PROPERTIES OF THE HEART MUSCLE. 505 



defined this refractory period as falling within the first part of the 

 systole, and stated that its duration varies with the actual strength 



Fig. 209. To show the effect of a short electrical stimulus applied at different times 

 in the heart beat. (Marey) The record is taken from the frog's heart. In 1, 2, and 3 the 

 stimulus (e) falls into the heart during systole (refractory period) and has no effect. In 

 4, 5, 6, 7, and 8 the stimulus falls into the heart toward the end of systole or during diastole, 

 and is followed by an extra systole and corresponding compensatory pause. It will be 

 noted that the latent period (shaded area) between the stimulus and the extra systole is 

 shorter the longer the diastole has proceeded before the stimulus is applied. 



