The effect of injecting morphin in right vagus dogs is to slow or practically to stop auricular systoles, the circulation being maintained by ectopic, independent, ventricular contractions. The function of conduction is, relatively speaking, undisturbed. The fact that after morphin injections there is little or no disturbance of conduction in right vagus dogs and a profound one in left vagus dogs may be used as a factor in distinguishing between them, especially when there is a reduction in rate greater than usual in left vagus dogs. The effect on left vagus dogs is to slow the rate of the auricles moderately and to increase the length of conduction, so that partial auriculoventricular block or complete auriculoventricular dissociation results. These are precisely the results that have been obtained by faradic stimulation of one vagus nerve, the other being divided. There are slight differences in the results obtained between the two methods, but the explanation for these is probably to be found in the differences in the nature of the experiments. When faradic stimulation is employed, the stimulus is applied a short time only,—in our experiments for periods not longer than ten seconds. The occurrence of the maximum effect is sudden and does not provide for a gradual introduction of the ventricles to new conditions. In right vagus stimulation the ventricles usually stop beating. A gradual introduction is, however, not always necessary, as may be seen in figures 5 and 6 of the paper (4) already quoted, where idioventricular rhythms began without delay. Another difference between the two methods is found in the degree of effect produced on stimulating the left vagus; in the faradic method the change in conduction varies from mere lengthening to a condition so profound that complete dissociation results; these degrees have been described. The morphin method usually produces only incomplete dissociations. Twice only was complete dissociation produced.
The similarity between the results of the two methods is sufficiently close to render it likely that in obtaining them an identical mechanism in the heart is involved. It may, therefore, be concluded that the inconstant action of the vagus resulting from morphin injections, called "ungleichartig" by Einthoven and Wieringa, appears so on account of the fact that the predominating effect of morphin may be exercised now on one and now on the opposite cardio-inhibitory system, and not, as was suggested, on account of a shifting of predominance from one to another of the fiber tracts in the vagi themselves. The explanation offered as the results of this series of experiments differs from theirs. The results obtained substantiate the conclusion reached in the former series, that the two vagi act differently. A fact relating to the mechanism of the right vagus nerve can now be added, from a consideration of the cardiac mechanism in right vagus dogs, namely, that derangements in conduction result, to a very slight extent only, as the effect of the influence of the right vagus nerve
Although no solution of the production of the extreme grades of sinus irregularity seen in morphin intoxication is offered, the failure of this rhythm to be converted into a more profound irregularity may be explained. There seems little doubt that the occasion for the occurrence of an independent ventricular rhythm in these experiments was due to the slowness of the auricular rate resulting from morphin injection. But in the cases of sinus irregularity (Nos. 683, 685, and 700), the rate of the auricles was never sufficiently low for the ventricles to initiate an independent rhythm. The lowest rates recorded in them were 66.3 (No. 683), 85.7 (No. 685), and 91.4 (No. 700), and it appears that before an independent ventricular rhythm occurred in any of the experiments, the auricular rate had always fallen below 41 (Nos. 686, 688, 697, 698, 706, 715, and 721). On the other hand, the ventricular rates in the cases of sinus irregularity were higher than the highest idioventricular rates observed and no advantage could consequently have been gained by the onset of a new rhythm. No. 683 was an exception, but the rate in this case exceeded the idioventricular rates of all but Nos. 702 and 706 and was only 11.1 beats below that of the highest recorded. Whether sinus irregularity itself is a morphin irregularity the result of a sino-auricular block, in the sense of Eyster and Meek, has already been discussed.