THE CIRCULATION OF THE BLOOD. 277 



moment, the tricuspid and mitral valves must be suddenly and com- 

 pletely closed. This is readily accomplished by reason of the position 

 of the valves, which have been floated up and placed almost in apposi- 

 tion 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 is prevented by the contraction and shortening of the papil- 

 lary muscles, which in consequence exert a traction on their under 

 surfaces. 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 contraction, there comes a moment when the intra- 

 ventricular pressure exceeds that in the aorta and pulmonary artery. 

 Immediately the semilunar valves of both vessels are thrown open and 

 the blood discharged. Both contraction and outflow continue until 

 the ventricles are practically empty, after which ventricular relaxa- 

 tion 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. With the accumulation of blood in 

 the auricles the cardiac cycle is completed. 



Relative Functions of Auricles and Ventricles. Though 

 both auricles and ventricles are essential to the continuous movement 

 of blood, they possess unequal values in this respect. The passage 

 of the blood through the pulmonary and systemic vessels is accom- 

 plished by the driving power of the right and left ventricles respec- 

 tively, aided, however, by minor extra-cardiac forces. They may 

 bejggarded therefore as force-pumps. 



It the heart consisted of ventricles only, the flow of blood from 

 the venae cavae and pulmonary veins would be temporarily arrested 

 during their systole and their subsequent refilling delayed. This is 

 obviated, however, by the addition of the auricles; for during the 

 ventricular systole the blood continues to flow into the auricles, in 

 which it is temporarily stored until the ventricular relaxation sets in. 

 With this event the accumulated blood passes into the ventricles, which 

 are thus practically filled before the auricular systole occurs by which 

 the filling is completed. By this means there is no delay in the filling 

 of the ventricles, and hence their effective working as force-pumps 

 is more readily secured. The auricles may therefore be regarded 

 as jeed-pwmpSk For this reason it~isT probable, notwithstanding the 

 cc^ntractio"ri T of the circular muscle-fibers at the terminations of the 

 venous system, the flow of blood into the auricles is never entirely 

 arrested. Regurgitation in these vessels does not occur for the reason 

 that the pressure in the auricles is not higher than, if as high as, in 

 the great veins. 



