. THE CIRCULATION OF THE BLOOD 279 



however, the auricular diastole comes to an end, the auricular walls again 

 contract and force some of their contained blood into the ventricles and 

 thus rapidly complete the filling. The ventricular systole immediately 

 follows, during which the blood is driven into the pulmonic artery and 

 aorta. This having been accomplished, the ventricles relax, and the blood 

 that has been accumulating in the auricles begins to flow into the ventri- 

 cles, after which the same series of events follows as in the previous cycle. 



The Action of the Valves During the Cycle. As previously stated, 

 the forward movement of the blood is permitted and regurgitation prevented 

 by the alternate action of the semilunar and the auriculo-ventricular valves. 

 As a point of departure for a consideration of the action of the valves and 

 their relation to the systole and diastole of the heart, the close of the ventricu- 

 lar systole may be conveniently selected. 



At this moment, if the blood is not to be returned to the ventricles, the 

 semilunar valves must be instantly and completely closed. This is accom- 

 plished in the following manner: During the outflow of blood from the 

 ventricles the valves are pushed outward toward the walls of the vessels, 

 though not coming into contact with them, for behind them are the pouches 

 of Valsalva, containing blood, continuous with and under the same pressure 

 as that in the vessels themselves. With the cessation of the outflow and the 

 beginning of the relaxation the pressure of the blood behind the valves 

 suddenly forces them inward until their free edges, including the lunulae, 

 come into complete apposition. By this means the orifices of the pulmonic 

 artery and aorta are securely closed and a return flow prevented. Reversal 

 of the valves is prevented by their mode of attachment to the fibrous rings of 

 the orifices. 



During the ventricular systole the relaxed auricles have been filling with 

 blood. With the ventricular relaxation this volume, or its equivalent, flows 

 readily into the empty and easily distensible ventricles, its place being taken 

 by an additional volume of blood flowing from the venae cavae and pulmonic 

 veins. Whether the ventricles exert a suction power at the moment of 

 their relaxation is an undecided question. A steady stream of blood into 

 the auricles and ventricles continues throughout the entire period of rest until 

 both cavities are filled. The tricuspid and bicuspid valves which hang down 

 into the ventricular cavities are now floated up by currents of blood welling 

 up behind them until they are nearly closed. The auricles now contract, 

 forcing their contained volumes, or at least the larger portions of them, into 

 the ventricles, which become fully distended. 



With the cessation of the auricular systole the ventricular systole begins. 

 If the blood is not to be returned to the auricles at this moment, the tricuspid 

 and mitral valves must be suddenly and accurately closed. This is readily 

 accomplished by reason of the position of the valves, which have been 

 floated up and placed almost in apposition by the blood itself. With the 

 beginning of the ventricular pressure the blood is forced upward against the 

 valves until their free edges are brought together and the orifices closed. 

 Reversal of these valves into the auricles is prevented by their attachment to 

 the chorda tendinece, and the latter are kept from moving bodily upward 

 during the ventricular contraction by the compensatory downward pull of 

 the papillary muscles. The blood now confined in the ventricle between the 

 closed auriculo-ventricular and semilunar valves is subjected to pressure 



