THE PUMPING ACTION OF THE HEART 157 



space. A third sound, much less distinct, is sometimes heard in diastole 

 a short time after the second. To study the exact time relationship of 

 the sounds the vibrations which they set up can be recorded graphically 

 alongside cardiac tracings by means of a microphone attachment to the 

 electrocardiograph (see page 270). 



Causes of Sounds 



It has been found that the first sound consists of two distinct elements, 

 one high pitched and the other of a dull character. The former element 

 is believed to be the result of vibrations set up in the flaps of the auric- 

 uloventricular valves, and therefore in the blood in the heart, by the 

 sudden rise in systolic pressure. The dull element on the other hand 

 is undoubtedly of muscular origin. The evidence for these conclusions is 

 as follows: (1) When the auriculoventricular valves are prevented from 

 closing properly either by disease or by pushing a loop of wire down the 

 large veins, the high pitched quality_disam)ears < and nothing but a rush- 

 ing sound accompanies the dull bruit produced by the contracting muscle. 

 (2) In a heart that has been rendered bloodless by an incision near the 

 apex, or even in an excised but still beating heart, the dull element of 

 the first sound still continues to be heard for a short time. That con- 

 tracting muscle produces a sound is a well-established fact. 



There are, however, many obscure phenomena connected with the 

 causation of the first sound, but we can not go into such controversial 

 matters here. A close inspection of the electrophonographic tracing 

 shows that the sound starts at the beginning of the presphygmic period, 

 and that it lasts with gradually declining but variable intensity until 

 well into the sphygmic period (Fig. 37). 



The second sound occurs accurately at the beginning of diastole and 

 can readily be shown to be caused by the sudden shutting and stretching 

 of the semilunar valves, which throws them, the blood in contact with 

 them, and the neighboring walls of the aorta into vibration. Proof of 

 this conclusion is furnished by the following facts: The second sound 

 immediately disappears if the blood is let out of the heart by opening 

 the apex, and it is replaced by a rushing "bruit" if the flaps are pre- 

 vented from closing as a result of disease or of hooking them back by 

 passing a wire down the carotid artery. The third sound, although audi- 

 ble only in some individuals, can nevertheless be shown to exist by the 

 electrophonograph, and since it occurs at the time when the auriculo- 

 ventricular valves open, it is believed to depend upon the sudden inrush 

 of blood from auricles to ventricles. 



The greatest importance of the sounds is in the clinical diagnosis of val- 

 vular and other lesions of the heart. When a valve leaks, for example, 



