THE VASCULAR SYSTEM 1 79 



mesentery a hepatico-subcardinal anastomosis (P'ig. 89B). 

 The result of this diversion of the blood stream through the 

 liver results in the dwindling of the postcardinals craniad of 

 this point. The growth of the pelvic regions of the embryo 

 due to the developing limbs is accompanied by an increase 

 in the size of the posterior part of the postcardinals (Fig. 89C). 

 There now appears a new pair of vessels in the prevertebral 

 regions, the supracardinal veins, which are destined to play a 

 very important part in the formation of the postcava. They 

 make connections with the postcardinals in the caudal regions 

 and also cranially (Fig. 89D). In the pig according to Sabin, 

 the supracardinal are more directly related to the duct of 

 Ciivier than to the postcardinals in that region. An anasto- 

 mosis is also made with the subcardinals at about the level of 

 the intersub cardinal anastomosis, the subcardino-supracardinal 

 anastomosis. In the same region there is also an intersupra- 

 cardinal anastomosis. The aorta is thus entirely surrounded 

 by a venous ring, known as the renal collar. The renal collar, 

 therefore, ''is formxcd by the pars subcardinalis of the postcava, 

 the intersubcardinal anastomosis, the right and left subcardino- 

 postcardinal anastomoses, the right and left supracardinals, 

 and the anastomoses between the supracardinals dorsal to the 

 aorta at this point, at which level the renal veins enter the col- 

 lar." (Fig. 89E.) Further development results in the complete 

 degeneration of the anterior segment of the postcardinal except 

 such part as may enter into the formation of the azygos and 

 such other changes as may be seen by consulting the diagrams 

 in figure 89. 



''The embryonic veins which typically enter into the for- 

 mation of the adult postcava are the right supracardinal 



bar division); d, duct of Cuvier; e, pars hepatica of postcava (hepatic communis 

 and ductus venosus Arantii);/, subcardinal; g, gonad; h, pars subcardinalis 

 of postcava (intersubcardinal anastomosis); i, supracardinal; 7, kidney; ;^, 

 renal collar (subcardino-supracardinal anastomosis); /, adrenal gland; m, vein to 

 adrenal; n, precava; o coronary sinus; p, left innominate; q, internal jugular; r. 

 subclavian; s, external jugular; t, external iliac; u, internal iliac. 



