Why a Wedding Ring is Worn on the Third Finger of the Left Hand:
It was once believed that a vein of blood ran directly from the third finger on the left hand to the heart. The vein was called "vena amori" or "the vein of love." Early writings on matrimonial procedure suggested that it would be appropriate for one's wedding ring to be worn on that special finger.
Source: Ever Wonder Why? Douglas B. Smith / Mass Market Paperback / Published 1992