Henry Gray (18251861). Anatomy of the Human Body. 1918.
a plexus of small capillary-like vessels termed sinusoids (Minot). The branches conveying the blood to this plexus are named the venæ advehentes, and become the branches of the portal vein; while the vessels draining the plexus into the sinus venosus are termed the venæ revehentes, and form the future hepatic veins (Figs. 475,476). Ultimately the left vena revehens no longer communicates directly with the sinus venosus, but opens into the right vena revehens. The persistent part of the upper venous ring, above the opening of the superior mesenteric vein, forms the trunk of the portal vein.
FIG. 476 Human embryo with heart and anterior body-wall removed to show the sinus venosus and its tributaries. (After His.) (See enlarged image)
The two Umbilical Veins fuse early to form a single trunk in the body-stalk, but remain separate within the embryo and pass forward to the sinus venosus in the side walls of the body. Like the vitelline veins, their direct connection with the sinus venosus becomes interrupted by the developing liver, and thus at this stage the whole of the blood from the yolk-sac and placenta passes through the substance of the liver before it reaches the heart. The right umbilical and right vitelline veins shrivel and disappear; the left umbilical, on the other hand, becomes enlarged and opens into the upper venous ring of the vitelline veins; with the atrophy of the yolk-sac the left vitelline vein also undergoes atrophy and disappears. Finally a direct branch is established between this ring and the right hepatic vein; this branch is named the ductus venosus, and, enlarging rapidly, it forms a wide channel through which most of the blood, returned from the placenta, is carried direct to the heart without passing through the liver. A small proportion of the blood from the placenta is, however, conveyed from the left umbilical vein to the liver through the left vena advehens. The left umbilical