THE NERVOUS REGULATION OF THE HEART 159 



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 stimula- 

 tion 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 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 inhibi- 

 tion, and this greater rate of working may be manifested by increased 



Fig. 71. Activity of Vagus on Verge of Heat Standstill. Auricular and ventricular 

 contractions of toad's heart recorded. Heart at 34'5 C. t v 50, stimulation of 

 vagus (distance of coils 50 mm.). The ventricle was already in heat standstill; 

 the auricle was at once inhibited. Then follows secondary augmentation (due 

 to the sympathetic fibres), during which the ventricle also resumes beating. 

 An interval of a minute elapsed between the first and second parts of the 

 tracing, during which the heart remained at 34*5 C. The auricle was almost 

 in standstill (contractions can still be seen on the curve with a lens), when the 

 vagus was again stimulated at v 50 with the same distance between the coils. 

 Complete inhibition followed by secondary augmentation. 



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 of 

 secondary augmentation. * The cause of this secondary augmentation, 

 and of the primary augmentation sometimes seen in fresh prepara 

 tions and often in hearts that have been long exposed (Fig. 73), 

 excited much speculation before it was known that sympathetic 

 fibres existed in the vagus. There is no longer any doubt that it is 

 due to the stimulation of these accelerator or, as it is better to call 



* Augmentation is termed ' secondary ' when it is preceded by inhibition, 

 ' primary ' when it is not so preceded. 



