630 



Comparative Morphology of Chordates 



Veins 



In the arrangement of the veins, the one major difference between 

 mammals and reptiles has to do with the renal-portal system. In 

 Anamnia the blood from the caudal and pelvic regions is collected 

 into a pair of renal-portal veins, each of which passes into the cor- 

 responding mesonephric kidney and distributes the blood throughout 

 the renal capillary system (Fig. 76). Emerging from the capillaries, it 

 passes into efferent veins which return it to the heart by way of the 

 postcardinal (most fishes) or the postcava (Dipnoi and Amphibia). 

 The blood-supply to the mesonephros is chiefly 

 venous, the renal arteries being relatively 

 small. In embryos of all Amniota the posterior 

 region of each postcardinal vein is in close rela- 

 tion to the mesonephros, and the two veins 

 early establish a typical renal-portal arrange- 

 ment in which blood from the posterior part of 

 the embryo passes through a plexus of fine 

 vessels in each mesonephros. When, in later 

 course of development, the mesonephros is 

 replaced by the metanephros as a definitive 

 kidney, the renal-portal arrangement may ex- 

 tend backward into the territory of the meta- 

 nephros, but with more or less modification 

 and reduction. In adult reptiles there is a renal- 

 portal system, but more or less of the blood 

 which it carries into the kidney passes directly 

 through by way of large veins which do not 

 break up into capillaries. The arterial supply 

 to the metanephros is correspondingly in- 

 creased. In birds there is merely the outer 

 semblance of a renal-portal system (Fig. 478). A large vein commonly 

 called "renal-portal" enters the posterior end of each metanephric kid- 

 ney, carrying blood from the posterior region of the animal, but the 

 vein passes forward through the kidney without dividing into capil- 

 laries and joins the iliac vein, which empties into the postcava. The 

 so-called "renal-portal" may give off a few very small branches into 

 the posterior lobe of the kidney, but it is certain that most, if not quite 

 all, of its blood passes into the postcava without having entered renal 

 capillaries. The blood-supply to the kidne\ is therefore mainl> or 

 entirely arterial. In mammals development of the metanephros is ac- 

 companied by complete obliteration of the renal-portal system. In the 

 adult the veins from the caudal and pelvic regions do not enter the kid- 



circu- 



Fig. 478. Renal 

 lationof bird, (cd) Caudal 

 vein; (cm) coccygeomes- 

 enteric vein; (cr) crural 

 vein; (h) hypogastric 

 vein; (po) postcava. (Af- 

 ter Gegenbaur. Courtesy, 

 Kingsley: "Comparative 

 Anatomy of Vertebrates," 

 Philadelphia, The Blakis- 

 ton Company.) 



