448 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



between the appearance of the extra systole and what would have been the end 

 of the cardiac cycle in which the extra systole fell. The extra length of this 

 pause restored the normal frequency or rhythm. It was called the compensa- 

 tory pause (see Fig. 113). 



FIG. 113. The refractory period and compensatory pause. The curves are recorded by a writing lever 

 resting on the ventricle of the frog's heart. They read from left to right. A break in the horizontal line 

 below each curve indicates the moment at which an induction shock was sent through the ventricle. In 

 curves 1, 2, and 3 the ventricle proved refractory to this stimulus ; in the remaining curves, the stimulus 

 having fallen outside the refractory period, an extra contraction and compensatory pause are seen. 

 Many of the phenomena mentioned in the text are illustrated by this figure (Marey, 1876, p. 72). 



If the heart, or the isolated apex, is beating at a rate so slow that an extra 

 contraction falling in the interval between two normal contractions has time to 

 complete its entire phase before the next normal contraction is due, there will 

 be no compensatory pause. 1 



The refractory phase disappears with sufficiently strong stimuli, especially 

 if the heart is warmed. 2 In such a case an artificial stimulus falling in the 

 beginning of a spontaneous contraction produces an extra contraction. This 

 extra contraction, however, comes first after the end of the systole during which 

 the artificial stimulation is made, 3 occurring in fact toward the end of the 



1 Kaiser, 1895, p. 449. 



2 Engelmann, 1882, p. 453 ; compare Burden-Sanderson and Page, 1880, p. 401. 



3 This is apparently true only of the whole heart, and not of the isolated apex (Engel- 

 mann, 1895, p. 317). 



