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, 



Fn:. 71. A, PMhysmoj-irnm oVitaiiiPil iintler normal ronlitiuns ; B. Plethysmogram 1'iuni satm' 



iln.u. ilunn.u (l\N]i]ni.'ir excitation of vagus. 



under different circumstances, is able more or less completely to 

 fill 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 tlic 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 plethysrnogram dine of systolic 



Fio. 7-. Plethysmogram showini; augmentation of canliac volunif ilurin 



of left va.^us. 



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.) 



