270 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



caudal ring, together with the median dorsal anastomosis, persist. A new vein 

 the superior mesenteric, develops in the mesentery of the intestinal loop and joins 

 the left vitelline vein just caudal to its dorsal middle anastomosis with the right 

 vitelline vein. Subsequently, with the atrophy of the yolk sac, the left vitelHne 

 vein degenerates caudal to its junction with the superior mesenteric vein. The 

 persisting trunk between the superior mesenteric vein and the Hver is the vena 

 porta, and thus represents: (1) a portion of the left vitelUne vein in the left limb 

 of the caudal ring; (2) the middle transverse anastomosis between the viteUine 

 veins; (3) the portion of the right vitelline vein which forms the right limb of the 

 cranial ring. 



Ductus venosus 



Right horn sinus venosus 



Right umbilical vein 



Caudal anastomosis of vitelline veins 



Right vilelliiic vein 



Left horn sinus venosus 

 Left vitelline vein 



Cranial anastomosis of vitelline 

 veins 



Middle anastomosis of vitelline 

 veins 



Left umbilical vein 



efl vitelline vein 



Fig. 280. — Reconstruction of the veins of the liver in a 4.9 mm. human embryo (after Ingalls). 



In the liver the portal vein through its cranial anastomosis between the vitel- 

 line veins is connected with the left umbiHcal vein. As the right lobe of the Hver 

 grows, the course of the umbiKcal and portal blood through the intrahepati'c 

 portion of the right vitelline vein becomes circuitous, and hence a new direct 

 channel to the sinus venosus is formed through the hepatic sinusoids. This is 

 the ductus venosus (Fig. 281), which is obliterated after birth and forms the 

 ligamentum venosum of the postnatal liver. 



According to Mall, the intrahepatic portion of the right vitelline vein persists proxi- 

 mally as the right ramus of the hepatic vein, and distally as the ramus arcuatus of the portal 

 vein. The intrahepatic portion of the left vitelline vein drains secondarily into the right horn 

 of the sinus venosus, and proximally forms later the left hepatic ramus. Distally, where it is 

 connected with the left umbilical vein, it becomes the ramus angularis of the vena portae. In 

 this way two primitive portal, or supplying trunks, and two hepatic, or draining trunks, 



