ACTION OF THE HEART. 73 



the contraction of the auricle, the contraction of the ventricle, 

 and the pause. 



During the pause the blood is steadily pouring into the 

 auricles; into the right auricle from the caval veins, into 

 the left auricle from the pulmonary veins. At this time the 

 curtain-like valves between the auricles and the ventricles are 

 open, and their flaps hang loosely beside the walls of the ven- 

 tricles. The blood, therefore, as it passes into the auricles, 

 passes on into the ventricles. As the ventricle fills, the valves 

 float up, as seen in the experiment of pouring water into the 

 ventricle. When the ventricle is full, but not stretched, and 

 the auricle partly full, the auricle suddenly contracts, thus 

 forcing more blood into the ventricle, and distending it. At 

 the same time the valves, which were already nearly closed, 

 are tightly closed by the pressure of the blood which is forced 

 up behind them. The flaps of the valves are kept from going 

 up too far by the chordce tendinece and by the papillary 

 muscles. 



Next comes the contraction of the ventricle, slower, but 

 more powerful, than that of the auricle. As the walls of the 

 ventricle are drawn together, the blood is subjected to pres- 

 sure. It cannot go back into the auricles, for the more it 

 presses against the valves, the more tightly they are closed. 

 The semilunar valves are closed by back pressure in the aorta 

 and pulmonary artery. But the pressure of the blood in the 

 ventricles is so much greater that the semilunar valves are 

 forced open, and nearly all the blood is driven out of the ven- 

 tricles ; from the right ventricle into the pulmonary artery, 

 and from the left ventricle into the aorta. 



While the ventricles are contracting and forcing their blood 

 out, the auricles are slowly filling by the steady inflow through 

 the veins. 



As soon as the ventricle has completed its contraction, it 



