GIL XX.] 



THE HEART SOUNDS 



235 



3 

 TF- 



is shorter and sharper, with a somewhat flapping character, and 

 follows close after the arterial pulse. The periods of time occupied 

 respectively by the two sounds taken together and by the pause, are 

 nearly equal. Thus, according to Walshe, if the cardiac cycle be 

 divided into tenths, the first sound occupies T %; the second sound, 



the first pause (almost imperceptible), T V ; and the second pause, 

 The sounds are often but somewhat inaptly compared to the 

 syllables, lubb diip. 



The events which correspond, in point of time, with the first 

 sound, are (1) the contraction of the ventricles, (2) the first part of 

 the dilatation of the auricles, (3) the tension of the auriculo- ventricular 

 valves, (4) the opening of the semilunar valves, and (5) the propul- 

 sion of blood into the arteries. 

 The sound is succeeded, in 

 about one-thirtieth of a second, 

 by the pulsation of the facial 

 arteries, and in about one-sixth 

 of a second, by the pulsation 

 of the arteries at the wrist. 

 The second sound, in point of 

 time, immediately follows the 

 cessation of the ventricular 

 contraction, and corresponds 

 with (a) the tension of the 

 semilunar valves, (b) the con- 

 tinued dilatation of the auricles, 

 (c) the commencing dilatation 

 of the ventricles, and (d) the 

 opening of the auriculo-ventri- 

 cular valves. The pause imme- 

 diately follows the second sound, 

 and corresponds 'in its first part with the completed distension of 

 the auricles, and in its second with their contraction, and the com- 

 pleted distension of the ventricles; the auriculo-ventricular valves 

 are open, and the arterial valves closed during the whole of the 

 pause. 



Causes. The exact cause of the first sound of the heart is a 

 matter of discussion. Two factors probably enter into it, viz., first, 

 the vibration of the auriculo-ventricular valves and the chordce tendinece. 

 This vibration is produced by the increased intraventricular pressure 

 set up when the ventricular systole commences, which puts the valves 

 on the stretch. It is not unlikely, too, that the vibration of the 

 ventricular walls themselves, and of the aorta and pulmonary artery, 

 all of which parts are suddenly put into a state of tension 

 at the moment of ventricular contraction, may have some part in 



FIG. 242. Scheme of cardiac cycle. The inner circle 

 shows the events which occur within the heart ; 

 the outer the relation of the sounds and pauses to 

 these events. (Sharpey and Gairdner.) 



