378 CIRCULATION 



cusp is mounted on a muscular septum which acts as a cushion, 

 absor})ing 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 atrio-ventricular valves when the ventricle 

 relaxes and the atrio-ventricular orifice 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 

 atrium to ventricle is narrowed. During the whole period when 

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

 flowing through the narrowed opening is set into ^'ibrations 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 

 narrozver 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. 



The efi'ect of the narrowing of the aortic orifice on the magnitude 

 of the velocity component of cardiac work is considerable. If the 

 orifice be decreased in area to 1 cm. 2, for instance, the velocity of 

 the blood in passing this very narrow orifice is so much increased 

 above the normal value of 0-4- metre per second that, in spite of the 

 decrease in heart rate characteristic of this condition, the velocity 

 component may reach a value of about half that of the total work 

 of the heart. An enormous hypertrophy of the left ventricle is, 



