lower extremities; and the coronary veins, from the heart itself.

2. _With the left auricle_, the four pulmonary veins. These receive blood
from the lungs and empty it into the left auricle.

3. _With the right ventricle_, the pulmonary artery. This receives blood
from the heart and by its branches distributes it to all parts of the
lungs.

4. _With the left ventricle_, the aorta. The aorta receives blood from the
heart and through its branches delivers it to all parts of the body.

*How the Heart does its Work.*—The heart is a muscular pump(18) and does
its work through the contracting and relaxing of its walls. During
contraction the cavities are closed and the blood is forced out of them.
During relaxation the cavities open and are refilled. The valves direct
the flow of the blood, being so arranged as to keep it moving always in
the same direction (Fig. 17).

The heart, however, is not a single or a simple pump, but consists in
reality of _four_ pumps which correspond to its different cavities. These
connect with each other and with the blood vessels over the body in such a
manner that each aids in the general movement of the blood.

[Fig. 18]


Fig. 18—Diagram illustrating the "cardiac cycle."