THE HEART BEAT. 557 



the animal is bled to death, and indeed as soon as the heart ceases 

 to throw out a supply of blood sufficient to maintain aortic tension. 

 It disappears also when cuts are made in the ventricles so that the 

 blood may escape otherwise than through the arteries. (2) When 

 the valves of the pulmonary artery and aorta are hooked back in the 

 n'ving animal the second sound is replaced by a murmur due to the 

 rushing back of the blood into the ventricle, and if the valves are 

 dropped back into place the normal second sound is again heard. 

 (3) Similar sounds may be produced if the root of the aorta with its 

 valves in place is excised and attached to a glass tube carrying a 

 column of water. With such an arrangement, if the valves are held 

 open for a moment and then closed sharply by the pressure of the 

 column of water a sound similar to that of the second heart sound 

 is heard. 



The physician uses this view of the cause of the second sound in 

 auscultation, and it is evident that the nature of the sound or its 

 replacement by murmurs will give useful testimony regarding the 

 condition of the semilunar valves. The first heart sound has of- 

 fered more difficulty. It occurs at or shortly before the closure of the 

 auriculo-ventricular valves, and it would seem natural, therefore, to 

 attribute it to the vibration of these valves when suddenly put under 

 tension by the ventricular systole. Most authors, indeed, believe 

 that this factor is at least partially responsible for the sound, 

 that is, that the sound contains a valvular element. But that this 

 is not the sole cause is shown by the fact that the bloodless beating 

 heart still gives a sound at the time of the ventricular systole. 

 Indeed, if the apex of the rabbit's heart is cut off, it continues 

 to beat for a few minutes and during this time gives a first heart 

 sound. It is usually said, therefore, that the first heart sound is 

 caused by the combination of at least two factors, a valvular 

 element due to the vibration of the auriculo-ventricular valves, and 

 a muscular element due to the vibration of the contracting muscular 

 mass. Accepting this view, there is a further difficulty in explain- 

 ing the origin of the muscular element. According to some, it is 

 due -to the fact that the contraction of the muscle fibers is not 

 simultaneous throughout the ventricle and the friction of the inter- 

 lacing fibers sets up vibration in the muscular mass; according to 

 others, the so-called muscular element is mainly a resonance tone of 

 the ear membrane of the auscultator, the shock of the contracting 

 heart sets the tympanic membrane to vibrating. It seems useless 

 to attempt a detailed discussion of these conflicting views, since no 

 convincing statements can be made. Practically, the time at which 

 the heart sounds occur is of great importance. A number of 

 observers have recorded the time upon a cardiographic tracing of 



