vault which is most highly domed up exerts the least outward thrust and is thus most easily supported. It is not surprising, therefore, to find this form a favorite where large windows were not required in the apse or where there was no ambulatory or but a low one. This may perhaps explain its use in the south of France in the cathedral of Béziers (Hérault), as well as its popularity throughout Italy, where it may be seen on an exceptionally large scale in the cathedral of Milan. Certain of these domed up chevets may also be attributed to the type of nave vault developed in the locality in which they are found, as, for example, the chevet of Angers cathedral (Fig. 74), which is very highly domed, with the small torus ribs of the region forming the radiants beneath it. As a matter of fact, such a chevet as this differs from a ribbed-lobed-dome only in having its masonry courses running at right angles to the supporting walls. Its pressures are almost all downward with but little outward thrust though the arrangement of the masonry courses and the shape of the vault cells serves to concentrate both thrusts and pressures upon the ribs and piers instead of along the whole curve of the outer walls, thus rendering perfectly safe the introduction of large windows.[398] CHEVETS WITH PIERCED PANELS [Illustration: FIG. 75.--AUXERRE, CATHEDRAL.] Still another interesting characteristic of certain chevet vaults is the presence of openings from one cell to the next in the lower portion of the panels between them. The simplest of these are to be seen in the cathedral of Auxerre (choir finished 1234) (Fig. 75), and it seems very reasonable from their square shape, comparatively small size, and their position at the beginning of the curve of the vault cells to assume that they were intended to hold wooden beams, used, quite possibly, as supports for scaffolding or centering for the rest of the vault. Whatever their use, they may be the prototypes of such larger openings as those in the cathedral of Bourges (after 1215) (Fig. 76), which may not only have been used in a similar manner but which, from their circular shape and moulded character, supply a certain amount of decoration to this part of the vault and even serve in a slight degree to distribute the light from its windows over a larger area.[399] An even greater amount of decoration is obtained by the use of tracery in the similar openings in the cathedral of Orleans (begun 1630), which are of larger size and of a generally triangular shape.[400] The final development of such tracery panels may be seen in the Brunnenkapelle of Magdeburg cathedral (fourteenth century)[401] where the apse vault proper becomes practically a flat ceiling the entire space between it and each of the ribs being filled with tracery. [Illustration: FIG. 76.--BOURGES, CATHEDRAL.] CHAPTER IV AMBULATORY VAULTS EARLY AMBULATORIES