THE PORTAL SYSTEM. 921 



I. The Superior Mesenteric Vein. — The superior mesenteric vein (v. mesen- 

 terica superior) (Fig. 773) accompanies the artery of the same name, lying upon its 

 right side. It has its beginning somewhere in the neighborhood of the terminal 

 portion of the ileum and ascends in the line of attachment of the mesentery. Above, 

 it passes over the third portion of the duodenum and then between that portion 

 of the intestine and the lower border of the pancreas, uniting behind the head of the 

 pancreas with the splenic vein to form the portal vein. It possesses no valves. 



Tributaries. — The tributaries of the superior mesenteric vein correspond with the branches 

 of the corresponding artery, except that it receives in addition the pancreatico-duodenal and 

 right gastro-epiploic veins which accompany the similarly named branches of the hepatic artery. 



(a) The veins of the small intestine (vv. intestinales) have their origin in the walls of tlie 

 small intestine from the last portion of the duodenum to within a short distance of the ileo-csecal 

 valve. Their arrangement is essentially similar to that of the arteries of the small intestine, the 

 numerous small branches which emerge from the intestine being united by transverse anasto- 

 moses, as a rule more numerous than those of the arteries, and forming one or more series of 

 venous arcades lying between the two layers of the mesentery. From these arcades branches 

 arise which pass towards the superior mesenteric vein, gradually uniting to form about twenty 

 stems which open independently into the superior mesenteric. The branches of origin of the 

 intestinal veins, just after they emerge from the intestine are provided with valves in the child, 

 but they usually degenerate more or less completely before adult life. 



{d) The ileo-colic vein (v. ileocolica) arises at the junction of the ileum and caecum by the 

 union of a caecal and an ileal branch, the latter of which anastomoses with the origin of the 

 superior mesenteric. The caecal branch receives an appendicular vein from the appendix vermi- 

 formis, and the main stem passes upward between the two layers of the mesentery to open into 

 the superior mesenteric just before it passes over the duodenum. 



(r) The right colic veins (vv. colicae dextrae) Originate in the walls of the ascending colon 

 and are two or three in number. They anastomose by transverse branches with the ileo-colic and 

 middle colic veins and pass almost horizontally medially to open into the superior mesenteric. 



(d) The middle colic vein (v. colica media) emerges from the transverse colon by a 

 number of small branches which anastomose to the right and left with the right and left colic 

 veins, and unite to a single stem which opens into the superior mesenteric just before it passes 

 beneath the pancreas. 



{r) The right gastro-epiploic vein (v. gastroepiploica dextra) runs from left to right along 

 the greater curvature of the stomach, communicating directly with the left gastro-epiploic at 

 about the middle of the curvature. It receives tributaries from the lower portions of the 

 anterior and posterior surfaces of the stomach and from the greater omentum, and opens into 

 the superior mesenteric shortly before its union with the splenic. It occasionally receives a 

 pancreatico-duodenal vein, and may unite with the middle colic vein to form a gastro-colic 

 vein instead of opening directly into the superior mesenteric. 



{/) The pancreatico-duodenal veins (vv. pancreaticoduodenales), like the arteries, may be 

 two in number, one of which opens directly into the superior mesenteric and the other 

 into the right gastro-epiploic. Frequently, however, they are broken up into a number of 

 separate vessels arising independently from each of the two viscera concerned, the duodenum 

 (vv. duodenales) and the head of the pancreas (vv. pancreaticae). 



2. The Splenic Vein. — The splenic vein (v. lienalis) (Fig. 774) is formed by 

 the union of hve or six branches which emerge from the hilum of the spleen. It passes 

 almost horizontally to the right below the splenic artery, resting at first upon the 

 upper border of the pancreas, but later coming to lie behind that organ. Behind the 

 head of the pancreas it unites with the superior mesenteric to form the portal vein. 



Tributaries. — These correspond with the branches of the artery, and in addition it receives 

 near its termination the inferior mesenteric vein, which for purposes of description will, however, 

 be regarded as independent. 



{a) The short gastric veins (vv. gastricae breves) arise from the fundus of the stomach and 

 pass between the layers of the gastro-splenic omentum to open partly into the splenic vein and 

 partly into its branches of origin as they emerge from the hilum. 



{b) The left gastro-epiploic vein (v. gastroepiploica sinistra) passes from right to left along 

 the greater curvature of the stomach, communicating directly with the right gastro-epiploic about 

 half-way along the curvature. It receives branches from the lower portions of both surfaces of 

 the stomach and from the greater omentum, and opens into the splenic vein near its formation. 



{c) The pancreatic veins (vv. pancreaticae), which maybe five or more in number, open 

 into the splenic vein at various points in its passage behind the pancreas. 



