Beyond this, they are connected to one another by two anastomotic branches, one on the dorsal, and the other on the ventral aspect of the duodenal portion of the intestine.

This is encircled by two venous rings; into the middle or dorsal anastomosis the superior mesenteric vein opens.

The portions of the veins above the upper ring become interrupted by the developing liver and broken up by it into a plexus of small capillary-like vessels termed sinusoids.

The vessels draining the plexus into the sinus venosus are termed the venae revehentes, and form the future hepatic veins.

The vitelline veins drain the yolk sac during early embryonic development.