Legends to Figures
Figure 1. The images are made by reconstructing the developing systemic venous tributaries in developing human embryos, using the interactive pdf files available from Reference #2. The developing heart is shown as viewed from behind. At Carnegie stage 12 (panel A), both sinus horns are in direct continuity with the atrial component of the developing heart tube (white arrows with red borders). One stage later, as shown in panel B. the left sinus horn has become incorporated into the developing left atrioventricular junction, losing its connection with the left side of the developing atrial component (white star with red borders). The systemic venous sinus then connects only with the cavity of the developing right atrium. Note the location of the dorsal mesocardium, which has served as the fulcrum for the remodelling.
Figure 2. The images are from a three-dimensional episcopic dataset prepared from a human embryo at Carnegie stage 14. The venous valves now mark the boundary between the systemic venous sinus and the developing right atrium, as seen in the four chamber section shown in Panel A. Panel B is a section showing how the systemic venous tributaries now open within the confines of the venous valves. With further development, the left sinus horn will become the coronary sinus.
Figure 3. The images are four-chamber sections through three-dimensional episcopic datasets prepared from developing mouse embryos. Panel A shows the arrangement at embryonic day 14, when septation is complete, while panel B shows the situation at term. Both venous valves persist.
Figure 4. The images show persistence of the right venous valve in two human hearts. Panel A shows an example where the valve has persisted as the Eustachian valve, guarding the mouth of the inferior caval vein, as a prominent Thebesian valve guarding the mouth of the coronary sinus, but also as a valve guarding the mouth of the superior caval vein. The valve at the orifice of the superior caval vein is reinforced by a muscular strap, which is the spurious septum. The arrow shows how the strap passes in front of the entrance of the superior caval vein to the right atrium. Panel B shows an example where the valve has become aneurysmal, producing a windsock that has prolapsed through the tricuspid valve.
Figure 5. The images show various human hearts with persistence of the left venous valve (white arrow with red borders). Panel A shows a four chamber cut across the oval fossa. In panel C, there is also persistence of the venous valve guarding the mouth of the superior caval vein (red arrow with white borders).