320 PHYSIOLOGY CHAP. 



sarcoplasm), he thinks the oscillations in tonicity are probably due 

 to the contractions and expansions of the sarcoplasm, and the 

 ordinary and more frequent beats to the doubly refractive substance 

 of these elements. 



The oscillations of excitability in cardiac muscle recorded by 

 Bowditch with electrical stimuli were determined more exactly by 



the later work of Kro- 

 necker and Stirling. 

 They showed that the 

 heart becomes inexcit- *T\ 

 able during the time J 

 of its contraction, and j 

 that if cooled this in- J / 

 excitability persists 

 for some time after 

 the beat is completed. 

 These facts were 

 confirmed by Marey, 

 who analysed the 

 phenomenon of peri- 

 odic inexcitability to 

 electrical stimuli in 

 the automatically 

 beating heart, and 

 termed it the refcac- .'J 

 tory phase of the ^ 

 cardiac" cycle. It 

 corresponds with the 

 period of systole, and 

 its duration varies 



Fir;. 143. MyOgrams of frog's ventricle, obtained by Marey with With that of the 



apparatus of Fig. 126, and reduced one-half bv photography. afirrmli anrl ntVoT 



Shows effect of excitation by break of induction current, at S 1111 . an - . }X 



various moments of the cardiac cycle. The line O indicates extrinSIC Conditions. 



the commencement of all the beats, during which the TTT-^I i j_- i 



shock is sent in. In 1, 2, and 3 the heart is refractory to With Weak Stimuli 



the stimulus. From 4 to 8 the heart reacts by an extra i_"U vafvanfnrv -rVhaeo 



systole, by a delay or lost time which is progressivelyless, l .IdUlUl^ JJlld,St; 



assh~o\vn by the sections shaded obliquely to make them more riprsists throughout 



conspicuous. The extra-systoles increase in height from 4 fl_^ & ., , 



to 8, each being followed by a compensatory pause. At ee, the Systole : With 

 the line marked by the electric signal, the break induction , f i- , 



shocks were thrown in. Stronger Stimuli it IS 



limited to the first . 



period of systole, or obliterated altogether. Warming shortens)/ 

 or suppresses it ; cooling prolongs it. Each forced or extra- 

 sy stole is more ample in proportion as iT^appears later after 

 the spontaneous systole that preceded it. The_ extrasystole is 

 followed by a resting period longer than that which usually occurs 

 between two syiMes^cprnpejasafe^ipause), byjwhich the temporary 

 disturbance of cardiac rhythm is adjusted (Fig. 143). 



Later observers, who, after Marey, studied the compensatory 



