ON THE DIFFERENCES IN THE EFFECTS OF STIMULATION OF THE TWO VAGUS NERVES ON RATE AND CONDUCTION OF THE DOG'S HEART

It may be concluded from the results obtained in these experiments : 1. That stimulation of the right vagus nerve in the dog usually causes arrest of all the chambers of the heart. 2. That stimulation of the left vagus nerve exerts a moderate negative chronotropic effect on the auricles. 3. That stimulation of the left vagus nerve has a profound effect on the conduction of impulses over the auriculoventricular system. 4. That the degree of effect exercised on the auriculoventricular system by stimulation of the left vagus nerve varies. In some dogs conduction is depressed to an extent which causes only a delay in the conduction of impulses from auricles to ventricles (P-R time) ; in other dogs the conduction is depressed to a degree which results in incomplete heart-block; while in still other dogs conduction is so depressed that although the auricles continue to contract, no impulses pass from them to the ventricles. 5. That when stimulation of either the right or left vagus nerve causes asystole of nomotopic ventricular contractions, ectopic ventricular contractions may occur. 6. That the time which elapses before ectopic ventricular contractions occur depends upon the irritability of the ventricular muscle, and this may vary in different dogs. 7. That stimulation of the left vagus nerve may rarely cause sino-auricular block. Possibly stimulation of the right nerve may also produce this effect. 8. That there is consequently usually a great qualitative difference in the action of the two vagus nerves on the heart of the dog.