inches broad, and two inches thick. A man's heart usually weighs about eleven ounces and that of a woman nine ounces. It never leaks except from disease and such leakage is fatal.
The Cavities.—The heart contains four cavities, two auricles above and two ventricles below, with a longitudinal septum between the auricle and ventricle on the right and those on the left. The posterior surface is largely made up of the left ventricle and the anterior of the right ventricle. The right auricle, which receives the blood from the general circulation, has a capacity of about two fluid ounces and is larger than the left, which receives the blood returning from the lungs, though its walls are thinner. Of the ventricles the left is the larger and its walls are about three times as thick as those of the right, for it has to send the blood all over the body. All the cavities are lined with smooth, transparent, serous membrane, the endocardium, which is continuous with the intima of the great vessels.
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Fig. 41.—Orifices of the heart, seen from above, both the auricles and the great vessels being removed: PA, Pulmonary artery and its semilunar valves; Ao, aorta and its valves; RAV, tricuspid, and LAV, bicuspid valves; mv, segments of mitral valve; lv, segment of tricuspid valve. (Huxley.)
The Valves.—The opening from the auricle into the ventricle on either side is guarded on the ventral side by a valve formed of folds of endocardium. The valve on the right side has three flaps or cusps and is called the
