Although it is established that a complete transverse lesion of the stem of the auriculoventricular conducting system or of its two main branches results in auriculoventricular dissociation, and although a few years ago the problem of heart-block seemed almost settled by the discovery of lesions in the conducting system, it is now apparent that the solution of this problem is not so simple as it appeared at that time. There are cases on record in which heart-block was proved beyond a doubt, but still no lesions in the hearts adequately explaining the failure of the conduction of impulses from auricle to ventricle were to be found.†
The three cases to be presented show three very different histological conditions in the conducting system; and it is somewhat unfortunate that the clinical data do not give us such definite information of the mechanism of the heart as can be positively