180 THE CIRCULATION OF THE BLOOD 



have one until the muscle has reached the full extent of its contraction 

 and is about to relax. Since a latent period must supervene upon the 

 application of this second stimulus, it follows that no complete fusion of 

 the contractions is possible. Complete tetanus therefore, does not occur 

 in cardiac muscle, however frequently the stimuli may be applied (Fig. 

 46). 



The refractory phase is a property of extreme importance in under- 

 standing many of the peculiar irregularities observed in cardiac action. 

 If we observe the effect of stimuli applied at varying periods after the 



Fig. 47. Myograms of frog's ventricle, showing effect of excitation by break induction 

 shocks at various moments of the cardiac cycle. The line O indicates the commencement of 

 all the beats during which the shock is sent in. It will be noted that in /, 2 and 3, the heart 

 is refractory to the stimulus. The signals indicate the moments at which the stimuli were ap- 

 plied. From 4 to 8 the heart reacts by an extrasystole, after a delay, which is progressively less 

 the later in diastole the stimulus enters, as shown by the sections shaded obliquely to make them 

 more conspicuous. The extrasystolcs increase in height from 4 to 8, each being followed by 

 a compensatory pause. (From J^uciani's Human Physiology.) 



termination of the refractory phase of a previous stimulus, we shall find 

 that the height of the extra contraction is directly proportional to the 

 time after the end of the refractory period at which it is applied. If a 

 stimulus is applied at the very beginning of diastole, the extra contrac- 

 tion will be small, whereas if it is applied at the end of diastole, the 

 extra contraction will be at least as high as that of the preceding. It 

 may be higher because of the treppe. 



