into the auricles, while the semilunar valves open to let the blood out. The cardiac cycle is, therefore, as follows:
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Fig. 42.—Diagram of the circulation. (After Kirke.)
Circulation.—The blood, after it has given off its oxygen and collected carbon dioxide, returns to the heart through two main channels,the superior and inferior venæ cavæ, the former bringing the blood from the upper part of the body, including the head, neck, and arms, and the latter from the lower part below the diaphragm. The two vessels empty along with the coronary sinus, which is guarded by the coronary valve, into the right auricle. At the same time that they empty into this auricle the four pulmonary veins, the only veins that carry arterial or oxygenated blood, are emptying the fresh blood from the lungs into the left auricle. When both auricles are full,
