CAUSE OF THE SOUNDS OF THE HEART. 209 



by some the muscular murmur, and the sound produced by 

 the impulse of the heart against the walls of the chest. 



The muscular sound has been recognized by Wollaston, 

 Laennec, and others, and by Laennec was supposed to be 

 the sole cause of the first sound of the heart. This observer 

 attributed the first sound to the muscular action of the ven- 

 tricles, and the second to the action of the auricles. There 

 can be no doubt but that this is one of the elements of the first 

 sound ; and it is this which gives it its prolonged character, 

 when the stethoscope is applied over the body of the organ, 

 as the sound produced in muscles continues during the whole 

 period of their contraction. Admitting this to be an element 

 of the first sound, we can understand how its duration must 

 necessarily coincide with the ventricular systole. We can 

 appreciate, also, how all but the valvular element is eliminated 

 when the stethoscope is moved from the body of the heart, 

 the muscular sound not being propagated as completely as 

 the sound made by the closure of the valves. 



The impulse of the heart against the walls of the thorax 

 also contributes to produce the first sound. This is demon- 

 strated by noting the difference in the sound when the sub- 

 ject is lying upon the back, and when he is upright ; or by 

 interposing any soft substance between the stethoscope and 

 the chest, or by auscultating the heart after the sternum has been 

 removed. Under these circumstances the first sound loses its 

 booming character, retaining, however, the muscular element, 

 when the instrument is applied to the exposed organ. It was 

 thought by Magendie that the shock of the heart against the 

 chest was the sole cause of the first sound. 1 This observer 

 maintained that when the sternum is removed in a living ani- 

 mal, the first sound cannot be heard over the heart. This, 

 however, is not the fact ; and though the element of impul- 

 sion enters into the composition of the first sound, the view 

 that it is the sole cause of this sound is not tenable. 



The first sound of the heart is complex. It is pro- 



1 MILNE-EDWARDS, Lemons sur la Physiologic, etc., tome iv., p. 3S. 

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