ogy of the circulatory organs._ 352. What do
the contraction and relaxation of the muscular walls of the heart
produce? How do the auricles and ventricles contract and dilate? 353.
What is said of the contraction and dilatation of the ventricles in
the heart? How is the right ventricle adapted to its function? How the
left? 354. What is the use of the valves in the heart? Those of the
aorta and pulmonary artery?

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355. The function of the different parts of the heart will be given,
by aid of fig. 74. The blood passes from the right auricle (3) into
the right ventricle, (5,) and the tricuspid valves (6) prevent its
reflux; from the right ventricle the blood is forced into the
pulmonary artery, (7,) through which it passes to the lungs. The
semilunar valves (9) prevent this circulating fluid returning to the
ventricle. The blood, while passing over the air-cells in the lungs,
in the minute divisions of the pulmonary artery, is changed from a
bluish color to a bright red. It is then returned to the left auricle
of the heart by the pulmonary veins, (11, 11.)

[Illustration: Fig. 74. 1, The descending vena cava, (vein.) 2, The
ascending vena cava, (vein.) 3, The right auricle. 4, The opening between
the right auricle and the right ventricle. 5, The right ventricle. 6, The
tricuspid valves. 7, The pulmonary artery. 8, 8, The branches of the
pulmonary artery that pass to the right and left lung. 9, The semilunar
valves of the pulmonary artery. 10, The septum between the two ventricles
of the heart. 11, 11, The pulmonary veins. 12, The left auricle. 13, The
opening between the left auricle and ventricle. 14, The left ventricle.
15, The mitral valves. 16, 16, The aorta. 17, The semilunar valves of the
aorta.]

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355. Describe the course of the blood from the right auricle in the
heart to the lungs.

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_Observation._ If the blood is not changed in the lungs, it will not
flow to the pulmonary veins. This phenomenon is seen in instances of
death from drowning, strangling, carbonic acid, &c. The same is true,
but in a less degree, of individuals whose apparel is tight, as well
as of those who breathe impure air, or have diseased lungs.

356. The left auricle, (12,) by its contraction, forces the blood into
the left ventricle, (14.) The mitral valves (15) prevent its
reflowing. From the left ventricle the blood is forced into the aorta,
(16,) th