Effects of Stimulation of Dog's Heart. 



9 



and spread in an abnormal manner; the structures normally con- 

 cerned with these functions remained inhibited. 



When the left vagus nerve was stimulated in the same dog, with 

 a current of the same strength, a difference from the effect of 

 stimulating the right nerve was observed in 88 per cent, of the 

 fifty-four experiments. The auricles did not cease to beat, they 

 were merely slowed, — sometimes 100 or more beats. In one group, 

 normal ventricular contractions ceased entirely in twenty-four 

 cases. In a second of twenty-four other cases, a ventricular 

 contraction followed every second, occasionally every third, fourth 

 or more auricular beats, the mechanism being one of incomplete 

 dissociation. In a third group, the only effect of stimulation was 

 an increase in the time occupied in conduction from auricles to 

 ventricles. 



In the first group when the left vagus was stimulated, as has 

 been stated, normal ventricular activity ceased, but abnormal 

 activity occasionally continued. The rate of the abnormal ven- 

 tricular contractions differed from that of the slowed auricles and 

 complete As-Vs dissociation resulted. Similar ventricular activity 

 occurred also when the right vagus was stimulated, but the auricles, 

 as is usual, ceased to contract. 



The explanation offered for the phenomena occurring on left 

 vagus stimulation is that the main effect consists in depressing 

 the conduction system. In the first group, the ventricles cease to 

 beat, because, on account of the great depression of conduction, 

 they receive no impulses from the auricles. If the ventricular 

 muscle is irritable, abnormal stimuli may be formed and an 

 idioventricular rhythm result. When a slighter degree of depres- 

 sion of conduction has taken place, every second or third beat 

 passes along the A-V bundle and initiates a ventricular contrac- 

 tion. A still slighter degree of depression results in a mere 

 lengthening of the time of conduction. 



The mechanism which results from stimulating the right vagus 

 nerve and the negative chronotropic effect of the left nerve are 

 not discussed at present. 



