The study of the heart beat by means of modern instruments of precision, such as the polygraph and the string galvanometer, has revealed the fact that the graver forms of cardiac arrhythmia, as heart block and auricular fibrillation, are much more common than was previously suspected. It has also been recognized that such arrhythmias as paroxysmal tachycardia, flutter and premature contractions (extrasystoles), are notoriously transient in most cases, but until very recently heart block (especially complete block) and auricular fibrillation were looked on as being nearly always permanent conditions due to extensive organic changes. In 1910, however, I was able to show that complete heart block may exist for years without demonstrable lesion of the bundle of His at autopsy, and similar cases have occasionally been reported since that time (see Ref. 8). In the case of auricular fibrillation, I have recently shown 2 that transient