THE CIRCULATION OF THE BLOOD. 275 



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 pulmonary 

 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 ca\ae and pulmonary 

 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 completely 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 

 from all sides. As the pressure rises proportionately to the vigor of the con- 

 traction, there comes a moment when the intra-ventricular pressure exceeds 

 that in the aorta and pulmonary artery. As soon as this occurs the semi- 

 lunar valves of both vessels are thrown open and the blood discharged. The 

 discharge of the blood by the contraction of the ventricular walls is probably 

 aided by the simultaneous downward displacement of the more central 

 portion of the auriculo-ventricular septum, due to the contraction of the 

 papillary muscles. Both contraction and outflow continue until the ven- 

 tricles are practically empty, after which ventricular relaxation sets in, 

 attended by a rapid fall of pressure. Under the influence of the positive 

 pressure of the blood in the sinuses of Valsalva the semilunar valves are 

 again closed, the column of blood supported, and regurgitation is prevented. 

 In the meantime and while the ventricles are contracting, blood is again 

 flowing into, and accumulating in the auricles and thereby distending them 

 preparatory to the next systole. With the accumulation of blood in the 

 auricles the cardiac cycle is completed. 



The approximate changes in the shape of the heart, the variations in the 



