550 OF THE CIRCULATION OF BLOOD. 



the sudden filling-out of the Semilunar valves of the aorta and pulmonary 

 artery, with blood ; when the outward current through them has ceased, and 

 the incipient dilatation of the ventricles occasions a vacuum behind them. If 

 one of these valves be hooked back by a curved needle against the side of the 

 artery, so that a reflux of blood is permitted, the sound is entirely suppressed. 

 The first sound cannot be so readily or satisfactorily accounted for. That it 

 is partly due to the Impulse of the apex of the Heart, seems proved by the 

 fact, that, when this impulse is prevented, the sound is much diminished in 

 intensity ; and also by the circumstance that, when the Ventricles contract 

 with vigour, the greatest intensity of the sound is over the point of percussion. 

 But that it is not entirely due to this cause is also evident from the fact, that 

 a sound may still be heard, when the Heart is contracting out of the body; as 

 in the case observed by Prof. Cruveilhier. This sound has been attributed, 

 by some experimenters, to the flapping-back of the auriculo-ventricular valves ; 

 by others to the muscular contraction of the walls of the ventricles ; by others 

 again to the rush of blood along the irregular walls of the ventricles, and 

 through the comparatively narrow orifices of the aorta and pulmonary artery. 

 This last is probably the most consistent with truth ; as would appear from 

 the following interesting observations made by Cruveilhier. By applying the 

 finger to the origin of the pulmonary artery (which is situated in front of the 

 aorta, and completely conceals it), a perfectly distinct vibratory fremissement 

 corresponding with the ventricular diastole, was perceived; but no such vibra- 

 tory thrill could be felt by the finger, when applied to any part of the base of 

 the ventricles : whence it was evident, that no action takes place in the mitral 

 and tricuspid valves, which can give rise to the same palpable effects, as those 

 produced by the semilunar valves. The same was ascertained regarding the 

 valvular sound, which could be distinctly heard, by laying the finger against 

 the origin of the pulmonary artery, and applying the ear to it as to a stetho- 

 scope: whilst nothing of the kind could be perceived in the region of the 

 auriculo-ventricnlar valves. Hence it seems quite certain, that the natural first 

 sound cannot be dependent in any way upon the action of the mitral and tri- 

 cuspid valves. It appeared, on the contrary, that the maximum intensity of 

 the first sound was in precisely the same situation as the maximum intensity 

 of the second, namely, at the origin of the large arteries ; and that it dimin- 

 ished, as the ear was carried from the base, towards the apex of the heart. 

 Moreover, the first sound was observed to be of exactly the same character 

 with the second (the complicating effect of the impulse being here withdrawn) ; 

 except as to its intensity, which was less, and its duration, which was greater. 

 721. Hence, although these observations do not entitle us to deny the par- 

 ticipation of the muscular contraction, and of the movement of the blood over 

 the ventricular walls, in the production of the first sound, they establish (if 

 correct), that the principal cause of it exists at the entrances to the arterial 

 trunks; and it does not seem that any other reason can be assigned for it, than 

 the prolonged rush of blood through their orifices, and the throwing back of the 

 Semilunar valves ; which, in suddenly flapping clown again, produce the second 

 sound. That an exaggeration of the first sound, not essentially differing from 

 it in character, is often produced by disease of the sigmoid valves, which 

 causes an obstruction of their orifice, has long been known ; and in such 

 cases, the character of the second sound is also changed. Indeed, M. Cru- 

 veilhier states it as, in his opinion, an uniform occurrence, that disease of the 

 Semilunar valves alters both sounds. When this disease is such as to prevent 

 the valves from effectually closing, a reflux of blood takes place into the ven- 

 tricle at the time of its diastole; causing a rushing sound, more or less pro- 

 longed, to be heard in the intervals of the pulse, instead of with it. These 

 considerations appear to prove almost incontestably, that the cause of the first 



