THE CIRCULATION OF THE BLOOD AND LYMPH 135 



regard to this inhibition : (i) It does not begin for a little 

 time after stimulation has begun. In other words, there is 

 a distinct latent period ; and the length of this latent period 

 is related to the phase of the heart's contraction at which 

 the stimulus is thrown in, and to the rate at which the heart 

 is beating. As a general rule, the heart makes at least one 

 beat before it stops. 



(2) The inhibition does not continue indefinitely, even if 

 stimulation of the nerve is kept up. Sooner or later, and 



FIG. 46. TRACING FROM FROG'S HEART. 



A, auricular, V, ventricular tracing. Sinus stimulated (primary coil 70 mm. from 

 secondary). Heart at temperature ii'2 C. Complete standstill. The time tracing 

 between the curves marks intervals of two seconds. 



usually, in fact, after an interval of a few seconds, the heart 

 begins again to beat if it has been completely stopped, or to 

 quicken its beat if it has only been slowed, or to strengthen 

 it if the inhibition has only weakened the contraction, 

 and it soon regains its old rate of working. Not only 

 so, but very often there follows a longer or shorter period 

 during which the heart works at a greater rate than it did 

 before the inhibition, and this greater rate of working may 

 be manifested by increased frequency of beat, or increased 



