The =portal vein= (V. portæ) is a large trunk which returns the blood
carried to the viscera by the gastric, splenic, and mesenteric arteries.
Its peripheral tributaries correspond closely with the branches of the
arteries, but the terminal trunks do not. The vein is formed behind the
pancreas and below the posterior vena cava by the confluence of the
=anterior= and =posterior mesenteric= and =splenic veins=. It passes
forward, traverses the posterior part of the pancreas very obliquely,
inclines ventrally and a little to the right, and reaches the portal
fissure of the liver. Here it divides into three branches which enter
the liver and ramify in the substance of the gland like an artery,
terminating in the lobular capillaries. From the lobules the blood
passes into the hepatic veins and through these to the posterior vena
cava. Thus the blood which is distributed to the stomach, nearly the
entire intestinal tract, the pancreas, and the spleen, passes through
two sets of capillaries prior to its return to the heart, viz., the
capillaries of these viscera and of the liver.
1. The =anterior mesenteric vein= (V. mesenterica cranialis) is the
largest of the portal radicles. It is situated to the right of the
artery of like name, and its tributaries correspond in general to the
branches of the artery. Usually a single colic vein corresponds to the
two arteries of the right portions of the colon.
2. The =posterior mesenteric vein= (V. mesenterica caudalis) is the
smallest of the radicles of the portal. It accompanies the artery in the
colic mesentery and its rectal branches anastomose with those of the
internal pudic vein.
3. The =splenic vein= (V. lienalis) is the very large satellite of the
splenic artery. It is formed by the union of two radicles at the base of
the spleen. On leaving the hilus of the spleen it passes inward between
the anterior pole of the left kidney and the saccus cæcus of the stomach
and above the left end of the pancreas, receives commonly the =posterior
gastric vein= (V. gastrica caudalis), and unites with the anterior
mesenteric at the posterior border of the pancreas.
The collateral tributaries of the portal veins are as follows:
(1) =Pancreatic veins= (Rami pancreatici).
(2) The =gastro-duodenal vein= (V. gastroduodenalis) corresponds mainly
to the extrahepatic branches of the hepatic artery.
(3) The =anterior gastric vein= (V. gastrica cranialis) joins the portal
at the portal fissure.