side. On each side are two cavities, the one being directly above the
other. The upper cavities are called _auricles_ and the lower ones
_ventricles_. To distinguish these cavities further, they are named from
their positions the right auricle and the left auricle, and the right
ventricle and the left ventricle (Fig. 15). The auricles on each side
communicate with the ventricles below; but after birth there is no
communication between the cavities on the opposite sides of the heart. All
the cavities of the heart are lined with a smooth, delicate membrane,
called the _endocardium_.

[Fig. 15]


Fig. 15—*Diagram showing plan of the heart.* 1. Semilunar valves. 2.
Tricuspid valve. 3. Mitral valve. 4. Right auricle. 5. Left auricle. 6.
Right ventricle. 7. Left ventricle. 8. Chordæ tendineæ. 9. Inferior vena
cava. 10. Superior vena cava. 11. Pulmonary artery. 12. Aorta. 13.
Pulmonary veins.


*Valves of the Heart.*—Located at suitable places in the heart are four
gate-like contrivances, called valves. The purpose of these is _to give
the blood a definite direction_ in its movements. They consist of tough,
inelastic sheets of connective tissue, and are so placed that pressure on
one side causes them to come together and shut up the passageway, while
pressure on the opposite side causes them to open. A valve is found at the
opening of each auricle into the ventricle, and at the opening of each
ventricle into the artery with which it is connected.

The valve between the right auricle and the right ventricle is called the