SUMMARY During anatomical practice between 2001-2002 at the Tokyo Women’s Medical University, a

defect in the hepatic segment of the inferior vena cava was encountered in a 94-year-old male cadaver. Although this type of defect has often been observed clinically using imaging diagno-sis, the topological changes in the small vessels and surrounding structures have not been des-cribed previously. Here, the topological changes in small vessels and surrounding structures, in addition to the morphology of a defect in the hepatic segment of the inferior vena cava, are described in detail. Normal variations in the venous system were also examined using 26 human cadavers for comparison. The following results and interpre-tations were obtained: The thoracic portion of the anomalous vein originated from the azygos system, judging from the topological relationships among the main venous trunk, small vessels, and the autonomic nerves. Although a close relationship between the inferior vena cava and the azygos system was recognized, the development of each venous system should be regarded as independent, based on the normal variations observed in 26 human cadavers and the currently available medical literature. The boundary between the cardinal vein (inferior vena cava) and the azygos system might be the lower border of the renal vein, since the renal vein drained into the ventral aspect of the inferior vena cava and the left renal artery ran superficially across the abdominal portion of the anomalous vein. Examination of topological changes in the surrounding structures may provide important clues regarding the morphogenesis of defects in the hepatic segment of the inferior vena cava and normal venous systems