To include the Gymnasium in the same building, Nero made a wooden floor over the reservoir, which he could remove and replace at pleasure; this was covered with sand for the athletes to wrestle upon, and became THE ARENA. Around this great oval basin galleries were erected for the spectators, which were gradually enlarged and raised higher as the seats were further off; and the great stone arcades of the Flavian Emperors, with the corridors in them, are built round those older galleries, which were chiefly faced with brick. Several of the arches of the galleries, in the fine brickwork of Nero, remain in the Colosseum[48]; and the stone arcades were evidently built up against them without any junction between the two in any part. The bricks on the side of the arch next the stone piers of the corridors, are in many places cut in half, to make way for the stone piers. The straight vertical joints between the brick galleries and the stone arcades are often two or three inches wide; this may be the effect of an earthquake, but there is no bonding between one and the other. The enormous arcades and corridors are in themselves a gigantic work, and it was evidently difficult to obtain so great a supply of materials. The external wall and corridor are of three periods. The first is the ground floor only, with the Doric order of columns; the first and second floor, with the Corinthian columns, belong to the second period, and it is a little later than the lowest part, but not with any long interval; the upper floor is an addition of the third century, and replaces a wooden storey.