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



We may now proceed to a detailed consideration of the 

 atypical conditions of the vena cava posterior as found in 

 the adult cat and in man. 



1. Persistence of right posterior cardinal vein 



Type A (A in fig. 1). Figures 6 and 7, cat (Darrach Series), and figure 8, 

 man (Gladstone, '12) 



A single main axial vein, to the right of the aorta, mani- 

 fests itself as the persistent right posterior cardinal vein 

 (vena cava posterior) by its relation to the right ureter, 

 which lies at first dorsal to the vein, emerges between the 

 latter and the right side of the aorta, and descends caudo- 

 lateral over the ventral surface of the cava. 



As shown in the reconstruction (fig. 2) and in the com- 

 posite diagram (fig. 1), the right side of the renal collar 

 (Subc.Sprc.Anast.) does not enter into the formation of the 

 lumbar cava when in the adult the posterior cardinal vein 

 (A) alone persists to form the cava. A vena cava posterior 

 of Type A develops along the following pathways in figure 1 : 

 pars hepatica (P. Hep), pars subcardinalis (P.Subc.), inter- 

 subcardinal anastomosis (Int.Subc.Anast.), subcardino-pos- 

 terior cardinal anastomosis (Subc.Pc.Anast.) and right 

 posterior cardinal vein (A). In the examples of Type A rep- 

 resented by figures 6, 7, and 8, the iliac veins of the left side 

 have established their connection with the cava (A, fig. 1) by 

 the persistence of an anastomosis, dorsal to the aorta, between 

 the main axial veins of opposite sides. 



In figures 6, 7, and 8, the sex vein on the right side opens 

 into the right posterior cardinal (cava) and on the left side, 

 into the left renal vein. The anastomosis between the sex 

 veins of opposite sides in figure 6 is due to the persistence of 

 an embryonic intersubcardinal anastomosis (Int.Subc.Anast., 

 fig. 1), ventral to aorta, somewhat caudal to the renal level. 

 Darrach observed this anastomosis in only two cases of his 

 series of 605 adult cats. In Type A of the cat, the right sex 

 vein (V . spermatic a] is derived from the subcardinal plexus 

 (S.V., fig. 1) of the right sex gland, together with one (fig. 6) 



