CHAP, iv.] THE VASCULAR MECHANISM. 149 



had not only completed its contraction but was far advanced in 

 relaxation. Such a view is not only cb priori improbable but is 

 directly contradicted by the fact that when we compare a tracing 

 obtained by placing a lever directly on the heart or indeed a tracing 

 of the cardiac impulse with a pulse tracing, that is a tracing of the 

 expansion of an artery, we find that the ventricle continues con- 

 tracted after its contents have entirely left the cavity. That is to 

 say, the actual flow of blood takes place only during the middle 

 portion of the time during which the muscular fibres of the ven- 

 tricle are contracting and engaged in carrying on the systole. 

 During the first part, pressure is being got up, during the second 

 the blood is being propelled, during the third the ventricle continues 

 to remain empty and contracted. By this means the complete 

 emptying of the ventricle is effectually secured. And others have 

 urged that the closure of the semilunar valves, being entirely due 

 to the reflux spoken of above, follows close upon the emptying of 

 the ventricle ; in other words that it takes place while the ven- 

 tricle is still contracted. It is very difficult to point out indications 

 on the ventricular curve which indubitably correspond to this event. 

 In tracings of the cardiac impulse, and in tracings taken by a lever 

 placed directly on the heart, a notch, followed by a rise, is some- 

 times observed in that part of the curve which intervenes between 

 the first large rise and the final sudden fall ; and this secondary 

 rise has been taken to indicate the closure of the semilunar valves ; 

 but, if this be the case, the time during which the ventricle 

 remains contracted after the closure of the valves forms a very con- 

 siderable fraction of the whole period of the systole; and this 

 presents difficulties. Sometimes two such notches and peaks are 

 seen, and the occurrence of the two has been attributed to a want of 

 synchronism in the closure of the pulmonary and aortic semilunar 

 valves, the latter closing some little time before the former. But 

 it is by no means clear that these notches and peaks are thus due 

 to the closure of the valves; they may possibly have another 

 origin, they are not always present, and the attempt to fix the time 

 of the closure of the semilunar valves by them cannot be regarded 

 as satisfactory. On the other hand, the second sound of the heart 

 is undoubtedly due to the complete closure and sudden tension of 

 the semilunar valves ; and not only is this second sound separated 

 from the first sound by a distinctly appreciable interval (from which 

 we may infer either that the systole of the ventricle ceases before 

 the complete closure and sudden tension of the semilunar valves or 

 that the first sound does not last so long as the systole itself and 

 is therefore not a muscular sound) but the time elapsing between 

 the beginning of the first sound and the second sound is, as we shall 

 see, remarkably constant. Now we have reason to believe that the 

 quantity of blood expelled at any one beat, and hence the time 

 taken up in its escape, does vary very considerably ; whereas the 

 duration of the actual systole is probably much more constant. 



