(a) Four-chamber section through the heart showing the offset arrangement of the mitral valve and tricuspid valve which produces the so-called muscular atrioventricular septum () and the deep infolding of the atrial wall superior and inferior to the floor of the oval fossa (dotted lines). (b) Short axis section across the atrial chamber to show the thin flap valve () and the muscular rim of the oval fossa (arrow). Note the atrioventricular valves, the vestibule of the left atrium (dotted line), and the different shape and size of the atrial appendages. (c) and (d) Longitudinal sections through the pulmonary venous component showing the orifices of the right and left PVs and the ostium of the left atrial appendage; the flap valve of the oval fossa overlaps () the rim to form the septal aspect of the left atrium. (e) A magnification of the left aspect of the interatrial septum. Note that, apart from a small crescent-like edge (arrows), the left atrial side of the septum can be seen by transillumination of the oval fossa () in the right side. In the case of patent foramen oval, the LA can be accessed from the right atrium (RA) through a crevice () that is the last part of the valve to be sealed to the rim. (f) Short axis section across the left atrial chamber. Note the nonuniform thickness (arrows) of the left atrial wall. MV = mitral valve, TV = tricuspid valve, SCV = superior cava vein, ICV = inferior cava vein, RAA = right atrial appendage, LAA = left atrial appendage, PT = pulmonary trunk, Ao = aorta, LI = left inferior pulmonary vein, LLR = left lateral ridge, LS = left superior pulmonary vein, RI = right inferior pulmonary vein, RS = right superior pulmonary vein, and CS = coronary sinus.
