des the heart into two
similar portions, named from their positions the right side and the left
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. Chordae tendineae. 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
_tricuspid_ valve. It is suspended from a thin ring of connective tissue
which surrounds the opening, and its free margins extend into the
ventricle (Fig. 16). It consists of three parts, as its name implies,
which are thrown together in closing the opening. Joined to the free edges
of this valve are many small, tendinous cords which connect at their lower
ends with muscular pillars in the walls of the ventricle. These are known
as the _chordae tendineae_, or heart tendons. Their purpose is to serve as
_valve stops_, to prevent the valve from being thrown, by the force of the
blood stream, back into the auricle.

The _mitral_, or bicuspid, valve is suspended around the opening between
the left auricle and the left ventricle, with the free