The dimensions of the two churches do not differ very much; Santiago has one bay less in its nave, but one bay more in each transept; it has only one aisle, whilst S. Sernin has two on each side of the nave; and its two towers are placed north and south of the west front, instead of to the west of it, as they are at S. Sernin. The arrangement of the chevet and of the chapels on the east of the transepts was the same in both churches. Here they still exist in the chevet, but in the transepts traces of them are only to be found after careful examination. Three of them, indeed are quite destroyed, though slight traces still exist of the arches which opened into them from the aisles, but the fourth has been preserved by a piece of vandalism for which one must be grateful. It has been converted into a passage-way to a small church which once stood detached to the north-east of the cathedral, and the access to which was by a western doorway. The erection of a modern chapel blocked up the access to this doorway, and an opening was then made through the northern chapel of the north transept, which has thus been saved from the fate which has befallen the others. The position and size of these chapels are indicated in the ground-plan. The proportions of the several parts of the plans of the two churches are also nearly identical; and owing in part to the arrangement of the groining piers of the transepts, in which the aisles are returned round the north and south ends, the transept fronts in both churches have the very unusual arrangement of two doorways side by side--a central single doorway being impossible. The triforium galleries surround the whole church, being carried across the west end and the ends of the transepts, so that a procession might easily ascend from the west end, by the tower staircases--which are unusually broad and spacious--and make the entire circuit of the church. Finally, the sections of both these great churches are as nearly as possible the same; their naves being covered with barrel-vaults, their aisles with quadripartite vaults, and the triforia over the aisles with quadrant vaults, abutting against and sustaining as with a continuous flying buttress the great waggon-vaults of their naves.[155] [Illustration: No. 17 SANTIAGO CATHEDRAL p. 147. INTERIOR OF LOWER CHURCH]