The way by which the “Great Baptistery” was reached from the bema, as mentioned in the _Ceremonies_ was probably by this cloister, which perhaps inclosed one of the courts on the sides of the church, spoken of by Procopius and the Silentiary. The portion drawn by Salzenberg still remains, although sadly plastered over and mutilated. § 4. STRUCTURAL SYSTEM AND VAULTING. The geometrical scheme of this building, which in its final form must be the result of hundreds of adjustments, modifications, and expedients, to meet newly discovered emergencies, is withal so seemingly simple, that it may be read as a bare mechanical solution of the primary conditions. The great central area, excepting only the narrow bema, is surrounded by two stories of vaults; the thrust of the dome over the square of about 100 feet is not only resisted by these, but by the four immense buttressing masses (or rather chambers for they are built hollow) which, pierced by arches, pass right across the aisles. East and west the dome is sustained by the semidomes of the great hemicycles, and these in turn by the vaults of the three subdivisions of the hemicycles. The thrusts are thus distributed in a regular pyramid. The external wall, which incloses the whole, being built out to the extremity of the great buttress piers of the north and south sides, and the lesser piers east and west, is thus little more than a screen, inclosing the more active parts of the structure. One of the most remarkable expedients of this marvellously planned building is that by which the vaults of the side aisles,--which, having large spans, necessarily spring comparatively low down--are received on the secondary order of columns, standing behind the pillars of the great order. This allows of the stately colonnade on either side of the central space and those in the four exedras being only controlled by the height of the upper floor, which is forty-four feet above the area as is explained by Figs. 36, 38. These secondary pillars also transform the spaces left by the exedras into square compartments.