The identity and effectiveness of the collaterals which compensate for occlusion of a large blood vessel are of perennial interest. The collaterals for the human vena cava have been considered in some detail by older writers, especially Hallett,7 Sappey-Dumontpallier,13 and Pleasants11; but these studies are based largely on postmortem dissection, and leave unanswered the question of choice of pathway during life. For this reason alone, a reconsideration of the problem, by means of venography during life, would appear desirable.
The problems posed by occlusion of the inferior vena cava are quite different when the closure is of the hepatic, renal, or infrarenal segments of the vessel. Here we shall address ourselves to occlusion below the renal veins. Patients available for study were those who had had a low ligation of the vena cava, performed mainly because of pulmonary embolism, and others in whom thrombosis alone had caused