The whole interior surfaces of the walls, up to the level of the springing of the gallery vaulting, and the nave walls, up to the gallery level, were once faced with marble. This is proved by the presence in the walls of many marble plugs and some iron holdfasts, as well as by remains of the moulded base of the facing. At the eastern extremity of the aisles there are chambers formed by walls built, as the vertical straight joints and difference of materials employed indicate, at various periods. The chamber at the end of the northern aisle has an archway, now built up, in its eastern wall, and seems to have served as a vestibule. It is in these chambers that Salzenberg supposes the staircases leading to the galleries stood, but it is evident from the character of the walls and vaulting that no such staircases could ever have existed there. The galleries extend over the narthex and over the whole length of the aisles. Access to them is now obtained by a wooden staircase and landing of Turkish construction, but how they were reached in Byzantine times is not evident. Possibly the fragments of wall on the exterior face of the south wall of the narthex and the traces of vaulting beside them may be the remains of a staircase. Or a staircase may have stood to the west of the narthex over the vaulting of the atrium, where projecting spurs of walls appear. The vaulting of the gallery over the narthex was originally similar to that of the narthex itself, but only the cross-groined vaults at the corners are Byzantine; the three central compartments are Turkish. Five windows in the western wall looked into the atrium, and as many openings in the eastern wall into the nave and side galleries. Below the former range is a string-course corresponding to that which runs round the interior of the building at gallery level. The gallery over each aisle consists of two open portions under the dome arches, divided from each other by the dome piers, which are pierced to connect the different parts of the gallery with each other, and with the gallery over the narthex. In the side walls there is a range of windows at gallery level; five on each side of the eastern nave bay, three in the south wall of the western nave bay, but none, at present, in its northern wall. Above these windows are two ranges of windows in each lunette under the dome arches, a system of five and three in the eastern bay, and of four and two in the western bay. All these windows, now square-headed, had originally semicircular heads. The lunette filling the western dome arch had doubtless a similar window arrangement, though at present it has only one window. [Illustration: PLATE XXI. (1) S. IRENE. INTERIOR, LOOKING NORTH-WEST. (2) S. IRENE. DOOR AT THE EAST END OF THE NORTH AISLE. _To face page 98._]