Why is a wedding ring always worn on the third finger?

Modern "authorities" on etiquette follow their predecessors in matrimonial procedure, in urging that the wedding ring always be worn on the third finger.Before medical science discovered how the circulatory system functioned, people believed that a vein of blood ran directly from the third finger on the left hand to the heart. Because of the hand-heart connection, they chose the descriptive name vena amori, Latin for the vein of love, for this particular vein.
Based upon this name, their contemporaries, purported experts in the field of matrimonial etiquette, wrote that it would only be fitting that the wedding ring be worn on this finger. By wearing the ring on the third finger of the left hand, a married couple symbolically declares their eternal love for each other.