CAUSES OF THE SOUNDS OF THE HEART. 49 



Both sounds are generally heard with distinctness over any part of the prcecordia. 

 The first sound is heard with its maximum of intensity over the hody of the heart, a little 

 below and within the nipple, between the fourth and fifth ribs, and is propagated with 

 greatest facility downward, toward the apex. The second sound is heard with its maxi- 

 mum of intensity at the base of the heart, between the nipple and the sternum, at about the 

 locality of the third rib, and is propagated upward, along the course of the great vessels. 



The rhythm of the sounds bears a certain relation to the rhythm of the heart's action, 

 which we have already discussed ; the difference being, that we here regard the heart's 

 action as commencing with the systole of the ventricles, while, in following the action of 

 different parts of the organ, we followed the course of the blood and commenced with 

 the systole of the auricles. Laennec was the first to direct special attention to the rhythm 

 of these sounds, although they had been recognized by Harvey, who compared them to the 

 sounds made by the passage of fluids along the oesophagus of a horse when drinking. He 

 divided a single revolution of the heart into four parts : the first two parts are occupied 

 by the first sound ; the third part, by the second sound ; and in the fourth part there is 

 no sound. He regarded the second sound as following immediately after the first. Some 

 authors have described a " short silence " as occurring after the 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. When we 

 come to consider the mechanism of the production of the two sounds, we shall see that, 

 if our views on that point be correct, the first sound should occupy the period of the ven-. 

 tricular systole, or four-tenths of the heart's action, the second sound about three-tenths, 

 and the repose three-tenths. 



The first sound is relatively dull, low in pitch, and is made up of two elements ; one, 

 a valvular element, in which it resembles in character the second sound, and the other, an 

 element which is 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 clear element, 

 which we have already alluded to as 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 Rouanet, 

 published in 1832, settled beyond a doubt that it is due to a closure of the aortic and 

 pulmonary semilunar valves. In his essay upon this subject, Rouanet acknowledges his 

 indebtedness for the first suggestion of this explanation to Mr. Carswell, who was at 

 that time prosecuting his studies in Paris. The experiments by which this is demon- 

 strated are as simple as they are conclusive. First we have the experiments of Rouanet, 

 who imitated the second sound by producing 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 with- 

 drawn and the valves permitted to resume their action, the normal sound returned. 



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 semilunar valves at the orifices of the aorta and pulmonary artery. We remark, 

 however, that the sound is heard with its maximum of intensity over the site of these 

 valves, and is propagated along the great vessels, to which they are attached. It also 

 occurs precisely at the time of their closure ; viz., immediately following the ventricular 

 systole. 



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