32 CHARLES F. W. McCLURE AND GEORGE S. HUNTINGTON 



of this intersupracardinal anastomosis, dorsal to the aorta, 

 between the cavae of opposite sides, one can in the lumbar 

 region (figs. 35 and 36) distinguish between a cranial and a 

 caudal section of the right and the left caval veins. This 

 distinction is one of considerable importance, as it enables us 

 to interpret correctly those cases in which the left lumbar 

 supracardinal is retained only in part, and not in its entirety, 

 as in figures 33, 34, 35, and 36. If we compare figure 

 37 with figure 36, it is evident that the paired condition of 

 the caval veins in figure 37 has been brought about by the 

 persistence of the cranial section of the right and the left 

 supracardinal veins as seen in figure 36. It is quite possible, 

 however, that the unpaired caudal section of the lumbar cava 

 in figure 37, as is usually the case in the cat, may have been 

 formed by a fusion of both lumbar supracardinal veins (see 

 figs. 2 and 3, in which an extensive anastomosis, dorsal to 

 the aorta, is present between the supracardinal veins of 

 opposite sides). Be this as it may, a fusion between the 

 supracardinals cranially has not occurred, so that we have 

 here a condition in which a paired cava is present, due to the 

 persistence of a cranial section of the right and the left 

 supracardinal veins as shown in figure 37. 



In a similar manner the series of variants of Type BC, 

 illustrated by figures 38 to 43, inclusive, can be interpreted 

 as cases in which the entire right lumbar supracardinal and 

 only a caudal section of the left supracardinal vein have been 

 retained. In figures 38, 39, and 40, in which an anastomosis 

 at the iliac level, between the iliac veins of opposite sides, 

 has not been formed, it is seen that the left supracardinal 

 joins the right supracardinal, dorsal to the aorta, at a level 

 somewhat caudal to that of the renal veins. * If we compare 

 the elongated left common iliac vein in these three variants 

 (figs. 38, 39, and 40) with the caudal section of the left supra- 

 cardinal (left common iliac vein) in figure 36, we observe 

 that, as in the latter, the elongated left common iliac must 

 have been formed to a large extent by the left supracardinal 

 vein, and that its point of union with the right supracardinal, 



