THE CIRCULATION OF THE BLOOD AND LYMPH 145 



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 

 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 



FIG. 60. FROG'S HEART : VAGUS STIMULATED. 



Temperature of heart 8 C. ; 78 mm. between the coils. Diminution in force of 

 auricle and ventricle, but not complete standstill. Time-tracing shows two-second 

 intervals. 



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 strength of beat, or by both. 

 When the temperature of the heart is low, increased frequency ; 

 when it is high, increased strength, is generally seen during this 

 period oj secondary augmentation* The cause of this secondary 

 augmentation, and of the primary augmentation sometimes seen 

 in fresh preparations and often in hearts that have been long 



* Augmentation is termed ' secondary ' when it is preceded by inhibi- 

 tion, ' primary ' when it is not so preceded. 



IO 



