THE INHIBITORY NERVES 2OI 



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 



FIG. 179. Tracing Showing Action of the Vagus on the Heart of the Terrapin. Aur, 

 Auricular; vent, ventricular tracing. The part between words "on" and "off" indicates 

 a period of vagus stimulation. The part of tracing to the left shows the regular contractions 

 before stimulation. During stimulation, and for some time after, the beats of the auricle 

 and ventricle are arrested. After they commence again the auricle contracts weakly at 

 first, but soon acquire a much greater amplitude. The ventricular contractions that 

 follow the first weak auricular contractions are maximal in the terrapin, but not so in the 

 frog. See next figure. Time in seconds. New figure. 



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. 180. Tracing Showing Diminished Amplitude and Slowing of the Pulsations of 

 the Auricle and Ventricle of the frog without Complete Stoppage during Stimulation of the 

 Vagus. (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. 



