PORTAL VENOUS SYSTE31.—One remarkable class of the veins of the abdomen, those of the chylopoictic Viscera, do not terminate directly in the vena cava, but unite to form a large ports', which enters the }iier, and, branching out like an artery (hence the name vena arteriosa), forms a capillary system in the substance of that organ.
The trunk of the vena ports' is formed by the coalescence of several large veins, which return the blood from the stomach, spleen, pancreas, and from the entire of the intes tinal tract, with the exception of the lower part of the rectum, many of the veins of which open into the internal iliac vein, as has been already mentioned.
. The veins which form the porta are the following: — the superior and inferior mesen teric, the splenic, gastric, duodenal, and pan creatic, veins. All these are destitute of valves.
1. Inferior mesenteric The veins of the upper portion of the rectum (superior luenzorrhoidal veins), sigmoid flexure of the colon, and descending colon, unite to form this vein, which ascends behind the inferior transverse portion of the duodenum and the pancreas, and opens into the splenic vein. By means of the free communications which -exist between the inferior hmmorrhoidal veins and the hmmorrhoidal and vesical plexuses, the system of the vena ports: is connected with that of the general systemic circulation.
2. The splenic vein arises in the spleen, and, accompanying the splenic artery (which lies superior to the vein), follows a transverse direction from left to right, and passes behind the pancreas to unite with the superior me senteric vein.
Besides its proper splenic roots the splenic vein receives the veins which correspond to the " vasa brevia," as also the epiploic, pancre atic, and infrior mesenteric veins.
The gastric veins, of which the coronary (coronaria ventriculi) is the most considerable, likewise terminate in the splenic vein.
3. The superior mesenteric vein (grande mesaraique, Cloquet) receives the venous ca Fillaries from the duodenum and all the small intestines, also from the ascending and trans verse portions of the colon, and passes behind the pancreas, where it unites with the splenic vein. The superior mesenteric vein corre sponds to the artery of the same name, and is placed anterior and to the right of it.
The trunk of the vena porky, formed by the union of these two great veins (splenie and superior mesenteric), is placed at first behind the head of the pancreas, to the right of the spinal column, and a little to the left of the inferior vena cava : it next ascends, -with an obliquity backwards and to the right side, between the layers of the gastro hepatic omentum to the transverse fissure of the liver ; after becoming slightly enlarged (sinus of the vena portm), it there divides into two branches, which pass horizontally into the right and left hepatic lobes respectively. Its
principal anterior relations are the follow ing : the head of the pancreas and the duode num, the hepatic artery, and the ductus cho ledochus ; to the two latter vessels it is related, whilst traversing the gastro-hepatic omentum ; it is surrounded by branches of lymphatic vessels and of the hepatic plexus of nerves.
The portal veins, and their subdivisions in the liver, surrounded by a prolongation from the capsule of Glisson, are contained within the" portal canals," and are each accompanied by a branch of the hepatic artery, and by a branch of the hepatic duct. From these arise the vaginal and interlobular branches, which, by their inoseulations, form plexuses which are similarly named. The interlobular veins sur round the lobules on their capsular surfaces, and ultimately, having been joined by the terminal branches of the hepatic artery, enter the lobules (forming the interlobular veins), and terminate by inosculating with the intralobu, lar (hepatic) veins: plexus interposed between the interlobular portal veins and the interlobular hepatic vein constitutes the venous part of the lobule, and may be called the lobular venous - " The portal vein collects the venous blood from the chylopoietic viscera, and then circu lates it through the lobules : it likewise re ceives the venous blood, which results from the distribution of the hepatic artery to the struc tures of the liver: these two sources of supply constitute the two origins of the portal vein, the abdominal origin and the hepatic origin." The blood conveyed to the liver by the branches of the vena portm is conveyed hack again into the general circulation by the he patic veins.
Hepatic veins. — These veins arise in each lobule, from the lobular venous plexus, by a vein termed, from its position, intralobzdar. The interlobular veins unite with others termed sublobular, which, by their coalescence from the venm cave hepaticm, these latter, usually four in number, leave the liver at its thick margin, and open into the vena cava inferior, where that vein passes between the Spigelian and right lobes of the liver.