THE HEART BEAT. 553 



the closure of the semilunar valves. The usual view, therefore, is 

 that the sound is due ultimately to the vibrations set up in these 

 valves by their sudden closure. These vibrations are transmitted 

 to the column of blood in the aorta (or pulmonary artery) and then 

 to the intervening tissue of the chest wall. This view is made 

 probable by a number of experimental results, some of the most 

 important of which were brought out by Williams in a report (1836) 

 of a committee appointed by the British Association for the 

 special purpose of investigating the subject. It has been shown: 

 (1) That the second sound disappears before the first sound when 

 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 

 living 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 



