correspond to the contractions of the heart.

No other single discovery with reference to the human body has proved of
such great importance. A knowledge of the nature and purpose of the
circulation was the necessary first step in understanding the plan of the
body and the method of maintaining life, and physiology as a science dates
from the time of Harvey’s discovery.

*Organs of Circulation.*—The organs of circulation, or blood vessels, are
of four kinds, named the heart, the arteries, the capillaries, and the
veins. They serve as contrivances both for holding the blood and for
keeping it in motion through the body. The heart, which is the chief organ
for propelling the blood, acts as a force pump, while the arteries and
veins serve as tubes for conveying the blood from place to place.
Moreover, the blood vessels are so connected that the blood moves through
them in a regular order, performing two well-defined circuits.

[Fig. 13]


Fig. 13—*Heart* in position in thoracic cavity. Dotted lines show positin
of diaphragm and of margins of lungs.


*The Heart.*—The human heart, roughly speaking, is about the size of the
clenched fist of the individual owner. It is situated very near the center
of the thoracic cavity and is almost completely surrounded by the lungs.
It is cone-shaped and is so suspended that the small end hangs downward,
forward, and a little to the left. When from excitement, or other cause,
one becomes conscious of the movements of the heart, these appear to be in