THE VASCULAR SYSTEM 



349 



the two become reduced to a single one, not through so com- 

 plicated a process as in the former case, but through the com- 

 plete suppression of the right vein. The anterior portion 

 of the left, also, shares the same fate, and the umbilical vein 



ud orns us 

 omd 



b 



ud 



2 



\ 

 S 



s 

 \ 

 \ 



us 



ud ouid or 



FIG. 99. Development of the hepatic portal system in mammals. [After 



HOCHSTETTER.] 



j, anterior cardinal (jugular); s, posterior cardinal; c, ductus Cuvieri; omd, cms, 

 right and left omphalo-mesenteric veins; ud, us, right and left umbilical veins; 

 sz', sinus venosus; x v x v x y commissures between the omphalo-mesenteric veins of 

 the two sides; y, y, and v, v, beginnings of the hepatic and portal capillaries in 

 the liver; a, ductus venosus Arantii. 



finally establishes a direct connection with the heart through 

 the ductus venosus Arantii (Fig. 99, c). 



The fate of the veins anterior to the heart, when compared 

 with that of those posterior to it, is a very simple one, for 

 while in the latter region three veins in succession have held 

 the supremacy, the sub-intestinal, the posterior cardinal, and- 

 the postcava, anteriorly the first to appear are the anterior 

 cardinals, and it is these very vessels which in the higher mam- 



