LIVER 279 



cavity, fuse with one another so as to form a single trunk (Fig. 271, B, v.v.). On 

 reaching the duodenum, the trunk separates into its components, and they pass into the 

 liver as the right and left vitelline veins (Fig. 273, A). Before entering the liver they 

 anastomose with one another dorsal to the duodenum, as shown in the figure. Thus, 

 with the connections between the right and left veins within the liver, two complete 

 venous rings are formed around the intestine. Branches extend out from these rings, 

 notably the superior mesenteric vein which receives blood from the primary loop of 

 intestine, and the splenic vein which not only drains the spleen but receives the inferior 

 mesenteric vein together with pancreatic and gastric branches. The superior mesenteric 

 vein (Fig. 273, s.m.v.) is joined by the splenic (s.) to form the portal vein (p.v.}, and the 

 portal vein is a persistent portion of the peri-intestinal rings formed by the vitelline 

 veins. Other parts of the rings atrophy, and as the yolk-sac degenerates and becomes 

 detached, the main vitelline trunk disappears. The 

 portal system of veins is therefore a derivative of 

 the vitelline system; its blood flows through the liver 

 in the vitelline sinusoids. 



The formation of the rings as above described 

 takes place with great constancy, and apparently 

 the only variations observed in their atrophy are the 

 two cases described by Begg (Amer. Journ. Anat., 

 1912, vol. 13, pp. 105-110). 



The umbilical veins are at first a pair of vessels, 

 but they' early unite in the umbilical cord. The FlG 



single vein thus formed brings the embryonic blood The formation of the portal vein, p. v., 

 back to the body after its excursion to the placenta. JS SV?d L^.f&^S^ 

 On reaching the body, the vein divides into right r^ASS^tSc^S." 1 

 and left vessels, which are contained in the ventral 



body wall, and at first pass directly to the heart; later they anastomose with the 

 vitelline sinusoids in the liver, and the right umbilical vein then atrophies, leaving 

 the left vein to convey the blood to the liver. In Fig. 274, the left vein is larger 

 than the right, and is seen connecting with the hepatic sinusoids. Gradually it 

 shifts from the left side to the median line. It then passes from the umbilical 

 cord to the under surface of the liver along the free edge of the falciform ligament, 

 where, after the umbilical cord has been severed, it degenerates to form the round 

 ligament of the liver (Fig. 275). This extends to the porta or entrance to the liver, 

 where the portal vein goes in and the hepatic duct comes out. Beyond this point 

 the umbilical vein may be followed as the ductus venosus in the embryo, or the 

 ligament of the ductus venosus in the adult, to the vena cava inferior. The ductus 

 venosus may be defined as the channel made by the umbilical vein in passing to the 

 vena cava inferior across the under surface of the liver. It is sometimes completely 

 enfolded by the hepatic trabeculae, and it communicates with the hepatic sinusoids. It 

 follows the line of attachment of the lesser omentum, and empties into the vena cava 

 inferior. 



The vena cava inferior apparently does not send much blood into the liver but passes 

 along its dorsal surface. An essential part of this great vein is formed from the hepatic 

 sinusoids. Before the vena cava inferior has developed, the blood in the dorsal body 

 wall flows to the heart through the posterior cardinal veins, one on either side of the 

 aorta. Each posterior cardinal vein shows a ventral subdivision, the right and left 

 subcardinal veins respectively, which are seen in section in Fig. 274. As shown in the 

 figure, the stomach prevents the liver from approaching the dorsal body wall (at the 

 root of the mesentery) on the left, but on the right there is no such obstruction, and the 



