It consists of three pocket-shaped
strips of connective tissue which hang loosely from the walls when there
is no pressure from above; but upon receiving pressure, the pockets fill
and project into the opening, closing it completely (Fig. 16). The _left
semilunar_ valve is around the opening of the left ventricle into the
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.
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