332 



Sir R. Quain. On the Mechanism by which 



diagnosis and treatment of diseases of the heart. An explanation of 

 the mechanism by which phenomena so distinct, so constant, and so 

 remarkable are produced, has been sought for by many observers 

 amongst the classes just mentioned. 



The second sound is the result of the sudden tension of the semi- 

 lunar valves, caused by the resistance which these valves offer to the 

 retrograde flow of the blood from the aorta and pulmonary artery 

 respectively into the ventricles on the cessation of systole. The sound 

 is similar to that produced when a piece of tape or ribbon is suddenly 

 •made tense. It is needless to say more here on the subject of the 

 second sound and its causes, for the explanation just offered is, I 

 believe, almost universally accepted. 



The cause of the first sound is, on the contrary, still a subject much 

 discussed and undecided. Many explanations have been offered at 

 various times of this phenomenon. Professor Michael Foster, to 

 whose admirable exposition of the mechanism of the circulation I am 

 deeply indebted for guidance in working out the subject of this 

 communication, observes that " this sound presents many difficulties 

 in the way of a complete explanation."* The difficulties result from 

 the number of events which occur simultaneously with the systole of 

 the ventricle and the occurrence of the sound. It seemed to me to 

 be desirable, amidst such differences of opinion, to solve if possible a 

 problem which has its special interest and its special importance. 

 Two of the most striking events which take place during systole, 

 aamely, the closure of the auriculo-ventricular valves and the mus- 

 cular contraction of the ventricular walls, are regarded by many 

 authorities as the sources from whence the first sound proceeds. The 

 result of my investigations, on the one hand, leads me to the conclu- 

 sion that neither of these explanations is satisfactory ; and, on the 

 other hand, enables me to indicate what I believe to be the real 

 explanation of the phenomenon. f 



A. The action of the auriculo-ventricular valves is not the source of 

 the first sound of the heart. 



The mechanism of these valves (the mitral and tricuspid) and 

 their action do not possess the elements necessary for the production 

 of such a sound. The action of the valves commences when the 

 blood, pressed from behind by the contraction of the auricles, flows 

 into the ventricles. It is there reflected from the walls of these 

 cavities, and presses on the lamella? of the valves, which are thus 



* 'Text-book of Physiology,' 6th Eel. (London, 1893), p. 239. 



+ I have to acknowledge with thanks the assistance rendered me in arranging 

 my notes of this memoir by my rained friend, Dr. Mitchell Bruce, during my 

 recent severe indisposition. 



