THE THORAX 1007 



its blood from (i) the yolk-sac, and (2) the primitive intestinal tube, within 

 the abdomen. 



As the yolk-sac atrophies the portions of the vitelline veins between it 

 and the commencement of the portal vein £dso atrophy, and the tributaries 

 of the portal vein gradually assume their condition in adult Ufe. ' 



The following parts of the sinus annularis, or double duodenal venous ring, 

 undergo atrophy: 



1 . Right half of lower ventral anastomotic vesseL 



2. Right half of lower duodenal ring. 



3. Left half of upper duodenal ring. 



The following parts of the sinus annularis persist: 



1 . Left half of lower ventral anastomotic vesseL 



2. Left half of lower duodenal ring. 



3. Middle or dorsal anastomotic vessel. 



4. Right half of upper duodenal ring. 



5. Upper ventral anastomotic vessel. 



These persistent portions, with the exception of the upper ventral anasto- 

 motic vessel, form t\ie greater part of the trunk of the portal vein. The upper 

 ventral anastomotic vessel represents a part of the left division of the portal 

 vein. 



The portal vein has originally a spiral relation to the duodenal portion of 

 the primitive intestinal tube — that is to say, it winds round the left side and 

 dorsal aspect of the duodenum, and then appears on its right side. 



Divisions of the Portal Vein. — These are connected with the upper duodenal 

 venous ring. As previously stated, thp veins which convey the blood from 

 this ring to the hepatic sinusoids are known as the venee hepatioB advehentes, 

 right and left. The right division of the portal vein is formed by the right 

 vena hepatica advehens, which springs from the right half of the upper duodenal 

 venous ring. The left division is formed by (i) the upper ventrjil anastomotic 

 vessel, and (2) the left vena hepatica advehens. 



Summary of the Development of the Portal Vein. 



Root=Fusion of vitelline veins ventral to the primitive duodenum. 



fi. Left half of lower ventral anastomotic vessel. 



_ 2. Left half of lower duodenal ring, 



of Trunk 1 3. Middle or dorsal anastomotic vessel. 



V 4. Right half of upper duodenal ring. 



Right Division= Right vena hepatica advehens. 



Toftniuicinn — / ^- Uppcr vcutral anastomotic vesscl. 



i.ei; uivision -y^ j^eft vena hepatica advehens. 



According to some authorities the common stem formed by the fusion of 

 the vitelline veins in front of the duodenal portion of the intestinal tube, 

 together with the left half of the lower duodenal ring, gives rise to the superior 

 mesenteric vein, which is joined by the splenic vein. According to this view, 

 the portal vein is formed by (i) the middle or dorsal transverse anastomotic 

 vessel, and (2) the right half of the upper venous ring. 



Summary of the Vitelline Veins. 



The vitelline veins, which are developed first, return the blood from the 

 yolk-sac to the sinus venosus. They enter the body of the embryo along 

 the vitelline duct. At first they ascend in front of the duodenal portion of 

 the primitive intestinal tube, hing close to, and parallel with, each other. 

 Subsequently they ascend on either side of the duodenal gut. The portions 

 of the veins in front of the duodenal gut fuse and form the root or lower part 

 of the portal vein, which receives the veins of the primitive intestinal tube. 

 The portions on either side of the duodenal gut become connected by three 



