206 CIRCULATION. 



tended with a certain sound. To this is added an impulsion 

 element, which is produced by the striking of the heart against 

 the walls of the thorax. 



The second sound is relatively sharp, high in pitch, and 

 has but one clear, element, which we have already alluded to 

 as valvular. 



Cause of the Sounds of the Heart. There is now 

 scarcely any difference of opinion respecting the cause of the 

 second sound of the heart. The experiments of Rouanet, 

 published in 1832, settled beyond a doubt that it was due to 

 a closure of the aortic and pulmonary semilunar valves. In 

 his essay upon this subject, Rouanet acknowledges his indebt- 

 edness, for the first suggestion of this explanation, to Mr 

 Carswell, who was at that time prosecuting his studies in 

 Paris. 1 



The experiments by which this is demonstrated are as 

 simple as they are conclusive. First we have the experi- 

 ments of Rouanet, who imitated the second sound by produ- 

 cing sudden closure of the aortic valves by a column of water. 

 We then have the experiments, even more conclusive, of the 

 British Commission, in which the semilunar valves were 

 caught up by curved hooks introduced through the vessels 

 of a living animal, the ass, with the result of abolishing the 

 second sound, and substituting for it a hissing murmur. 

 When the instruments were withdrawn, and the valves per- 

 mitted to resume their action, the normal sound returned. 2 



It is unnecessary to discuss the various theories which 

 have been advanced to explain the second sound, as it is now 

 generally acknowledged to be due to the sudden closure of the 



1 Cyclopedia of Anatomy and Physiology, vol. ii., p. 617. In this article, we 

 find Dr. Elliott, of Carlisle, alluded to as having stated in his thesis, published in 

 1831, " that the second sound of the heart is dependent upon the rush of blood 

 from the auricles into the ventricles during their diastole, and also upon the sud- 

 den flapping inward of the sigmoid valves at the origin of the large arteries by the 

 refluent blood." 



* Ibid., p. G18. 



