THE HEART BEAT. 555 



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 Third Heart Sound. Several observers* have called 

 attention to the fact that in certain individuals a third heart 



Fig. 234. 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.-) 



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. Einthoven has shown the existence of this 

 sound by objective methods. By means of a microphone 

 attachment the heart sounds can be transmitted to the string- 

 galvanometer, in which they cause deflections of the string that 

 can be photographed. In this way he has obtained records of 

 the third sound upon individuals in whom the stethoscope failed 

 to reveal its existence. The cause of this sound has been 

 explained differently by the several authors who have investi- 

 gated it. It occurs early in the diastole, and Einthoven suggests 

 that it is due to an after-vibration of the semilunar valves. 

 Thayer and Gibson suggest the more probable explanation that 

 it is due to a vibration of the auriculo ventricular 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 



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

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



