THE PHYSIOLOGY OF THE HEART 



131 



If an external stimulus be applied to the ventricle during the 

 period of contraction or systole of the heart, it has no effect for a 

 considerable period of the systole (Fig. 38). The heart-muscle is 

 said, therefore, to possess a refractory period. The varying excita- 



FlG. 39. To ILLUSTRATE THE VARYING EXCITABILITY OF A FROG'S HEART AT 



DIFFERENT PERIODS OF SYSTOLE AND OF DIASTOLE. (Waller.) 



The excitability is lowest during the first half of systole, greatest during the second 



half of diastole. 



bility of the heart at different periods of systole and diastole is repre- 

 sented in Fig. 39. If, however, the external stimulus be applied 

 towards the end of systole and during the period of relaxation of the 

 heart the diastole it responds with a contraction an extra systole. 



FIG. 40. EFFECT OF TETANIZING THE STANNIUSED HEART. (L. H.) 



The curve should be read from left to right. The time is marked in seconds. The 

 third line shows the period of stimulation. 



Such extra systoles are followed by a longer pause than normal 

 the compensatory pause. It is nearly equal in length to the normaj 

 pause plus the amount cut off from the previous pause by the 

 induction of the extra systole. The heart therefore continues to 



