This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Vena Portse is formed by the junction of the splenic vein, after it has received the inferior mesenteric, with the superior mesenteric vein. The trunk of this large vein commences on the front of the aorta, behind the superior margin of the pancreas, and opposite to the first lumbar vertebra: it then takes a direction upwards, and to the right side, to reach the transverse fissure of the liver, in which it divides into a right and left branch. In this course it is at first behind and between the hepatic artery and ductus choledochus, and higher up it is directly behind them. In the adult, the right branch, shorter and larger than the left, and more in the direction of the trunk itself, soon sinks into the right extremity of the transverse fissure, to supply the right lobe; the left proceeds in the opposite direction, takes a longer course, forming nearly a right angle with the trunk, and sinks into the left lobe.
We shall now endeavor to explain the distribution of these vessels in the foetus, and the manner in which the subsequent changes in their arrangement are effected.