Fig. 23-5. The external and internal anatomy of the heart. A, Right atrium and right ventricle, showing the tricuspid orifice. The arrows indicate the circulation of the blood. An arrow in the fossa ovalis represents "probe patency" of the foramen ovale. The membranous part of the interventricular septum lies mostly under cover of the septal cusp. The asterisk indicates the supraventricular crest. The septomarginal trabecula is frequently called the moderator band. B, Interior of the left atrium, dorsal view. The openings of the pulmonary veins (variable in number) and the left side of the fossa ovalis can be seen. C, Cross section through the ventricles, showing the much thicker wall of the left ventricle and the curved shape of the interventricular septum. D, Left ventricle, showing the mitral valve. E, Aortic valve, spread out. F, Tricuspid valve, showing the papillary muscles, chordae tendineae, and cusps (or leaflets). A., anterior cusp of aortic valve; Ao., aorta; C.S., opening of coronary sinus; I.V.C., inferior vena cava; I.V.S., interventricular septum; L., left cusp of aortic valve; L.C., opening of left coronary artery; L. V., left ventricle; P., posterior cusp of tricuspid valve; P. T., pulmonary trunk; R., right cusp of aortic valve; R.C., opening of right coronary artery; R.V., right ventricle; S., septal cusp of tricuspid valve; S.V.C., superior vena cava. (Based on Testut and other sources.) Click for high resolution image.