272 THE DIGESTIVE SYSTEM 



duct (p. 262). At the upper border of the duodenum, the 

 vein enters the lesser omentum, in which it ascends to the 

 porta hepatis (transverse fissure of the liver) in company with 

 the bile duct and the hepatic artery (PL I.). 



At the porta hepatis, the portal vein divides into right and 

 left branches, which enter the right and left lobes of the 

 liver, respectively. These branches eventually break up into 

 capillaries, and thus the blood of the portal circulation passes 

 through two sets of capillaries before it finally returns to the 

 heart. 



The effects of portal obstruction can be fully appreciated 

 only after a study of the organs which drain their blood into 

 the portal circulation. 



The Superior Mesenteric Vein receives tributaries from the 

 following sources : (i) The terminal part of the duodenum 

 and the whole length of the jejunum and ileum ; (2) the 

 caecum and the vermiform process (appendix) ; (3) the 

 ascending colon and rather more than the right half of the 

 transverse colon ; and (4) the greater curvature of the stomach, 

 through the right gastro-epiploic vein. 



The Splenic Vein commences at the hilum of the spleen, 

 where the veins which issue from that viscus are joined by 

 the left gastro-epiploic vein, from the greater curvature, and 

 some small veins from the fundus of the stomach. It passes 

 to the right behind the pancreas and receives numerous 

 pancreatic veins. In addition, the splenic vein is joined by 

 the inferior mesenteric vein, which receives tributaries from 

 (i) the left half of the transverse colon ; (2) the descending colon ; 

 (3) the iliac colon; (4) the pelvic colon ; and (5) the rectum. 



The portal vein itself receives tributaries from (i) the 

 lesser curvature of the stomach ; (2) the head of the pancreas 

 and the duodenum ; and (3) the gall-bladder. 



When these various tributaries are summarised, it is 

 found that the portal system drains the spleen, the pancreas, 

 the gall-bladder and the whole of the abdominal part of 

 the alimentary canal with the exception of the anal canal. 



