed by the capillaries, return the blood to the heart. The branches
unite into larger and larger vessels and finally flow into one main
vein, the _vena cava_. This extends along in front of the backbone and
opens into the heart.

=Why the Blood flows in only one Direction.=--The heart causes the
flow of the blood. It does this by squeezing together its walls so as
to make the blood go out into the arteries. When once in the arteries,
the blood must go forward because there are little doors at the mouths
of the arteries in the heart. These doors, called _valves_, open in
only one direction, so that the blood cannot flow backward (Fig. 71).
There are other valves between the upper and lower cavities of the
heart, preventing the blood from being pushed back into the veins.

[Illustration: FIG. 71.--The heart wit