IV.—The Atrioventricular Pathway for Conduction of the Impulse for Cardiac Contraction in the Dog

Abstract

Synopsis: It is generally believed that in mammals the impulse for cardiac contraction is conveyed from the atria to the ventricles by the atrioventricular bundle and its branches. This belief is mainly based on the A-V. dissociation which is found to follow experimental injury of the bundle. Since the bundle contains nerves, as well as specialized muscular elements, these experiments do not conclusively prove which structures, nerves or muscle, actually conduct the impulse. In the dog it has been previously shown that the right limb of the bundle is not usually accompanied by nerves, and in the present work the attempt was made by means of very small incisions to cut the muscle (Purkinje) fibres of the limb without damaging nerves in the ventricular septum, the effect on the electrocardiogram being noted. In 5 out of 38 perfused hearts of dogs the experiment was successful, and resulted in a significant delay in the contraction of the right ventricle relative to that of the left. The experiments also revealed that very considerable damage can be inflicted on the nerves in the ventricular septum without altering the electrocardiogram, so long as the limb itself is not injured. The results are regarded as providing direct evidence that the cardiac impulse in the dog is transmitted to the right ventricle by the muscular elements of the right limb of the bundle, and it is submitted that there would seem to be no valid reason for supposing that conduction throughout the A-V. bundle and its limbs is other than myogenic.