Heart block is ordinarily understood to be a condition in which there is impairment in the conduction of impulses from auricles to ventricles. The first stage of this process is represented by a mere delay in the conduction time recognized by an increase in the a-c interval in jugular tracings and in the P-R time in electrocardiograms. In this condition all impulses reach the ventricle; but if the delay is sufficient or if the heart rate is rapid, an impulse may occasionally fail to reach the ventricle, giving partial heart block. Finally, there is a condition in which none of the auricular impulses reach the ventricle, and the auricles and ventricles each beat of their own rhythm and independently of one another. These three stages may be termed, first, delayed conduction, second, partial heart block, and third, complete heart block. The cause of this disturbance in