THE MAMMALIAN VENA CAVA POSTERIOR 25 



one on record in which a periureteric venous ring on the 

 left side has been described as occurring in an adult mammal. 



In the early stages of development a symmetrical arrange- 

 ment of the main axial veins is a marked feature. One of the 

 earliest changes which transposes the symmetrical embryonic 

 plan to an asymmetrical one is the discontinuance of the left 

 periureteric venous ring, through a break which occurs in the 

 left posterior cardinal, near the point where the vein is 

 crossed by the ureter (Pc. and Ur. in fig. 13). The early 

 disappearance in ontogeny of the left periureteric venous 

 ring undoubtedly accounts for the fact that this ring is rarely 

 carried as a variant into the adult stage. 



\Yhen compared with the composite ontogenetic plan of the 

 veins (fig. 1), as typical an example of Type ACD as could 

 exist is shown in figure 17. In the absence of an anastomosis, 

 dorsal to the aorta, between the right and the left common 

 iliac veins, all the blood from the right iliac veins is directed 

 into the right posterior cardinal (right cava), about which the 

 ureter winds. The right lumbar cava is composed only of 

 the right posterior cardinal (A, fig. 1) and the right subcar- 

 dino-posterior cardinal anastomosis (Subc.Pc.Anast., fig. 1). 

 The lateral portion of the renal collar (Subc.Sprc.Anast.) 

 does not enter into the formation of the cava when the cava 

 is formed by a posterior cardinal vein. A typical peri- 

 ureteric venous ring is present on the left side (fig. 17). The 

 dorsal circumference of the ring is formed exclusively by 

 the left supracardinal (C in figs. 1 and 17) and is of less 

 importance functionally than the ventral circumference of 

 the ring, which has been derived from the left posterior car- 

 dinal vein (D in fig. 1). 



S. Persistence of right and left posterior cardinal r< / 



Type AD (A and D in fig. 1). Figures 18, 19, 20, 21, 22, 23, 24, cat (Darrach 

 Series), and figure 25, man (Gladstone, '12) 



Figures 18 to 24, inclusive, represent different variations 

 of Type AD, in which two caval veins are present in the 

 lumbar region, derived, respectively, from the right and the 



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|u_i LI B R A R Y 

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