ACTION OF THE HEART. 4V7 



and completely drained of its blood. 1 But that this is not its only source, is 

 shown by the great diminution in its intensity, which is observable under such 

 circumstances. 



c. That the sudden tension of the auriculo-ventricular valves, with the reflux 

 of the blood against them, at the commencement of the ventricular systole, is a 

 cause of sound, would seem to be indicated by the analogy of the semilunar 

 valves; and an experiment by Valentin, 2 in which a sound in some degree re- 

 sembling the first sound of the heart was produced by the impulse of fluid 

 against a tense membrane, has been adduced in confirmation of this view. But 

 it is to be borne in mind that these valves cannot close together with the same 

 suddenness as do the semilunar, being restrained by the spring-like tension of 

 the earner columnae; and, moreover, even admitting a sound to be produced 

 by their closure, such a sound would be momentary, and would not possess the 

 prolonged character of the true first sound. Still it is not improbable that the 

 tension of these valves serves to augment by resonance the sounds produced in 

 other ways. 



d. That the rush of blood through the narrowed orifices of the great arterial 

 trunks is really a cause of sound, is indicated by the results of experiments 

 made upon tubes out of the body, and upon large bloodvessels through which 

 the blood is circulating ; for any diminution of the caliber of a tube through 

 which fluid is rapidly moving, gives rise to a continuous murmur. And that 

 this cause is in operation in the heart, is specially indicated by the observations 

 of Cruveilhier upon the case already cited; for he noticed that (the effect of the 

 impulse being there in abeyance) the greatest intensity of the first sound was, 

 like that of the second, at the base of the heart, in the region from which the 

 great vessels originate, whilst he could discover no production of sound in the 

 region of the auriculo-ventricular valves. 



e. Lastly, that the collision of the particles of the blood with each other, and 

 with the tense muscular parietes of the heart,, together with its movement over 

 the inequalities of the internal surface of the ventricle, will become a cause of 

 sound, may be suspected from what happens elsewhere, and more especially from 

 the production of a very distinct sound by the movement of blood in the interior 

 of an aneurism; 3 but that this cause, if it have a real existence, is much inferior 

 in potency to the preceding, appears from the fact that it cannot be distinguished 

 from it ; and that neither separately nor combined do these give a sufficient 

 account of the phenomenon, is obvious from the persistence of a sound after the 

 heart has been completely emptied of its blood. 



507. It is only by thus regarding the first sound as made up by several fac- 

 tors, that we can adequately account for the operation of pathological causes in 

 modifying it; since the greater part of the bruits and murmurs that are pro- 

 duced by morbid changes in the heart and in its valves, are really modifications 

 of the natural sound, not additions to it. 



508. That the second sound is produced in the act of closure of the Semilunar 

 valves, is now almost universally admitted; the simple hooking-back one of these 

 valves by a curved needle against the side of the artery, so as to permit a reflux 

 of blood into the ventricle, being sufficient to suppress this sound altogether. 

 Whether it proceeds from the tension of the valves themselves, or from the recoil 

 of the blood against them, or from both causes combined, has not been clearly 

 determined; probably the last is the true account of it. When the first sound 

 is altered by disease of the semilunar valves, occasioning obstruction to the exit 

 of blood, the second sound also is affected in its character; and if the disease be 

 of such a kind as to prevent these valves from effectually closing, a reflux of 



1 See the Report of the London Committee upon the Sounds of the Heart, in the "Trans, 

 of Brit. Assoc." for 1836. 



2 " Lehrbuch der Physiologic," band i. p. 427. 



3 See the " Report of the Dublin Committee of the British Association," loc. cit. 



