Outside this central feature are two side-aisles, each having two stories, separated from the nave by arcading and formed of a series of columns and vaulting. As in all Early Christian and Byzantine churches which have upper and lower galleries, the former were occupied by women worshippers. The outer walls on the north and south sides, as the plan shows, are reinforced by immense buttresses, 25 feet wide and 75 long, which appear on the outside of the buildings like huge pylons. On the inside they are pierced with arches on each story. These buttresses withstand the thrust of the dome which is reinforced on the east and west by the semi-domes. The edifice, which occupies practically a square, is approached on the west side by a narthex of magnificent proportions, 200 feet long by 30 wide, which is divided like the aisles into an upper and lower story. So far “the plan resembles that of =S. Sergius=, if the latter were cut in half and a dome on pendentives inserted over the intervening square and the whole doubled in size.” In front of the narthex, however, extends a second one, opening, as in some of the basilican churches, into an atrium. The exterior walls are faced with alternate courses of brick and stone and the domes, all of which are visible, are covered with a sheathing of lead. =S. Mark’s, Venice.=--=S. Sophia= is a marvel not only of construction but also of unity of design. It is in this respect, among others, that it is superior to =S. Mark’s= in =Venice=, which was erected by Byzantine builders at the end of the eleventh century. Venice, like Ravenna, was in close touch with Constantinople and when she determined to build a cathedral to her patron saint, to replace an earlier basilican church destroyed by fire, it was natural that she should look to that city for the character of the design as well as for artists and artisans to execute it. The actual model was the =Church= of the =Holy Apostles=, in Constantinople, founded by Constantine, rebuilt by Justinian, and destroyed by the Turks in 1463 to make room for the mosque of Sultan Mahomet II. The plan is a Greek cross, that is to say, a cross with the four parts of practically equal length, grouped around a central square. Each of the five divisions is crowned by a dome, supported on pendentives and reinforced by transverse barrel vaults. The transept and choir domes are slightly smaller than the ones over the crossing and the nave, because of the restrictions of space caused by the chapel of S. Isadore in the north transept, the Ducal Palace on the south, and the retention of the apse of the ancient basilica. Originally all the domes were sheathed externally with lead, but at a later date were covered with the lead-sheathed wooden lanterns now existing. With their high-pitched curves and ornamental terminals they represent a serious deviation from the true Byzantine style.