These pieces are called "Vierge Ouvrante," which roughly translates to "The Opening Virgin." They are also known as "Madonna Shrines." They were popular in the 15th and 16th centuries.The idea behind these is wonderful. "God" - the Eternal - is within each of us, but is only known through Her as She carries the Fount of Life. The mystical symbolism and meaning of these pieces of art are amazing.