VENOUS SYSTEM 



429 



A renal portal system occurs in connection with the embryonic 

 kidney in all Sauropsida, and traces of it can also be recognised in 

 Mammalian embryos, being particularly well seen in those of 

 Echidna. In adult Reptiles and Birds other connections of the 

 vessels are set up, so that only indications of the renal portal 

 system are usually retained, and in Mammals are entirely wanting. 



As in Fishes, the first veins to appear in the embryo are the 



FIG. 330. DIAGRAM SHOWING THE RELATIONS OF THE POSTERIOR CARDINAL 



AND POSTCAVAL VEINS IN A, THE RABBIT, AND B, MAN. (After 



Hochstetter.) 



V.c.i, postcaval ; V.c.p.d, V.c p.s, right and left posterior cardinals; 

 VM.int.comm, common internal iliac vein; V.il.x.c, common iliac vein; 

 V.f.I, lumbar vein ; V.r.d, V.r.s, renal veins. 



vitelline or omphalo-mesenteric veins (Fig. 331, A), bringing back 

 the blood from the yolk-sac, and uniting into a single trunk before 

 opening into the sinus venosus. As the liver becomes developed 

 it surrounds this trunk, which sends advehent branches into it, 

 revehent branches returning the blood into the anterior section 

 and eventually giving rise to a right and a left hepatic vein. 

 Thus a portal circulation arises, and the main trunk of the vein, 



