The Predominant Influence of the Left Vagus Nerve upon Conduction between the Auricles and Ventricles in the Dog

PLATE 74. It has been shown by one of us (I) that the normal production of impulses in the region of the sino-auricular node is inhibited by stimulation of the right vagus nerve to a greater extent than by stimulation of the left. Earlier observations, which are fully discussed in that paper, had indeed indicated that such might prove to be the case ; but a clear demonstration of the preponderating influence of the right vagus upon the sino-auricular node, in which structure the normal impulses are almost certainly generated, had not been published. In the same paper, as in the discussed observations of previous workers, it was stated that the left vagus had an influence upon the tissues uniting auricle and ventricle which was not obtained on stimulating the right vagus nerve, stimulation of the left vagus nerve producing high grades of heart-block. A comparison of the right and left vagi in this respect has not been possible in previous experiments , for the reason that the right vagus slows the auricle to a greater extent than does the left. It seemed possible to us that the profound effect of left vagal stimulation upon conduction over the A-V junction, and the absence of such effect or the observation of a lesser effect upon stimulation of the right vagus upon the same region of the heart, may have been due to the usual difference in the retardation of auricular rate. For we know that auriculoventric-ular heart-block is favored by high auricular rate (2 and 3). We * Dr. Lewis's expenses have been defrayed by grants from the Graham Research Fund and the Royal Society. The observations were undertaken in London. 740 Influence of Left Vagus Nerve upon Conduction. therefore determined to examine the influence of right and left vagal stimulation upon A-V conduction while the auricular rate was maintained artificially at a constant level. METHOD. Dogs I were employed exclusively and were anesthetized with morphia, paraldehyde, and ether. Records of the heart beat were takefi electrocardiographically, lead II being utilized. The cut vagi have been stimulated by means of a faradic current, the primary circuit of the inductorium being excited at a constant voltage, the secondary coil being placed at fixed points, and the leads from secondary coil to right and left nerves, running through a simple mercurial commutator, having equal resistances. The right and left vagus nerves have been …