CIRCULATION OF THE BLOOD. 133 



The Cardiac Cycle. The entire period of the heart's pulsation 

 may be divided into three stages, viz. : 



1. The auricular contraction and relaxation. 



2. The ventricular contraction and relaxation. 



3. The pause or period of repose during which both auricles and ven- 

 tricles are at rest. These three stages constitute collectively a 

 cardiac cycle or a cardiac revolution. 



The duration of a cycle, as well as the duration of its three stages, 

 varies in different animals in accordance with the number of cycles 

 which recur in a minute. In human beings in adult life there are 

 about 72 cycles to the minute ; the average duration is, therefore, 

 0.80 sec. From this it follows that the time occupied by any one of 

 the three stages must be extremely short and difficult of determina- 

 tion. From experiments on animals and from observations 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 auriculoventricular 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 relaxed 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 



