THE MAMMALIAN VENA CAVA POSTERIOR 31 



10. Persistence of right and left supracardinal veins 



Type BC (B and C in fig. 1). Figures 33, 34, 35, man (Huntington Collection), 

 36, 37, 38, 39, 40, 41, 42, and 43, cat (Darrach Series) 



The atypical venae cavae illustrated under Type BC pre- 

 sent a variety of forms in which both the right and the left 

 supracardmal veins of the embryo persist in the lumbar 

 region, either wholly or only in part, in the adult cat and in 

 man. 



Figure 33 is perhaps a typical example of a case in which 

 both lumbar supracardinal veins of the embryo have in their 

 entirety been carried into the adult stage. In such cases the 

 left cava (C in fig. 1) joins the cava of the right side (B), 

 ventral to the aorta, and approximately at the level of the 

 renal veins. As in all examples of Type BC, the ureters in 

 figure 33 extend from the kidneys to the bladder without pass- 

 ing between the aorta and the caval (supracardmal) veins. 



In figure 34, also of man, both lumbar supracardinals are 

 present in their entirety and, as in figure 33, the left cava 

 joins the cava of the right side, ventral to the aorta, at the 

 level of the renal veins. A distinguishing feature of this 

 variant, however, is that, functionally, the cava of the left 

 side is the more important vessel of the two. Also, at about 

 the middle of the lumbar region, the right cava receives a 

 large spinal vein and, cranial to its union with this vein, the 

 right cava is a vessel of much greater caliber than is that 

 section of the cava which lies caudal to the spinal vein. 



In figures 35 (man) and 36 (cat), both lumbar supra- 

 cardinals are present in their entirety and, as in figures 33 

 and 34, the left cava joins the cava of the right side, ventral 

 to the aorta, at the level of the renal veins. A distinguishing 

 feature of these two variants is that, dorsal to the aorta, the 

 right cava and the left again unite with each other, at a level 

 somewhat caudal to that of the renal veins. This dorsal union 

 between the two veins is due to the persistence of an inter- 

 supracardinal anastomosis (Int.Sprc.Anast., figs. 1 and 3), 

 which is present in the embryo, at different levels, between 

 the supracardinals of opposite sides. Through the presence 



