THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



265 



from the body wall and passes directly from the umbilicus to the ventral side of 

 the liver. During foetal life it conveys all the blood from the placenta to the 

 liver. A part of the blood is distributed in the liver, a part is carried directly 

 to the inferior vena cava by the ductus venosus (Fig. 240). After birth the 

 placental blood is cut off and the umbilical vein degenerates to form the round 

 ligament of the liver. 



The venous rings around the intestine also undergo marked changes. The 



Heart 



Ductus venosu 



Left lobe of liver- 



Hepatic veins 



'-', Rightdobe of liver 



Umbilical ring 



Gall bladder 



- Portal vein 



(omphalomesenteric) 



Intestine 



Inf. vena cava 



Fig. 240. — Veins of the liver (seen from below) of a human foetus at term. Kollmann's Atlas. 



right side of the most caudal and the left side of the most cranial disappear; the 

 remaining vessel finally loses its connection with the ductus venosus and becomes 

 the portal vein (Figs. 237, 238, 239 and 240). The portal vein is thus a deriva- 

 tive of the omphalomesenterics. After birth, when the placental blood is 

 cut off, blood is distributed in the liver by branches of the portal vein, which 

 represent the advehent hepatic veins; it is collected again by branches which 

 unite to form the revehent hepatic veins, or hepatic veins proper, and the latter 



