vii MECHANICS OF THE HEART 217 



the third a reflux." The most important results which Stefani 



obtained from analysis of cardiac plethysmograms are as follows : 



(a) Their magnitude varies considerably not merely in different 



animals, but also in the same animal, showing that the heart, 



Fie. 71. A, Plethysmogram obtained under normal renditions ; B, Plethysmogram from same 

 dog, during dyspnoeie excitation of vagus. 



under different circumstances, is able more or less completely to 

 nil and empty itself of blood, " so that it may assume a volume 

 considerably greater than that attained in the preceding cycles, 

 independent of any modification in the frequency of the beat" 

 (This comes out clearly in Figs. 69, 70, 71.) 



(&) There is no perceptible difference in the duration and form 

 of the two first periods of the plethysniogram (line of systolic 



FIG. 72. Plethysmogram showing augmentation of cardiac volume during excitation 

 of left vagus. 



evacuation and active diastolic refill) with changes in frequency 

 of the heart's beat. It is only the third period (of slow passive 

 refilling) that varies in duration and form with the acceleration 

 or retardation of the cycle. (This is demonstrated in the tracings 

 of Figs. 71, 72, 73.) 



