The floor of the ARENA for the gymnastics and the slaughter of wild beasts was of wood covered with sand; on one occasion (as we have shewn) placed upon wine-barrels in the parts that had been flooded; the boards could be removed or replaced with facility. It was full of trap-doors with lifts under them, some large for the animals to jump through, these are over the passage round the outer line, in front of the dens under the _podium_; others smaller for men and dogs for the hunt; these are on each side of the central passage. Large corbels, with bold projections for placing the boards upon when removed from the floor of the stage, are provided over the stream of water, in front of the _podium_, but at a lower level; they are in pairs six feet apart, and also served to stiffen the lower end of the masts or poles for the awning; on the surface of them a notch is cut to place the boards upon, and ready access was given by the passage in front of the _podium_. The gymnasium and the _stagna_ were in one and the same building used for both purposes, and Nero probably built galleries of brick round it for the accommodation of the people, according to the fashion of his time. The exterior was left unfinished for some years, and completed of stone by Titus and the Flavian emperors on a more magnificent scale. The space for the upper galleries was afterwards very much enlarged by building them upon the magnificent double arcades of stone round it, and so completing the great building known as the Colosseum. The straight vertical joint, which is plainly seen between the old brickwork within and these stone galleries and corridors, and the want of any bond between them, is thus accounted for[74]. The upper gallery for the common people was an addition to the original design over these arcades and corridors, and was originally of wood; it was destroyed by fire caused by lightning in the time of the Emperor Macrinus, A.D. 217, and restored in stone in about twenty-three years, having been completed by Gordianus III., A.D. 240. To support this upper gallery of stone at that enormous height vertical piers of travertine are introduced, cutting through the walls of the lower galleries from top to bottom[75]; these walls are of tufa, faced with brick. In several instances portions of these piers of travertine have been removed for building purposes in the Middle Ages, and the space that had been occupied by the piers is left empty. The brick facing of the walls on either side stands just as firm without these travertine piers as with them, a clear proof that their object was to support the upper gallery when it was rebuilt in stone, and not to support the brick walls of the lower one through which they were cut, although they appear to do so. This accounts for the fact that in the brick arches of construction (as they are called), the bricks, originally two feet square, are cut down to a few inches[76].