(a) Longitudinal section through the roof of the left atrium showing the endocardial view of the left atrium and right ventricle. Note the prominent left lateral ridge between the ostium of the left atrial appendage and the orifice of the left superior pulmonary vein. In this specimen, extra pectinate muscle trabeculations are extending inferiorly from the appendage to the vestibule of the mitral valve (arrows). (b) Posterolateral view of the left atrium and left ventricle to show the myofiber arrangement in the subepicardium of the left lateral ridge, left pulmonary veins, and the trajectory of the oblique vein of Marshall. Note that the vein of Marshall is in direct contact with the myocardium of the ridge. (c), (d), and (e) Cross-histological sections of the left lateral wall of the left atrium stained with Masson’s trichrome. (c) shows the oblique vein of Marshall in direct contact with the myocardium of the septoatrial bundle. (d) and (e) show the epicardial ganglion () and autonomic nerve bundles (arrows) in the vicinity of the oblique vein of Marshall. LAA = left atrial appendage, PT = pulmonary trunk, Ao = aorta, MV = mitral valve, RV = right ventricle, CS = coronary sinus, LI = left inferior pulmonary vein, LS = left superior pulmonary vein, LLR = left lateral ridge, RI = right inferior pulmonary vein, RS = right superior pulmonary vein, OVM = oblique vein of Marshall, and LCPV = left common pulmonary vein.
