The vault proper is no longer formed of concentric courses, as in the mother cupola. It consists thenceforward of voussoirs cut normally to the curve, and filling the triangles (A, B, C, D, Fig. 7) determined by the longitudinal, the diagonal or intersecting, and the transverse arches. These arches form a stone skeleton, no less solid though far less ponderous than the cupola pendentives, and sustain the vault by distributing its thrusts over four points of support. The triangular fillings no longer imprison the ribs, or, more exactly speaking, the intersecting arches, nor do they any longer neutralise their active functions. These fillings, on the other hand, have, like the intersecting arch, gained a new independence. They now contribute to the elasticity of the divers organs of the vault, a most essential element in its solidity. The peculiar arrangement of the intersecting arches in the nave of Angers gives incontrovertible proof of the direct filiation of this building to the Aquitainian cupola. The voussoirs of the intersecting arches are about equal in horizontal section to those of the transverse arches, while their vertical section equals the thickness of the filling plus the internal salience which marks their function. They look in fact like slices cut from the pendentives of a cupola (A, Fig. 8). It must be remarked, too, that at Angers the stones of the filling do not yet rest upon the extrados of the ribs, in the fashion adopted some years later in the Ile-de-France and elsewhere (see B, Fig. 8), but embrace them (as at A). [Illustration: 8. SECTION OF AN INTERSECTING ARCH] The identity of function in the pendentive and in the Gothic intersecting arch, both constructed, as they are, of stones dressed normally to their curves, shows that they sprang from a common origin, which is as much as to say that the Aquitainian cupola begat the intersecting vault. CHAPTER III THE FIRST GROINED VAULTS The first application of the system of intersecting vaults appears in the great churches of Angers and Laval. It is probable that the new methods propagated by the religious architects of Aquitaine and neighbouring provinces had excited the emulation of the Northern builders, more especially those of the Ile-de-France. Evidences to this effect are to be found in certain subordinate portions of their buildings at this period, such as side aisles or apsidal chapels. Their timid arrangement seems, however, reminiscent of the Roman system of ribbed vaulting, with a slightly increased prominence of the ribs superadded, rather than of the revolution that had been effected in church vaulting generally. [Illustration: 9. PLAN OF A BAY IN THE NAVE OF ST. MAURICE AT ANGERS] [Illustration: 10. TRANSVERSE SECTION OF THE NAVE OF ST. MAURICE AT ANGERS]