valves which open like folding doors between the two
rooms. If these doors are closed there are two separate rooms, but if open
there is practically only one room. The heart thus has four chambers, two
on each side. The two upper chambers are called auricles from their
supposed resemblance to the ear. The two lower chambers are called
ventricles, and their walls form the chief portion of the muscular
substance of the organ. There are, therefore, the right and left auricles,
with their thin, soft walls, and the right and left ventricles, with their
thick and strong walls.

185. The Valves of the Heart. The heart is a valvular pump, which
works on mechanical principles, the motive power being supplied by the
contraction of its muscular fibers. Regarding the heart as a pump, its
valves assume great importance. They consist of thin, but strong,
triangular folds of tough membrane which hang down from the edges of the
passages into the ventricles. They may be compared to swinging curtains
which, by opening only one way, allow the blood to flow from the auricles
to the ventricles, but by instantly folding back prevent its return.

[Illustration: Fig. 70.--Lateral Section of the Right Chest. (Showing the
relative position of the heart and its great vessels, the oesophagus
and trachea.)

A, inferior constrictor muscle (aids in conveying food down the
oesophagus);
B, oesophagus;
C, section of the right bronchus;
D, two right pulmonary veins;
E, great azygos vein crossing oesophagus and right bronchus to empty
into the superior vena cava;
F, thoracic duct;
H, thoracic aorta;
K, lower portion of oesophagus passing through the diaphragm;
L, diaphragm as it appears in sectional view, enveloping the heart;
M, inferior vena cava passing through diaphragm and emptying into
auricle;
N, right auricle;
O, section of right branch of the pulmonary artery;
P, aorta;
R, superior vena cava;
S, trachea.
]

The valve on the right side is called the tricuspid, because it
consists of three little folds which fall over the opening and close it,
being kept from falling too far by a number of slender threads called
chordae tendinae. The valve on the left side, called the mitral,
from its fancied resemblance to a bishop's mitre, consists of two folds
which close together as do those of the tricuspid valve.

The slender cords which regulate the valves are only just long enough to
allow the fold