THE HEART BEAT. 559 



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 



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.) 



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 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 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 



