250 



THE OUTPUT OF THE HEART. 



[BOOK i. 



and rod of the piston being placed by means of the indiarubber spring d, 

 in such a position that the pressure within the box is some few mm. Hg 

 below that of the atmosphere. 



By these methods it has been determined that the diminution 

 of the volume of the heart at a systole, the " contraction volume " 

 as it has inconveniently been called, that is to say the quantity 

 of blood discharged at a systole, the output of a systole, or the 

 " pulse-volume " as we may call it, for it is this which causes the 

 pulse, varies very much under various circumstances. We shall 

 have to discuss later on some of the inGuences bearing on its 

 amount. Meanwhile \ve merely call attention to the fact that it does 

 vary largely, and that any numerical statement as to a normal 

 pulse-volume has relatively little value. 



Another fact of considerable importance brought to light by 

 these methods is that under certain circumstances at all events the 

 output by the left ventricle during a number of beats may be less 

 than the intake through the right auricle. This means that under 

 these circumstances the ventricle does not at the systole discharge 

 the whole of its contents ; some of the blood remains behind in 

 the cavity of the ventricle at the close of the systole. Hence the 

 assumption that the ventricle, in its systole, always discharges 

 the whole of its contents, so as to be quite empty at the onset of 

 diastole is not true ; the ventricle may completely empty itself 

 but it by no means always does so. 



The Mechanism of the Beat. 



133. We may now attempt to consider in rather more 

 detail what we may call the mechanism of the beat, that is to say 

 the exact manner in which the heart receives and ejects the blood. 

 For this purpose we shall need certain data in addition to those 

 on which we have already dwelt. 



In addition to the curve obtained by placing a light lever on 

 the exposed heart (Fig. 46), a method which though useful is open 



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FIG. 46. (Repeated from Fig. 37.) 



