190 THE SOUNDS OF THE HEART. [BOOK r. 



and to vibrate so soon as the contents of the ventricle are driven 

 out. This suggests that the sound is caused by the sudden 

 tension of the auriculo-ventricular valves, and this view is sup- 

 ported by the facts that the sound is obscured, altered or 

 replaced by murmurs when the tricuspid or mitral valves are 

 diseased, and that the sound is also altered or, according to 

 some observers, wholly done away with when blood is prevented 

 from entering the ventricles by ligature of the venae cavse. On 

 the other hand, the sound has not that sharp character which 

 one would expect in a sound generated by the vibration of 

 membranes such as the valves in question, but in its booming 

 qualities rather suggests a muscular sound. Further, according 

 to some observers, the sound, though somewhat modified, may 

 still be heard when the large veins are clamped so that no blood 

 enters the ventricle, and, indeed, may be recognized in the few 

 beats given by a mammalian ventricle rapidly cut out of the 

 living body by an incision carried below the auriculo-ventricular 

 ring. Hence the view has been adopted that this first sound 

 is a muscular sound. In discussing the muscular sound of skeletal 

 muscle (see 75), we saw reasons to distrust the view that this 

 sound is generated by the repeated, individual, simple contrac- 

 tions which make up the tetanus, and hence corresponds in tone 

 to the number of those simple contractions repeated in a second, 

 and to adopt the view that the sound is really due to a repetition 

 of unequal tensions occurring in a muscle during the contraction. 

 Now, the ventricular systole is undoubtedly a simple contraction, a 

 prolonged simple contraction, not a tetanus, and, therefore, under 

 the old view of the nature of a muscular sound, could not produce 

 such a sound ; but accepting the other view, and reflecting how 

 complex must be the course of the systolic wave of contraction 

 over the twisted fibres of the ventricle, we shall not find great 

 difficulty in supposing that that wave is capable in its progress of 

 producing such repetitions of unequal tensions as might give rise 

 to a ' muscular sound,' and, consequently, in regarding the first 

 sound as mainly so caused. Accepting such a view of the origin of 

 the sound we should expect to find the tension of the muscular 

 fibres, and so the nature of sound, dependent on the quantity of 

 fluid present in the ventricular cavities and hence modified by liga- 

 ture of the great veins, and still more by the total removal of the 

 auricles with the auriculo-ventricular valves. We may add that 

 we should expect to find it modified by the escape of blood from 

 the ventricles into the arteries during the systole itself, and might 

 regard this as explaining why it dies away before the ventricle has 

 ceased to contract. 



Moreover, seeing that the auriculo-ventricular valves must be 

 thrown into sudden tension at the onset of the ventricular systole, 

 which, as we have* seen, is developed with considerable rapidity, 

 not far removed at all events from the rapidity with which the 



