256 A Study of the Structural Unit of the Liver 
sequently they are converted into veins. All of the blood from the um- 
bilical veins now passing through the liver increases the circulation 
through the small liver so much that a venous channel (right omphalo- 
mesenteric vein) remains open, or in case it be closed it is opened up 
again. The two new branches within the liver care for the circulation 
through its left lobes, and may have been formed directly from the left 
omphalo-mesenteric vein. At any rate, we see in them two permanent 
main trunks of the liver,—the vena hepatica sinistra and the ramus 
angularis arising from the recessus umbilicalis.~ In the next stage which 
Fic. 9. Section through a human embryo 4.5 mm. long (No. 76). X 25. 
VC, cardinal vein; a, aorta; vom, omphalo-mesenteric vein; vu, umbilical 
vein; h, heart. 
Fic. 10. Semidiagrammatic reconstruction of the veins of the liver of a 
human embryo 4.5 mm. long (No. 76). JL, liver; wv, umbilical vein; vom, 
omphalo-mesenteric vein; 7, intestine. 
is found during the fifth week the right omphalo-mesenteric vein is ob- 
literated and the ductus venosus is formed as a new and more direct chan- 
nel. In place of the obliterated omphalo-mesenteric vein there are two 
new permanent veins, the ramus dextra of the hepatic vein and the ramus 
arcuatus et descendens of the portal system. We now have a liver of two 
** An excellent description of the vascular system of the mammalian liver 
is given by Rex (Morph. Jahrb., XIV, 1888). As much as possible I have 
used his nomenclature. 
