"The choir and sanctuary symbolize Heaven; the nave is the emblem of the earth; as the gulf that divides the two worlds can only be passed by the help of the Cross, it was formerly the custom, now, alas, fallen into desuetude, to erect an enormous Crucifix over the grand arch between the nave and the choir. Hence the name of triumphal arch was given to the vast space in front of the High altar. It may also be remarked that a railing or screen marks the limits of these two parts of the cathedral. Saint Gregory Nazianzen regards this as the border line traced between the two parts--that of God, and that of man. "There is, however, a different explanation given by Richard de Saint Victor, as to the sanctuary, the choir, and the nave. According to him, the first symbolizes the Virgins, the second the chaste souls, and the third the married hearts. As to the altar, or, as old liturgical writers call it, the _Cancel_ (chancel), it is Christ Himself, the spot whereon His Head rests, the Table of the Last Supper, the Stake whereon He shed His blood, the Sepulchre that held His body; and again, it is the Spiritual Church, and its four angles the four corners of the earth over which it shall reign. "Now behind this altar we find the apse, assuming in most cathedrals the form of a semicircle. There are exceptions; to mention three: at Poitiers, at Laon, and in Notre Dame du Fort at Etampes the wall is square, as in the ancient civic basilicas, and does not describe the sort of half-moon, of which the significance is one of the most beautiful inventions of symbolism. "This semicircular end, this apsidal shell, with the chapels that surround the choir, simulates the Crown of Thorns on the Head of Christ. Excepting in Sanctuaries which are wholly dedicated to Our Lady--this one, Notre Dame de Paris, and some others--one of these chapels, that in the centre and the largest, is dedicated to the Virgin, to show by the place that it occupies at the end of the church that Mary is the last refuge of sinners. "She, in person, is again symbolized by the Sacristy, whence the priest comes forth as Christ's representative after putting on his sacerdotal vestments, as Jesus came forth from His Mother's womb after clothing Himself in flesh. "It must constantly be repeated; every part of a church and every material object used in divine worship is representative of some theological truth. In the script of architecture everything is a reminiscence, an echo, a reflection, and every part is connected to form a whole.