488 CIRCULATION OF BLOOD AND LYMPH. 



base of the heart (second intercostal space) the first sound is heard 

 (6 to c-d) just at the time when the semilunar valves are opened 

 (/)> that is, at the beginning of the period of emptying according 

 to the classification given on p. 485. The first sound ceases long 

 before the ventricular contraction itself is over, a fact which 

 would seem to indicate that the muscular element in the first sound 

 is not a muscular sound, such as is given out by a contracting 

 skeletal muscle. The beginning of the second sound seems to mark 

 exactly the time of closure of the semilunar valves. The character 

 and the time relations of the murmurs that accompany or replace 

 the heart sounds form the interesting practical continuation of this 

 theme; but the subject is so large that the student must be referred 

 for this information to the works upon clinical methods. 



The Events That Occur during a Single Cardiac Cycle. 

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



Fig. 207. Schematic representation of the relation of the heart sounds to the ventric- 

 ular beat: C, The cardiogram; 1, to show the duration of the first heart sound; 2, the 

 duration of the second heart sound; S, the time record, each division corresponding to 

 0.02 sec. In 1, a-a' marks the instant that the first heart sound is heard over the apex, 

 and b-b' the moment that it is heard at the second intercostal space. (Einthoven and 

 Geluk.) 



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 auriculo-ventricular valves are still closed, 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 

 ventricular relaxation is complete the pressure of blood in the 

 auricles opens the auriculo-ventricular valves, and from that moment 

 until the beginning of the auricular systole the blood from the large 

 veins is filling both ventricles and auricles. The ventricles become 

 more tense and the auriculo-ventricular valves are floated into 

 position ready for closure. The auricular systole sends a sudden 

 wave of blood into the ventricles, dilating them still further and 



