SOUNDS OF THE HEART 291 



absorbing the shock when the pressure falls on the valve and the 

 other two cusps shut down on it. 



The sudden stretching of these semilunar valves by the impact 

 of the high arterial pressure sets the valves in vibration like the 

 blow of a drum-stick on a drum-head. It produces a clear 

 sharp high-pitched sound, the so-called second sound of the 

 heart. 



A third sound has been described. It has been attributed to 

 the rebound of the semilunar valves when the ventricle relaxes 

 and the ventricular exit again becomes patent. 



When the valves are diseased certain more or less continuous 

 sounds or murmurs are heard. They are in the main due to 

 either of two causes. 



(1) Stenosis. When a fluid flows along a tube of uniform bore 

 or a tube where the bore alters gradually no vibrations are set up. 

 On the other hand, if the cross-section is altered suddenly and 

 appreciably, the fluid is set into vibrations. These vibrations 

 are transmitted to the solid tube and to the material in which it 

 is set and a sound is produced. Most people have heard the 

 rather irritating purr emitted by the domestic water supply when 

 there is " air in the pipe." The vibrations may not only be heard 

 but they may be felt at the tap and seen in the water issuing. 

 Something similar takes place when, by disease, the opening from 

 auricle to ventricle is narrowed. During the whole period when 

 the ventricle is filling up from its auricular reservoir, the blood 

 flowing through the narrowed opening is set into vibrations which 

 are transmitted through the more solid tissues to the inner ear 

 this is the murmur of mitral or of tricuspid stenosis, according 

 to whether the fault lies on the systemic or pulmonary side 

 respectively. The narrowing does not need to be absolute. If 

 the previous part is dilated, the orifice will become relatively 

 narrower and will produce the result. 



Similarly the murmur caused by stenosis of the aortic or of the 

 pulmonary valves will be heard during the expulsion of blood 

 from the ventricles. 



(2) Incompetence. The failure of any of the valves to (lose- 

 completely, allows blood to trickle back into the empty expelling 

 chamber. This regurgitation throws the tightly stretched cusps 

 into vibration and produces a murmur. If this sound is heard 

 during ventricular systole it may be ascribed to incompetence of 

 either of the auriculo-ventricular valves if during ventricular 

 diastole, the aortic or pulmonary valves are at fault. 



