THE HEA.RT BEAT. 



551 



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 prac- 

 tical 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 Third Heart Sound.^ — Several observers* have called 

 attention to the fact that in certain individuals a third heart 

 sound may be heard very shortly (0.13 sec.) after the beginning 

 of the second sound. Thayer describes this sound as being "softer 

 and of lower pitch" than the second sound, and in some cases 

 as resembling rather a dull thud or hum. In those persons in whom 

 it can be detected it is heard most distinctly over the apex of the 

 heart. The cause of this sound has been explained differently by 

 the several authors who have investigated it. It occurs early in 

 the diastole, and Einthoven suggests that it is due to an after- 



iig. 236. — 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' n arks the instant that the first heart sound is heard over the a|>ex, 

 and 6-6' the moment t!iat it is heard at the second intercostal space. — (Einthoven and 

 Geluk ) 



vibration of the semilunar valves. Thayer and Gibson suggest 

 the more probable explanation that it is due to a vibration of the 

 auriculoventricular valves which is set up by the sudden inrush of 

 blood from the auricles at the beginning of diastole. This inflow 

 of venous blood distends the ventricle sharply and throws the 

 valves into a position of closure with some suddenness. The sound 

 occurs at about the beginning of the period of diastasis, at the 

 point marked 4 in the diagram in Fig. 234. 



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 



* Thayer, "Boston Med. and Surg. Journal," 158, 713, 1908; Einthoven, 

 "Archiv f. d. ges. Physiol.," 120, 31, 1907; Gibson, "Lancet," 1907, II, 1380. 



