s to close together, and no force of the blood pushing
against the valves can send them farther back, as the cords will not
stretch The harder the blood in the ventricles pushes back against the
valves, the tighter the cords become and the closer the folds are brought
together, until the way is completely closed.

From the right ventricle a large vessel called the pulmonary artery
passes to the lungs, and from the left ventricle a large vessel called the
aorta arches out to the general circulation of the body. The openings
from the ventricles into these vessels are guarded by the semilunar
valves. Each valve has three folds, each half-moon-shaped, hence the
name semilunar. These valves, when shut, prevent any backward flow of the
blood on the right side between the pulmonary artery and the right
ventricle, and on the left side between the aorta and the left ventricle.

[Illustration: Fig. 71.--Right Cavities of the Heart.

A, aorta;
B, superior vena cava;
C, C, right pulmonary veins;
D, inferior vena cava;
E, section of coronary vein;
F, right ventricular cavity;
H, posterior curtain of the tricuspid valve;
K, right auricular cavity;
M, fossa ovalis, oval depression, partition between the auricles formed
after birth.
]

186. General Plan of the Blood-vessels Connected with the Heart.
There are numerous blood-vessels connected with the heart, the relative
position and the use of which must be understood. The two largest veins in
the body, the superior vena cava and the inferior vena cava,
open into the right auricle. These two veins bring venous blood from all
parts of the body, and pour it into the right auricle, whence it passes
into the right ventricle.

From the right ventricle arises one large vessel, the pulmonary
artery, which soon divides into two branches of nearly equal size, one
for the right lung, the other for the left. Each branch, having reached
its lung, divides and subdivides again and again, until it ends in
hair-like capillaries, which form a very fine network in every part of the
lung. Thus the blood is pumped from the right ventricle into the pulmonary
artery and distributed throughout the two lungs (Figs. 86 and 88).

We will now turn to the left side of the heart, and notice the general
arrangement of its great vessels. Four veins, called the pulmonary
veins, open into the left auricle, two from each lung. These veins
start from very minute vessels the continuation