THE VELSS. 355 



They are direct continuations of the 

 femoral veins, which lie along the inner or 

 preaxial borders of the thighs. 



vii. The internal iliac veins return blood from the 

 backs of the thighs, and, running forwards 

 through the pelvic cavity, unite about half 

 an inch behind the external iliacs to form the 

 commencement of the posterior vena cava. 



2. The portal system consists of the veins conveying blood 

 from the whole length of the alimentary canal 

 from the stomach backwards, and also from the 

 pancreas and spleen. These veins lie in the mesen- 

 tery, and converge to form the main trunk, or 

 portal vein : this lies close alongside the posterior 

 vena cava and, entering the liver, divides into 

 branches supplying its several lobes. 

 The portal system is best seen in a freshly hilled animal 

 on opening the abdomen and turning the viscera aside. The 

 veins are large and usually gorged with blood, and care must 

 be taken not to injure them, as they bleed very freely. 



The following are the main trunks which unite 

 to form the portal vein. 



i. The lieno-gastric vein returns blood from the 

 walls of the stomach and from the spleen. 



ii. The duodenal vein returns blood from the duo- 

 denum and pancreas. 



iii. The anterior mesenteric vein is formed by the 

 union of veins from the whole length of the 

 small intestine, and from the caecum, the colon, 

 and the greater part of the length of the 

 rectum. 



iv. The posterior mesenteric vein lies in the meso- 

 rectum or hindmost part of the mesentery, 

 and returns blood from the terminal portion 

 of the rectum. 



