270 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



anastomosis with the right vitelline vein. Subsequently, with the atrophy 

 of the yolk sac, the left vitelline vein degenerates caudal to its junction 

 with the superior mesenteric vein. The persisting trunk between the 

 superior mesenteric vein and the liver is the vena portcs, and thus represents : 



(1) a portion of the left vitelline vein in the left limb of the caudal ring; 



(2) the middle transverse anastomosis between the vitelline veins; (3) 

 the portion of the right vitelline vein which forms the right limb of the 

 cranial ring. 



In the liver, the portal vein, through its cranial anastomosis between 

 the primitive vitelline veins, is connected with the left umbilical vein 



Efferent Common Ductus Efferent 



hepatic vein hepatic vein, venosus Pancreas hepatic vein 



R. umbilical vein 

 Stomach 

 Afferent hepati 



Portal vein 



L. umbilical vein 



nl hepatic vein 



R. umbilical vein 



'Obliterated L. vitelline vein 



L. umbilical vein 



PIG. 281. A diagram showing the development of the portal vein as illustrated in a human 

 embryo of about 7 mm. (modified after His). 



(Fig. 281). As the right lobe of the liver grows, the course of the umbilical 

 and portal blood through the intrahepatic 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 ligamen- 

 tum 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 portas. In this way two primitive portal, or supplying trunks, and two hepatic, or 

 draining trunks, originate. Later there are differentiated first four, then six, such opposed 

 trunks within the liver, and the six primary lobules supplied and drained by these trunks 

 may be recognized in the adult liver. 



