556 CIRCULATION OF BLOOD AND LYMPH. 



at about the time of the shoulder on the diastolic limb of the 

 volume curve, as is indicated in the diagram in Fig. 232. 



The Events that Occur During a Single Cardiac Cycle. 

 By a complete cardiac cycle is meant the time from any given 

 feature of the heart beat until that feature is again produced. 

 It may be helpful to summarize the events in such a cycle, both 

 as regards the heart and as regards the blood contained in it. 

 We may begin with the closure of the semilunar valves. At 

 that moment the second heart sound is heard and at that 

 moment the ventricle is quickly relaxing from its previous 

 contraction. Since the auriculoventricular valves are still 

 closed (see diagram, Fig. 219), the ventricles for a brief interval 

 are shut off on both sides. The blood is flowing steadily into 

 the auricles and dilating them. As soon as the ventricles relax 

 the pressure of blood in the auricles opens the auriculoven- 

 tricular valves, and from that moment until the beginning of 

 the auricular systole the blood from the large veins is filling 

 both ventricles and auricles. As stated on p. 551, the venous 

 blood which has been accumulating in the auricles during the 

 ventricular systole flows into the ventricles with some sudden- 

 ness on the opening of the auriculoventricular valves. The 

 ventricles, therefore, dilate rapidly and the auriculoventricular 

 valves are floated into a position ready for closure. This event 

 occurs at about the time that the third heart sound is heard. 

 In a slowly beating heart there may be quite an interval (period 

 of diastasis) between this point and the auricular contraction. 

 The auricular systole sends a sudden wave of blood into the 

 ventricles, dilating them still further and momentarily blocking 

 or retarding the flow from the large veins, and causing one of 

 the waves seen in the normal venous pulse as recorded in the 

 jugular veins. The ventricular systole follows at once upon 

 the auricular systole, the exact relations in this case depend- 

 ing somewhat upon the pulse rate. As the ventricle enters 

 into contraction the auriculo-ventricular valves are tightly closed, 

 the first sound is heard, and for a short interval the ventricular 

 cavity is again shut off on both sides. Soon the rising pressure in 

 the interior forces open the semilunar valves, and then a column 

 of blood is discharged into the aorta and pulmonary artery as long 

 as the contraction lasts. During this interval the flow at the 

 venous end of the heart continues, the blood being received into 

 the yielding auricles. Indeed, this capacity for receiving the 

 venous inflow during the comparatively long-lasting ventricular 

 systole may be considered as one valuable mechanical function 

 fulfilled by the auricles. The venous flow is never completely 

 blocked and at the most suffers only a slight retardation during 



