46 PHYSIOLOGY. 



cavity is much reduced in size, if not entirely obliterated, 

 and the blood is forced out. 



The complete action of the heart consists of three parts, 

 the contraction of the auricles, the contraction of the 

 ventricles, and the pause. 



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 ventricles. 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. 



The Contraction of the Auricle. 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 tendinous cords and by the papillary muscles to 

 which the cords are attached. 



The Contraction of the Ventricle. 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 pressure. 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 



