976 



PHYSIOLOGY 



rhythm or strength of contraction, or the conduction from auricle to ventricle, 

 according to the part of the heart-muscle which is affected. 



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 



FIG. 448. Tracings of ventricular (upper curve) and auricular 



contractions (lower curve). 

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



FIG. 449. Tracing to show effect of stimulation of the vago- sympathetic nerve on the 

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

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

 (GASKELL.) 



FIG. 450. A tracing similar to Fig 449. In this case, however, the stimulation caused 

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



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. 448). 

 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, and 

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

 than before (Figs. 449, 450). 



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





