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APPLIED PHYSIOLOGY 



substance of the ventricles, (2) that which results from 

 the stretching of the auriculo-ventricular valves. One 

 says advisedly the stretching of the valves, for the sound 

 is not due, as was once supposed, to the driving together 

 of the membranous flaps, but to their sudden tensioi 

 between the fibrous ring round the mouth of the auriculo- 

 ventricular orifice on the one hand and the papillary 

 muscles on the other. The flaps of the mitral and 

 tricuspid valves are at no period of the cardiac cycle in 

 anything approaching to a horizontal position. On the 

 contrary, they are always more or less vertical, and the 

 contraction of the musculi papillares merely serves to 

 counteract the tendency of the cusps to float up owing 

 to the constriction of the base of the heart during 

 systole. Hence it is that the valves are suddenly made 

 tense by being pulled on at both their extremities like 

 a sheet well shaken, and the characteristic flapping 

 sound results. Either the muscular or the membranous 

 element of the first sound may come to predominate in 

 conditions of disease. If the ventricular wall is hyper- 

 trophied, the sound is low and booming from an emphasis 

 of the muscle element ; if, on the other hand, the 

 muscle is thin and atrophied, the predominance of the 

 membranous factor imparts to the sound a high-pitched 

 and slapping character. When the ventricular systole 

 is feeble and inefficient, both the muscular contraction 

 and the stretching of the valve are so much reduced 

 that the first sound becomes very faint, or may even 

 be inaudible. 



The second sound, on the other hand, is due entirely 

 to the tension of the semilunar valves, and its loud- 



