I HE Hl'.l'AHC CIRCULATION 



'3^9 



the spleen to the thin freely anastomosing mucosal 

 arterioles in the gastrointestinal tract (22, 39, 184, 

 227, 317)- It seems probable that the major point 

 of splanchnic vascular resistance lies in these vessels, 

 but arteriovenous (A-V) anastomoses between mu- 

 cosal arteries and veins appear to be numerous. 

 There is evidence (22) that blood may be diverted 

 through these channels principally as a result of 

 changes in capillary resistance rather than active 

 changes in A-V cross section. 



The capillary nets that drain into the various tribu- 

 taries of the portal vein are also highly variable. In 

 the gastric and mesenteric beds (22, 317) thorough- 

 fare channels (A-V) may provide direct routing of 

 blood from the arterioles to the venules, the degree 

 of capillary filling outside the A-V capillary depending 

 upon selective "sphincteric" action. Similar vessels 

 have been described in the spleen. Here the capillary 

 system is made more than usually complex by the 

 presence of a venous sinusoidal system which has 

 been the cause for much disagreement (184, 227). 

 The presence of capillary sphincters and A-V channels 

 elsewhere also remains disputed, since it is possible 

 that the phenomena described may be artifactitious. 

 Muscular tissue is not obviously present in the capil- 

 laries or at the sites of the so-called "sphincters." 

 Capillary vasomotion and the closure of sphincters 

 may therefore be attributable to changes in intra- 

 luminal pressure secondary to arteriolar activity 

 rather than local contractions. Capillary nets could 

 contribute importantly to frictional resistance through 

 mechanisms such as these, but further anatomical 

 investigation is necessary. In addition, it must be 

 shown more satisfactorily that the manipulation of 

 tissues prior to or during microscopic examination 

 is not responsible for the changes observed. 



The portal vein enters the hilum of the liver in 

 close relationship to the hepatic artery and the 

 emerging common bile duct. It is a rather weakly 

 muscled vessel, most of the muscle fibers being ar- 

 ranged longitudinally with a sparse coat of circular 

 muscle (65). The structure of the portal vein sug- 

 gests limited distensibility and easy collapsibility. 

 Numerous communications between the portal 

 vein and the systemic veins have been demonstrated 

 by a variety of techniques. Edwards (119) has shown 

 (by roentgenography and dissection after injection 

 of a barium sulfate suspension into the femoral veins 

 of three cadavers) that the most important connec- 

 tions are to be found in man at the retroperitoneal 

 surfaces of the abdominal viscera, in the pelvis, 

 and in the mediastinum. Even in normal persons, 



the portal system fills with radiopaque material 

 introduced in this way. The anastomoses are rela- 

 tively small and are probably of no importance in 

 determining portal venous pressure. They play a 

 more prominent role when portal venous inflow is 

 blocked. In dogs and other vertebrates in which 

 mesenteries are better developed than in man such 

 retroperitoneal links appear to be lacking (85). 



At its entrance into the porta hepatis, the portal 

 vein displays a relatively uniform and constant ar- 

 rangement which contrasts sharply with the dis- 

 orderly configuration of the hepatic arterial supply. 

 According to Gilfillan (145) the portal vein is nearly 

 always formed behind the head of the pancreas at the 

 level of the second lumbar vertebra by the union of 

 the splenic and superior mesenteric veins to course 

 directly in the hepatoduodenal ligament to the hilum. 

 The hepatic artery (or arteries), on the other hand, 

 is highly variable in its origin, course, anastomoses 

 within the gastrohepatic ligament, and relation to 

 the portal vein. Within the liver, the venous and 

 arterial inflow tracts assure a fairly regular pattern 

 of distribution and relationships. At the hilum the 

 blood vessels and bile ducts do not penetrate the 

 capsule of the liver but are carried in a sheath of 

 connective tissue derived from it which accompanies 

 them in all their ramifications as the portal tract. 

 Recent studies of vascular casts have demonstrated 

 that the external fissuring or lobation of the liver is 

 not precisely followed by the vascular and ductal 

 system (173, 214). Instead the vessels are distributed 

 to the right or left lobar segments which are sepa- 

 rated by an avascular sagittal cleavage plane inter- 

 secting the visceral surface of the liver along a line 

 drawn from the fossa of the inferior vena cava through 

 the gallbladder. These "vascular lobes" are divided 

 into segments which are distinct and easily separable 

 in plastic corrosion casts. Communications between 

 the right and left hepatic arteries or between their 

 branches may be found within the liver (148) but 

 appear to be uncommon, at least in man (84, 205). 

 The portal venous system appears to have a similar 

 and parallel arrangement with even fewer intra- 

 hepatic anastomoses. 



The finer branches of the portal vein and hepatic 

 artery communicate indirectly through a capillary 

 network which appears to furnish the blood supply 

 to the bile duct and other tissues in the portal tract 

 (97, 120, 148, 185, 205, 214, 270, 299). The small 

 hepatic artery lies close to the portal vein in all the 

 vertebrate species studied. Arterioportal anastomoses 

 resembling rungs of a ladder have been described by 



