THE HEART SOUNDS 169 



in the partially filled auricles. At this moment the weight of the 

 blood which has accumulated in the auricles during the systole, 

 forces the valves of the auriculo-ventricular orifice open, and the 

 ventricle again begins to fill. The period between the closure of 

 the semilunar valves and the opening of the auriculo-ventricular 

 valves is known as the post-sphygmic period, and is the begin- 

 ning of the diastole of the ventricles. The above events com- 

 prise those taking place in a complete cardiac cycle. 



The Heart Sounds. If one applies his ear to the front of the 

 chest, or better still uses a stethoscope, which physicians use to 

 examine the sounds of the lungs and heart, two sounds will be 

 heard during each cardiac cycle. The first sound is dull, low 

 pitched, and long ; the second sharp, high and short. Following 

 the second sound is a short pause. It has been determined ex- 

 perimentally that the first sound is caused partly by the closure 

 and sudden tension of the auriculo-ventricular valves at the mo- 

 ment of cardiac systole, and partly by the muscular contraction 

 of the ventricle. Anything which interferes with the closure of 

 the valves causes an alteration in the sound; for instance, if the 

 valves are diseased there will be a leaking of blood back into the 

 auricles during systole, and this will cause a distinct murmur to 

 take the place of the sound. If the musculature of the heart is 

 weakened, the sound is also modified. Hence the first sound of 

 the heart is an important diagnostic sign in heart disease. The 

 second sound of the heart is due to the sudden tension exerted 

 on the semilunar valves at the moment the blood is forced back 

 on them, following ventricular systole. This sound is also sub- 

 ject to variations in heart disease, especially in disease of the 

 valves themselves, in which case because of roughening they may 

 offer resistance to the outrush of blood from the ventricles, or by 

 not closing tightly, allow the passage of blood in the wrong direc- 

 tion. In either case the sound is changed in character and is a 

 useful diagnostic sign. 



By using these heart sounds as signals of the events occurring 

 within the heart, it is possible to calculate the time relations of 

 the various phases of the cardiac cycle. The heart in the ordinary 

 individual beats about seventy times a minute, so that we may 



