236 PHYSIOLOGY OF THE HEART. [CH. XX. 



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

 sound, are (i) 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 propulsion of blood into the arteries. The sound is suc- 

 ceeded, in about one-thirtieth of a second, by the pulsation of the 

 facial arteries, and in about one-sixth of a second, by the pulsa- 

 tion of the arteries at the wrist. The second sound, in point of 

 time, immediately follows the cessation of the ventricular con- 

 traction, and corresponds with (a) the tension of the semilunar 

 valves, (b) the continued dilatation of the auricles, (c) the corn- 



Fig. 235. 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.) 



mencing dilatation of the ventricles, and (d) the opening of the 

 auriculo-ventricular valves. The pause immediately 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 completed 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 intraven- 

 tricular pressure set up when the ventricular systole commences, 

 which puts the valves on the stretch. It is not unlikely 



