THE REFRACTORY PERIOD 109 



the end of the period of relaxation. We see that the heart responded 

 by giving a second beat, but that there was no real summation of the 

 contraction. The apex of the second contraction is at exactly the 

 same height as that of the first. 



The one difference observable is that the second contraction is 

 carried out a little quicker than the first. There is less flattening of 

 the apex. The measurements of the ' apex time ' show this very 

 clearly. They are f^-ths sec. and f^ths sec. respectively. 



Fig. 91. — The Effect of Two Successive Stimuli upon the Ventricle of 

 the Frog's Heart. Time Tracing, 30 peb sec. 



In curve III. we have practically a repetition of curve II., but the 

 second point of stimulation falls a little earlier, f^ths sec. after the 

 first, and therefore nearer the commencement of the period of relaxa- 

 tion. Again we find no true summation of height. The second 

 contraction repeats the first, but starts with many fibres already con- 



