Longitudinal sections at the mitral isthmus to illustrate its anatomic relations with the great cardiac vein/coronary sinus and left circumflex artery. (a) This cut shows in profile the mitral isthmus between the mitral annulus and the orifice of the left inferior pulmonary vein. (b) and (c) These corresponding histological sections stained with Masson’s trichrome show the irregular length and thickness of the atrial wall and the relationship to the great cardiac vein/coronary sinus. Note in (b) the space between the pectinate muscles where the left posterior atrial wall becomes thinner (arrow). LI = left inferior pulmonary vein, MV = mitral valve, LCx = left circumflex artery, and GCV = great cardiac vein.
