120 HUMAN PHYSIOLOGY. 



and difficult of determination. From experiments on animals and from obser- 

 vations made on human beings, the following estimates have been made and 

 accepted as approximately correct for human beings: 



1. The auricular systole 0.16 sec.; the auricular diastole, 0.64 sec. 



2. The ventricular systole 0.32 sec.; the ventricular diastole, 0.48 sec. 



3. The period of rest for both auricles and ventricles 0.32 sec. 



The Action of the Valves. The forward movement of the blood is 

 permitted and regurgitation prevented by the alternate action of the auriculo- 

 ventricular and semilunar valves. As a point of departure for a consideration 

 of the action of these valves and their relation to the systole and diastole of 

 the heart, the close of the ventricular systole may be selected. At this moment, 

 the semilunar valves, which during the systole, are directed outward by the 

 blood current are now suddenly and completely closed by the pressure of 

 the blood in the aorta and pulmonary artery. Regurgitation into the 

 ventricles is thus prevented. 



During the ventricular systole, the relaxed auricles are filling with blood. 

 With the ventricular diastole this blood or its equivalent flows into the re- 

 laxed and easily distensible ventricles until both auricles and ventricles are 

 nearly filled. The tricuspid and mitral valves which are hanging 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 suddenly 

 contract, forcing their contained volumes 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 the tricuspid and mitral 

 valves must be instantly and completely closed. This is accomplished by 

 the upward pressure of the blood which brings their free edges in close 

 apposition. Reversal in the position of these valves is prevented by the con- 

 traction of the papillary muscles which exert a traction on their under sur- 

 faces and edges and hold them steady. 



The blood now confined in the ventricles between the closed auriculo- 

 ventricular and semilunar valves is subjected to pressure on all sides; as the 

 pressure rises proportionately to the vigor of the contraction there comes a 

 moment when the infra-ventricular pressure exceeds that in the aorta and 

 pulmonary artery; at once the semilunar valves are thrown open and the blood 

 discharged. Both contraction and outflow continue until the ventricles are 

 practically empty, when relaxation sets in attended by a rapid fall of pressure, 

 under the influence of the positive pressure of the blood in the aorta and 

 pulmonary artery, the semilunar valves are again closed. The accumu- 

 lation of blood in the auricles, attended by a rise in pressure, again forces 



