CHAP, iv.] THE VASCULAR MECHANISM. 237 



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, accord- 

 ing 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 80), we saw reasons to distrust the view that this 

 sound was generated by the repeated individual simple contractions 

 which made up the tetanus and hence corresponded in tone to the 

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

 adopt the view that the sound was 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 ligature 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 ven- 

 tricles into the arteries during the systole itself, arid 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 semilunar valves are closed, a rapidity therefore capable of 

 giving rise to vibrations of the valves adequate to produce a 

 sound, it is difficult to escape the conclusion that the closure 

 of these valves must also generate a sound, which in a normally 

 beating heart is mingled in some way with the sound of muscular 

 origin, although the ear cannot detect the mixture. 



If we accept this view that the sound is of double origin, 



