180 THE CIRCULATION OF THE BLOOD 



It appears that any kind of stimulus, either chemical, mechanical, elec- 

 trical, or thermal, produces the same effect, but that of these the most potent 

 is a rapidly interrupted induction current. A certain amount of confusion 

 has arisen as to the effects of vagus stimulation in consequence of the fact 

 that fibers of the sympathetic nerve run within the trunk of the vagus nerves 

 of some animals. 



The result of stimulation also depends, to some extent, upon the exact 

 position of the application, of the stimulus. Speaking generally, however, 



FIG. 178. Tracing Showing Actions of the Vagus on the Heart of the Frog. Aur, Auricular; 

 vent, ventricular tracing. The part between perpendicular lines indicates a period of vagus stimu- 

 lation. C. 8 indicates that the secondary coil was 8 cm. from the primary. The part of tracing 

 to the left shows the regular contractions of moderate height before stimulation. During stimu- 

 lation, and for some time after, the beats of auricle and ventricle are arrested. After they com- 

 mence again they are single at first, but soon acquire a much greater amplitude than before the 

 application of the stimulus. (After Gaskell.) 



excitation of any part of the trunk of the vagus produces inhibition, the 

 stimulus being particularly potent if applied to the points where the nerves 

 enter the substance of the heart at the situation of the sinus ganglia. The 

 stimulus may be applied to either vagus with like effect. 



The effect of the stimulus of the vagus is twofold to slow the rate, or 

 even to bring the heart to a complete standstill, and to produce a decrease 



FIG. 179. Tracing Showing Diminished Amplitude and Slowing of the Pulsations of the Auricle 

 and Ventricle without Complete Stoppage during Stimulation of the Vagus. (After Gaskell.) 



in the amplitude. The slowing does not take place until after the lapse of 

 a short latent period during which one or more contractions may occur. 

 The stoppage may be due either to prolongation of the diastole or to diminu- 

 tion of the systole. Vagus stimulation inhibits the spontaneous beats of 

 the heart only, it does not entirely suppress the irritability of the heart-muscle, 



