THE HEART 113 



would be alone sufficient to prevent the reflux of blood 

 into the auricles in two cases out of three were it not 

 that, as has been already stated, the diameter of the 

 base of the heart undergoes such a marked diminution 

 during systole in consequence of the contraction of the 

 circular muscle of the ventricular wall. In other words, 

 the muscular contraction of the ventricle is quite as im- 

 portant in preventing a backward flow of blood as is an 

 intact condition of the auriculo-ventricular valves. One 

 need not wonder, then, that mitral incompetence so often 

 results simply from disease of the muscular wall. The 

 closure of the aortic and pulmonary orifices, on the other 

 hand, is entirely brought about by the semilunar valves, 

 but it is well known that even extreme cardiac dilatation 

 does not lead to any regurgitation at these situations. 

 Kecent investigations by Keith* tend to show that the 

 backflow of blood into the venae cavse is also prevented, 

 to some extent at least, by a series of muscular arrange- 

 ments in the heart which are of too intricate an 

 anatomical nature to be described here. Enough has 

 been said, however, to show that the valvular mechanism 

 of the heart is as largely muscular as membranous in 

 character, a conclusion which is quite in harmony with 

 the results of clinical observation. 



Sounds of the Heart. 



It is generally agreed that the first sound of the heart 

 is the product of at least two factors : (1) the tone 

 produced by the contraction and vibration of the muscle 



* Proc. of AnaL Soc. Gt. Brit, and Irel., 1902, Nov. p. ii. (issued 

 with vol. xxxvii. of Journ. of Anat. and Physiol. ). 



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