Elements of animal physiology, chiefly human . transverse constrictions, each of which is strengthenedby a fibrous ring, termed the zona annularis: these formthe auriculo-ventricular openings, to the edges of whichthe valves of the heart, the mitral and the tricuspidvalves, v/hich close or open these cavities, are attached. 185. The lualls of the auricles are comparatively thin;the passage of the blood into the ventricles upon the j^iopulsive power exerted by the walls ofthe auricles, than upon the suction consequent upon thedilatation of the cavities of the ventricles. The righ
Elements of animal physiology, chiefly human . transverse constrictions, each of which is strengthenedby a fibrous ring, termed the zona annularis: these formthe auriculo-ventricular openings, to the edges of whichthe valves of the heart, the mitral and the tricuspidvalves, v/hich close or open these cavities, are attached. 185. The lualls of the auricles are comparatively thin;the passage of the blood into the ventricles upon the j^iopulsive power exerted by the walls ofthe auricles, than upon the suction consequent upon thedilatation of the cavities of the ventricles. The right auricle communicates directly with the 52^-perior and inferior venoe cav^; the inferior vena cavaonly being protected by a valve—the Eustachian orifice by which it communicates with the right ven-tricle is guarded by the tricuspid valve. (See fig. 43.) The left auricle communicates directly with the four THE VENTRICLES. 91 %>ulmonary veins which return purified blood from Ihc rdght SubclavianAlter J. Superior .?\BUaCava. -^. Outside of Apex of Ventricio. rig. 43. Eight Side of Heart,lungs. Its lower opening, by Avhich it communicateswith the left ventricle, is guarded by the mitral valve. 186. The Ventricles (from Lat. venter, stomach)are the two lower and thicker walled cavities of theheart. They are separated from each other by theventricular portion of the median septum of the walls of the left ventricle are much thicker thanthose of the right. Each ventricle is capable of contain-ing four to five ounces of blood. The ventricles are alittle laro;er than the auricles. 92 ANIMAL tllYSiOLOGlY. The pulmonary artery, the lower end of which isprotected by semi-lunar valves, opens into the rightventricle. When the right ventricle contracts, thetricuspid valve closes, and the venous blood is drivenout of the ventricle through the pulmonary artery intothe lungs, to be oxidized. 187. The left ventricle opens into the aorta, entranceof which is protected
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