DEVELOPMENT OF THE VASCULAR SYSTEM 



519 



a plexus of small capillary-like vessels termed sinusoids (Minot). The branches 

 conveying the blood to this plexus are named the venae advehentes, and become 

 the branches of the portal vein; while the vessels draining the plexus into the 

 sinus venosus are termed the venae 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 mes- 

 enteric vein, forms the trunk of the portal vein. 



Eight primitive jugular vein 

 Right cardinal vein 



Right duct of Cuvier 



Sinus venosus 

 Right hepatic vein 



Portal vein 



Portal vein 

 Right umbilical vein 



Umbilical cord 



Left primitive 

 jugular vein 



Left cardinal vein 



Left duct of Cuvier 



~ Left hepatic vein 

 Left umbilical vein 



Left umbilical vein 



G. 476. Human embryo with heart and anterior bodv-wall removed to show the sinus venosus and its tributaries. 



(After His.) 



The two Umbilical Veins fuse early to form a single trunk in the body-stalk, 

 ut 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 





