The myocardial fibres of the posterior wall of the atrio-venous junctions were examined in 35 large domestic mammals. In the majority of specimens a common pattern in the course and organization of the fibres could be observed. The most obvious features were the following: (1) a main circular fascicle surrounding the pulmonary trunks; (2) fibres encircling the atriovenous junctions; and (3) myocardial sleeves extending along the veins, occasionally as far as the lung. The superior part of the left atrial wall was consistently thicker than the inferior section. Individual variations of this wall between the various trunks followed one of four patterns—vertical, oblique, horizontal or criss-crossed. Differences between mammal and human hearts were found regarding the number of pulmonary trunks, the presence of the oblique vein of the left atrium, and the extension of the myocardial sleeves on the caval vein. This extension on the caval vein continues over the end of the azygos vein in animals. The functional significance of the structures described in this study is discussed.

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