Vein of the abdomen
The median sacral vein (or middle sacral veins) is a vein of the abdomen. It accompanies the median sacral artery along the front of the sacrum. It ends in the left common iliac vein. Sometimes, it ends in the angle of junction of the two common iliac veins.
References.
 "This article incorporates text in the public domain from the 20th edition of" Gray's Anatomy "(1918)"