46 CIRCULATION OF THE BLOOD ACTION OF THE HEART. 



first sound, and a " long silence," after the second sound. The short silence, 

 if appreciable at all, is so indistinct that it may practically be disregarded. 



.Most physiologists regard the duration of the first sound as a little less 

 than two-fourths of the heart's action, and the second sound as a little more 

 than one-fourth. AVhen the mechanism of the production of the two sounds 

 is considered, it will be seen that if the views on that point be correct, the 

 first sound should occupy the period of the ventricular systole, or four-tenths 

 of the heart's action. The second sound occupies about three-tenths, and 

 the repose, three-tenths. 



The first sound is relatively dull, low in pitch, and is made up of two ele- 

 ments ; one, a valvular element, in which it resembles in character the second 

 sound, and the other, an element which is directly due to the action of the 

 heart as a muscle. It has been ascertained that all muscular contraction is 

 attended 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 ele- 

 ment, which is purely valvular. 



Causes of the Sounds of the Heart. There is now scarcely any difference 

 of opinion with regard to the cause of the second sound of the heart. The 

 experiments of Eouanet (1832) settled beyond a doubt that it is due to 

 closure of the aortic and pulmonary semilunar valves. In these experiments, 

 the second sound was imitated by producing sudden closure of the aortic 

 valves by a column of water. In the experiments of the British Commission, 

 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. AVhen the instruments 

 were withdrawn and the valves permitted to resume their action, the normal 

 sound returned. 



The cause of the first sound of the heart has not been so well understood. 

 It was maintained by Rouanet that this sound was produced by the closure 

 of the auriculo- ventricular valves ; but the situation of these valves rendered 

 it difficult to demonstrate this by actual experiment. While the second sound 

 is purely valvular in its character, the first sound is composed of a certain 

 number of different elements ; but auscultatory experiments have been made 

 by which all but the valvular element are eliminated, when the first sound 

 assumes a purely valvular quality. These observations were made in 1858 by 

 the late Dr. Austin Flint : 



If a folded handkerchief be placed between the stethoscope and in- 

 tegument, the first sound is divested of some of its most distinctive feat- 

 ures. It loses the quality of impulsion and presents a well marked valvular 

 quality. 



In many instances, when the stethoscope is applied to the prsecordia while 

 the subject is in a recumbent posture and the heart is removed by force of 

 gravity from the anterior wall of the thorax, the first sound becomes purely 

 valvular in character and as short as the second. 



When the stethoscope is applied to the chest a little distance from the 



