Fig. 5. (A) Schematic representation of the valve when open. Blood travels from the anterior vena cava (AVC) into the lateral venae cavae (LVC). When blood pressure rises in the LVC relative to the AVC, the valve closes. (B) Blue tracing medium in the AVC (circled). (C) Once tracing medium was pushed from the AVC (circled) into the branch point, it could not be pushed back into the AVC. Scale bar, 1 cm. (D) Close to the lateral wall, the valve spanned the whole vessel (x6 magnification; scale bar, 0.5 mm). (E) Mid-sagittally a natural split occurred in the valve tissue. We verified that it was not an artifact through analysis of successive serial sections. The distal end of the larger portion of the valve tissue was reinforced by a polysaccharide-rich thickening (x6 magnification; scale bar, 0.5 mm). (F) The muscular, small side of the valve, indicated by a broken box in E. Muscle cells stained red (x60 magnification; scale bar, 50 µm). AVC, anterior vena cava; BP, branch point; LVC, lateral venae cavae; PAV, posterior azygos vein.