x.] THE CIRCULATING SYSTEM. 425 



meseraic veins, but also sends a branch directly to the sinus 

 venosus. 



Then the combined omphalo-meseraic and umbilical veins 

 break up into the portal circulation the branch direct from 

 the latter to the sinus remaining however for a time as the 

 dnctus venosus. The part of the omphalo-meseraic vein 

 near the heart which persists undivided thus, becomes the 

 hepatic vein. 



In the meantime the vena cava inferior arises as a large 

 median vein (superficial to the aorta), receiving accessions 

 from the pelvic limbs and from the kidneys. It intrudes, as 

 it were, upon the hepatic veins, and gives its name to the 

 vein directly entering the heart, into which the hepatic vein 

 is described as opening in the adult. 



After a time the posterior cardinal veins become discon- 

 tinuous with the anterior ones, and grow into and become the 

 azygos veins. 



The anterior cardinal veins become the jugular and inno- 

 minate veins. The left ductus Cuvieri aborts, and the left 

 anterior cardinal vein proceeds to join the right ductus Cuvieri ; 

 the united trunk is thus transformed into the vena cava 

 superior. 



While the sinus venosus becomes indistinguishably united 

 with the right auricle, the ductus venosus becomes ob- 

 literated, so that all the blood from the stomach and 

 intestines passes into the portal circulation on its way to the 

 heart. In the meantime the umbilical vein (which is, as it 

 were, the root of the portal and primitive hepatic veins) in 

 part aborts, becoming mere fibrous cord ; in part, however, 

 it persists, namely as the small internal iliac vein. 



Concomitantly the vena cava inferior greatly increases in 

 relative size, as also do its roots the external iliacs. No veins 

 are, on their way to the heart, re-distributed in the kidneys in 

 the same way that the portal vein is in the liver. 



10. Turning now to the general condition of the venous 

 system, we find that the main conditions existing in adult 

 man are those of his class, but that there may be certain 

 variations as to the union of the larger trunks, and also as 

 regards the presence of venous retia mirabilia similar to 

 those already noticed with respect to the arterial portion of 

 the circulating system. 



These retia seem to attain their maximum in the abdo- 

 minal region of Cetaceans, e.g. the Porpoise. The two 

 azygos veins may be much more equal in size than in man, 



