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- 

 able during the time 

 of its contraction, and 

 that if cooled this in- 

 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 refrac- 

 tory phase of the- 

 cardiac cycle. It 

 corresponds with the 

 period of systole, and 

 its duration varies 

 with that of the 

 stimuli and other 



extrinsic conditions. 

 With weak stimuli 

 the refractory phase 



Fio. 143. Myograins of frog's ventricle, obtained by Marey with 

 apparatus of Fig. 126, and reduced one-half by photography. 

 Shows effect of excitation by break of induction current, at 

 various moments of the cardiac cycle. The line O indicates 

 the commencement of all the beats, during which the 

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

 the stimulus. From 4 to 8 the heart reacts by an extra 

 systole, by a delay or lost time which is progressively less, 

 as shown by the sections shaded obliquely to make them more rt Pr a a t a 

 conspicuous. The extra-systoles increase in height from 4 P 61 S1StS 

 to 8, each being followed by a compensatory pause. At ee, the SVStole : with 

 the line marked by the electric signal, the break induction J ,. ,. ., . 



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- 

 systole 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 systoles (compensatory pause), by which the temporary 

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



Later observers, who, . after Marey, studied the compensatory 



