The images show the essence of the technique used to calculate the histological extent of the inferior extension of the atrioventricular node in serially sectioned human hearts. The left panel shows the gross features of the triangle of Koch and the cavotricuspid isthmus, with arrows indicating the anticipated sites of the fast and slow pathways. Lines B-B and C-C show the corresponding histological sections as shown in Panels B and C. Panel B shows the last input to the conduction axis prior to its insulation by the fibrous tissues of the atrioventricular junctions. This is suggested to be the substrate of the fast pathway. Panel C shows the compact atrioventricular node and its inferior rightward extension. We tracked the inferior extension until it could no longer be distinguished histologically from the vestibular myocardium.