The distributing artery of the conducting system of the heart is occasionally injured in cardiac surgery. The aim of this study was to define the anatomic characteristics of the principal arterial source of the sinu-atrial node and atrioventricular node. Furthermore, the morphology of the tendon of Todaro was clarified. Thirty hearts were studied by gross anatomic methods, and the exact area of the conducting system was supported by histologic observations of four hearts. The sinu-atrial node was supplied by the right coronary artery more frequently (73% of cases) than by the left (3%), and in 23% of cases this node was supplied by both coronary arteries. The atrioventricular node was supplied by the right coronary artery (80% of cases) more than by the left (10%), and in 10% of the cases this node was supplied by both coronary arteries. The atrioventricular bundle branch arose from the right coronary artery in 10% of cases, the left coronary artery in 73%, and both coronary arteries in 17%. Most of the blood to the right bundle (the moderator band) was supplied by the interventricular septal branches of the anterior interventricular branch from the left coronary artery. Finally, all the arteries of the right bundle and left bundle were defined to be derived from left coronary arteries.

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Three of resident cells of the hepatic sinusoidal wall and in connection with these, the hepatic parenchyma were electron microscopically observed in four species of birds. Avian hepatocytes were characterized by densely packed, abundant large mito-Hepatic Parenchyma and Sinusoidal Cells in Avian Livers 327

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