THE VENOUS SYSTEM. 



653 



to the liver. The liver, on its development, embraces the 

 subintestinal vein, which then breaks up into a capillary system 

 in the liver, the main part of its blood coming at this period 

 from the yolk-sack. 



The portal system is thus established from the subintestinal 

 vein ; but is eventually joined by the various visceral, and some- 

 times by the genital, veins as they become successively de- 

 veloped. 



The blood from the liver is brought back to the sinus veno- 

 sus by veins known as the hepatic veins, which, like the hepatic 

 capillary system, are derivatives of the subintestinal vessel. 



There join the portal system in Myxinoids and many 

 Teleostei a number of veins from the anterior abdominal walls, 

 representing a commencement of the anterior abdominal or 

 epigastric vein of higher types 1 . 



In the higher Vertebrates the original subintestinal vessel never attains a 

 full development, even in the embryo. It is represented by (i) the ductus 



FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL 

 OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F. 



A. is the posterior section. 



nc. neural canal; al. post-anal gut; alv. caudal vesicle of post-anal gut; x. 

 subnotochordal rod; mp. muscle-plate; c/i. notochord; cl.al. cloaca; ao. aorta; 

 v.cait. caudal vein. 



1 Stannius, Vergleich. Anat., p. 251. 



