The function of originating and transmitting the automatic impulses which result in the contraction of the heart belongs to certain specialized tissues which represent the remnant of the primitive cardiac tube. These excitation waves have been shown to arise at the junction of the superior vena cava with the right auricle, in the upper portion of the sino-auricular node. From here they pass through the node and are transmitted by numerous paths to the auricular musculature and to the atrioventricular node described by Tawara. From this node stimuli pass through the bundle of His and into its branches, which divide and subdivide until their ramifications form an intricate network by means of which the impulses are rapidly distributed to the ventricular myocardium. As impulses travel downward through these conducting fibers, the heart responds by sequential contractions of the auricles and ventricles. Under certain conditions, however, any portion of this conducting