40 CHARLES F. W. MCCLUBE AND GEORGE S. HUNTINGTON 



supracardinal system of veins, may be classed under two gen- 

 eral groups : 



Group I includes those cases in which blood from the iliac 

 veins is returned to the heart by the supracardinal system of 

 veins, as well as by the usual and normal route through the 

 liver. 



Group II includes those cases in which all blood from the 

 iliac veins is returned to the heart by the supracardinal sys- 

 tem of veins, and in which the usual and normal route through 

 the liver, via the prerenal division of vena cava, has not 

 been established. 



Figures 54 and 55 are excellent examples of cases which 

 fall within group I. In each instance blood from the iliac 

 veins has, in returning to the heart, a choice of two routes : 

 one through the liver, by the prerenal division of the vena 

 cava, and the other, by the supracardinal system of veins, 

 which open into the superior vena cava and into the left 

 innominate vein. As shown in figure 54 (fig. 5, Fawcett, '98), 

 the embryonic condition of a bilaterally symmetrical and con- 

 tinuous system of supracardinal veins in the lumbar and the 

 thoracic regions has actually been carried into the adult 

 stage. In this case the supracardinals are represented in the 

 lumbar region by the right and the left caval veins (Type 

 BC), and, continuous with the latter in the thoracic region, 

 by the azygos veins. In addition to the supracardinals, there 

 is present the prerenal division of the vena cava inferior. 

 This vessel communicates with the supracardinals at the 

 renal level, and from this point extends craniad through the 

 liver to open into the right auricle. The left azygos has been 

 designated by Fawcett (fig. 54) as a derivative of the left 

 cardinal vein. This view we now know to be incorrect. With 

 the exception of the proximal end of the right and the left 

 azygos veins, which are derived from the posterior cardinal 

 veins (Pc.l in fig. 1), the continuous channel in the lumbar 

 and the thoracic regions on each side of the body has been 

 derived from the supracardinal system of veins. In figure 55 

 (fig. 1, Lauber, '01) is shown a similar condition, less dia- 



