CH. LIX.] 



THE LIVER VEINS 



855 



common iliac vein, and the lower part of the inferior vena cava below 

 the right renal vein. Above the right renal vein it becomes the vena 

 azygos major. The parts of the left cardinal between the left lumbar 

 veins disappear, and blood from the left lumbar veins and the left 

 common iliac vein is carried across to the right cardinal, and subse- 

 quently to the inferior vena cava by a series of transverse anastomosing 

 channels, of which the lowest ^becomes the left common iliac vein. The 

 upper part of the left cardinal vein is also broken up, and its remains form 

 the vertical parts of the minor azygos veins and lower part of the left 

 superior intercostal vein. The transverse parts of the minor azygos veins 

 are also developed from transverse anastomosing channels (fig. 652). 



FIG. 653. Diagram showing the arrangement and transformation of some of the primitive veins. A, 

 Early stage ; B, later stage. 1, Primitive jugular vein ; 2, duct of Cuvier ; 3, cardinal vein ; 4, 

 right umbilical vein ; 5, right omphalo-mesenteric vein ; 6, common umbilical vein ; 7, sinus 

 venosus ; 8, liver ; 9, left umbilical vein ; 10, right vena revehens ; 11, left vena revehens ; 12, right 

 vena advehens ; 13, left vena advehens. 



In the early stages both the omphalo-mesenteric and the right and 

 left terminal branches of the umbilical vein end in the heart. When 

 the liver forms, the omphalo-mesenteric veins end in venae advehentes, 

 which break up into capillaries in the liver, and the capillaries end in 

 venae revehentes, which become the hepatic veins (fig. 653). The left 

 hepatic vein joins the right hepatic vein to form a common trunk, which 

 becomes the upper end of the inferior vena cava, and this is prolonged 

 down to unite with the right cardinal at the level of the right renal 

 vein ; but before joining the right cardinal it gives off a branch to join 

 the left cardinal at the level of the left renal vein, and thus the blood 

 from both kidneys enters the inferior vena cava. In the meantime 

 two transverse anastomoses have formed between the omphalo- 

 mesenteric veins below the liver, and still lower the two veins fuse 



