THE PORTAL SYSTEM. 675 



The renal veins are of large size, and placed in front of the renal arteries. 1 

 The left is longer than the right, and passes in front of the aorta, just below the 

 origin of the superior mesenteric artery. It receives the left spermatic, the left 

 inferior phrenic, and. generally, the left suprarenal veins. It opens into the vena 

 cava a little higher than the right. 



The suprarenal veins terminate, on the right side, in the vena cava ; on the 

 left side, in the left renal or phrenic vein. 



The phrenic veins follow the course of the phrenic arteries. The tu'o superior, 

 of small size, accompany the phrenic nerve and comes nervi phrenici artery, and 

 join the internal mammary. The two inferior phrenic veins follow the course of 

 the phrenic arteries, and terminate, the right in the inferior vena cava. the left in 

 the left renal vein. 



The hepatic veins commence in the substance of the liver, in the capillary 

 terminations of the portal vein and hepatic artery : these tributaries, gradually 

 uniting, usuallv form three large veins, which converge toward the posterior 

 surface of the liver and open into the inferior vena cava, whilst that vessel is 

 situated in the -groove at the back part of this organ. Of these three veins, one 

 from the right, and another from the left lobe, open obliquely into the inferior 

 vena cava. that from the middle of the organ and lobulus Spigelii having a straight 

 course. The hepatic veins run singly, and are in direct contact with the hepatic 

 tissue. Thev are destitute of valves. 



' 



The Portal System of Veins. 



The portal venous system is composed of four large veins which collect the 

 venous blood from the viscera of digestion. The trunk formed by their union 

 / portcp) enters the liver and ramifies throughout its substance, and its 

 branches, again emerging from that organ as the hepatic veins, terminate in the 

 inferior vena cava. The branches in this vein are in all cases single and destitute 

 of valves. 



The veins forming the portal system are the 



Superior Mesenteric. Inferior Mesenteric. 



Splenic. Gastric. 



The superior mesenteric vein returns the blood from the small intestines and 

 from the caecum and ascending and transverse portions of the colon, correspond- 

 ing with the distribution of the branches of the superior mesenteric artery. The 

 large trunk formed by the union of these branches ascends along the right side 

 and in front of the corresponding artery, passes in front of the transverse por- 

 tion of the duodenum, and unites, behind the upper border of the pancreas, with 

 the splenic vein to form the vena portse. It receives the right gastro-epiploic vein. 



The splenic vein commences by five or six large branches which return the 

 blood from the substance of the spleen. These, uniting, form a single vessel, which 

 passes from left to right, grooving the upper and back part of the pancreas below 

 the artery, and terminates at its greater end by uniting at a right angle with the 

 superior mesenteric to form the vena portte. The splenic vein is of large size, and 

 not tortuous like the artery. It receives the vasa brevia from the left extremity of 

 the stomach, the left gastro-epiploic vein, pancreatic branches from the pancreas, 

 the pancreatico-duodenal vein, and the inferior mesenteric vein. 



The inferior mesenteric vein returns the blood from the rectum, sigmoid flexure, 

 and descending colon, corresponding with the ramifications of the branches of 

 the inferior mesenteric artery. Ascending beneath the peritoneum in the lumbar 

 region, it passes behind the transverse portion of the duodenum and pancreas and 

 terminates in the splenic vein. Its haemorrhoidal branches inosculate with those 



1 The student may observe that all veins above the Diaphragm, which do not lie on the same 

 plane as the arteries which they accompany, lie in front of them, and that all veins below the 

 Diaphragm, which do not lie on the same plane as the arteries which they accompany, lie behind 

 them, except the renal and profunda femoris vein. 



