THE NERVOUS REGULATION OF THE HEART 1019 



The augmentor effect on the strength of the ventricular beats is often 

 very marked. The sympathetic fibres are much less easily tired than the 

 vagus fibres, and have a longer latent period. Whereas the latent period 

 of the vagus in the mammal is considerably less than one second, that of 

 the accelerator nerves may amount to ten or even twenty seconds (Fig. 457). 



FIG. 457. Tracings of ventricular (upper curve) and auricular 

 contractions (lower curve). 



From x tt) y the accelerator nerves stimulated. Lowest line = seconds. 



FIG. 458. Tracing to show effect of stimulation of the vago-sympathetic nerve on the 

 frog's heart. The rhythm is unaltered, but the beats of auricle and ventricle 

 are much decreased in size. On ceasing the stimulation the beats become augmented. 

 (GASKELL.) 



FIG. 459. A tracing similar to Fig. 458. In this case however, the stimulation caused 

 complete stoppage (inhibition) of both auricular and ventricular beats. (GASKELL.) 



Hence if the vago-sympathetic of the frog be stimulated, the first effect is 

 inhibition due to vagus action. The vagus nerve-endings then become 

 fatigued, and the influence of the accelerator fibres makes itself apparent; 

 the heart commences to beat, and the beats become more rapid and forcible 

 than before (Figs. 458, 459). 



Like the vagus, the sympathetic nerve fibres appear to exercise a tonic 

 influence on the heart so that, after extirpation of the stellate ganglion on 

 each side, the pulse frequently becomes permanently slowed. 



