THE VEINS. 345 



veins from the bladder [and in the female 

 from the uterus]. 



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 iUacs 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 mesentery, 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 killed 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 

 duodenum 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 csecum, 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, 



