aorta, and is similar in all respects to the right semilunar valve.

*Differences in the Parts of the Heart.*—Marked differences are found in
the walls surrounding the different cavities of the heart. The walls of
the ventricles are much thicker and stronger than those of the auricles,
while the walls of the left ventricle are two or three times thicker than
those of the right. A less marked but similar difference exists between
the auricles and also between the valves on the two sides of the heart.
These differences in structure are all accounted for by the work done by
the different portions of the heart. The greater the work, the heavier the
structures that perform the work.

[Fig. 17]


Fig. 17—*Diagram of the circulation*, showing in general the work done by
each part of the heart. The right ventricle forces the blood through the
lungs and into the left auricle. The left ventricle forces blood through
all parts of the body and back to the auricle. The auricles force blood
into the ventricles.


*Connection with Arteries and Veins.*—Though the heart is in communication
with all parts of the circulatory system, it makes actual connection with
only a few of the blood tubes. These enter the heart at its upper portion
(Fig. 15), but connect with its different cavities as follows:

1. _With the right auricle_, the superior and the inferior venæ cavæ and
the coronary veins. The superior vena cava receives blood from the head
and the upper extremities; the inferior vena cava, from the trunk and the