*Work of Auricles and Ventricles Compared.*—In the work of the heart the
two auricles contract at the same time—their contraction being followed
immediately by the contraction of both ventricles. After the contraction
of the ventricles comes a period of rest, or relaxation, about equal in
time to the period of contraction of both the auricles and the
ventricles.(19) On account of the work which they perform, the auricles
have been called the "feed pumps" of the heart; and the ventricles, the
"force pumps."(20) It is the function of the auricles to collect the blood
from the veins, to let this run slowly into the ventricles when both the
auricles and ventricles are relaxed, and finally, by contracting, _to
force an excess of blood into the ventricles_, thereby distending their
walls. The ventricles, having in this way been fully charged by the
auricles, now contract and force their contents into the large arteries.

*Sounds of the Heart.*—Two distinct sounds are given out by the heart as
it pumps the blood. One of them is a dull and rather heavy sound, while
the other is a short, sharp sound. The short sound follows quickly after
the dull sound and the two are fairly imitated by the words "lūbb, dŭp."
While the cause of the first sound is not fully understood, most
authorities believe it to be due to the contraction of the heart muscle
and the sudden tension on the valve flaps. The second sound is due to the
closing of the semilunar valves. These sounds are easily heard by placing
an ear against the chest wall. They are of great value to the physician in
determining the condition of the heart.

*Arteries and Veins.*—These form two systems of tubes which reach from the
heart to all parts of the body. The arteries receive blood from the heart
and distribute it to the capillaries. The veins receive the blood from the
capillaries and return it to the heart. The arteries and veins are similar
in structure, both having the form of tubes and both having three distinct