That destruction of the auriculo-ventricular bundle of His in the mammalian heart will always cause heart-block has been repeatedly proved both experimentally and by numerous clinico-pathological observations within the last five years.1 The converse of this proposition, however, that heart-block is always caused by destruction of the aforesaid bundle, has by no means been proved, and, in fact, it is most probable that there are several other causes that may produce this dissociation of auricular and ventricular rhythm. What these causes may be it is not yet possible to say ; this paper is merely to show that complete heart-block and the symptoms of Adams-Stokes' syndrome may exist, not only without destruction of the bundle of His, but with insignificant lesions no greater than those found in normal bundles. Furthermore, following the principles established by the experiments of Erlanger and Hering, it has generally been supposed that partial