o the other, toward the heart. When they are
relaxed, the veins refill, and are compressed by the recurring action
of the muscles.

_Observation._ The physician, in opening a vein, relies on the
energetic contractions and sudden relaxations of the muscles, when he
directs the patient to clasp the head of a cane, or the arm of a
chair; these alternate motions of the muscles cause an increased flow
of blood to the veins of the ligated arm.

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363. What is said of the influences that return the blood to the
heart? What is said of the propulsive power of the capillaries? Of the
suction power of the heart? 364. Give another influence. State another
agency. 365. What is one of the most powerful causes which influence
venous circulation? Give practical observation.

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[Illustration: Fig. 75. An ideal view of the circulation in the lungs and
system. From the right ventricle of the heart, (2,) the dark, impure
blood is forced into the pulmonary artery, (3,) and its branches (4, 5)
carry the blood to the left and right lung. In the capillary vessels (6,
6) of the lungs, the blood becomes pure, or of a red color, and is
returned to the left auricle of the heart, (9,) by the veins, (7, 8.)
From the left auricle the pure blood passes into the left ventricle,
(10.) By a forcible contraction of the left ventricle of the heart, the
blood is thrown into the aorta, (11.) Its branches (12, 13, 13) carry the
pure blood to every organ or part of the body. The divisions and
subdivisions of the aorta terminate in capillary vessels, represented by
14, 14. In these hair-like vessels the blood becomes dark colored, and is
returned to the right auricle of the heart (1) by the vena cava
descendens, (15,) and vena cava ascendens, (16.) The tricuspid valves
(17) prevent the reflow of the blood from the right ventricle to the
right auricle. The semilunar valves (18) prevent the blood passing from
the pulmonary artery to the right ventricle. The mitral valves (19)
prevent the reflow of blood from the left ventricle to the left auricle.
The semilunar valves (20) prevent the reflow of blood from the aorta to
the left ventricle.]

366. The muscles exercise an agency in maintaining the venous
circulation at a point above what the heart could perform. As the
pulsations are diminished by rest, so they are accelerated by
exercise, and very much quickened by violent effort. There can be
little doubt