228 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



The greater part of the left common iliac vein arises, like the 

 left innominate, as a communicating channel between the posterior 

 cardinals. It is formed as the post -renal part of the left cardinal 

 becomes obliterated (Fig. 186). 



The Inferior Vena Cava. — The post-renal part is formed from 

 the right cardinal vein (Fig. 186). The pre-renal part, with the 

 mesial part of the left renal vein, is quite a new formation which 

 opens up a short circuit to the heart for the blood of the lower 

 half of the body. The formation of this new channel leads to the 

 retrogression of the thoracic stages of the cardinal veins and their 

 formation into the azygos veins. The exact date of the origin of 

 the inferior vena cava in the human foetus is not known ; the new- 

 channel is said to grow out from the ductus venosus of the liver, 

 and growing downwards opens up a connection with the right 

 and left cardinal veins at the entrance of the renal vessels 

 (Fig. 188). Thus in the formation of the inferior vena cava are 

 included three elements — (1) the terminal or upper part of the 

 ductus venosus ; (2) the new channel ; (3) the inferior or post- 

 renal part of the right cardinal. 



Occasionally it happens that pressure on the hepatic part of 

 the vena cava, from tumours, cirrhosis of the liver, etc., leads to 

 the azygos veins, the early foetal channels, being again opened up. 



The Portal Vein. — The Portal Vein is formed out of the two 

 vitelline veins — the first of all the veins to be developed. They 

 end in the posterior chamber of the tubular heart of the embryo — 

 the sinus venosus. The vitelline veins, right and left, arise from 

 ramifications on the yolk sac and pass in the splanchnopleure to 

 the sinus venosus (Fig. 187). The nutriment within the yolk sac 

 is thus carried to the heart and distributed by the heart to the 

 tissues of the embryo and yolk sac. The commencement of the 

 left vitelline or omphalo-meseraic vein disappears. The right 

 (Fig. 187) forms the superior mesenteric vein. It commences 

 on the yolk sac, of which a remnant (the neck) may remain as 

 Meckel's diverticulum. 



The terminal parts of the two vitelline veins are joined by 

 three transverse communications, the upper two of which are in- 

 cluded in the portal vein. The uppermost of the three afterwards 

 lies in the transverse fissure of the liver (Fig. 188). The middle 

 communication lies on the dorsal aspect of the duodenum 



