This feature, as Willis remarks, "may have arisen either from the indifference of the artist as to the mixture of forms or else from deliberate contrivance, for as he was compelled, from the nature of his work, to retain round-headed arcades, windows, and arches, in the side-aisles, and yet was accustomed to and desirous of employing pointed arches in his new building, he might discreetly mix some round-headed arches with them, in order to make the contrast less offensive by causing the mixture of forms to pervade the whole composition, as if an intentional principle." [Illustration: THE CHOIR, LOOKING EAST (FROM A PHOTOGRAPH BY CARL NORMAN AND CO.).] Whatever the motive, this daring mixture renders the study of the architectural features of our cathedral peculiarly interesting. In the triforium we find a semicircular outer arch circumscribing two inner pointed ones. The clerestory arch is pointed, while some of the transverse ribs of the great vault are pointed and some round. The inward bend of the walls at the end of the choir was necessitated by the fact that the towers of St. Anselm and St. Andrew had survived the great fire of 1174. Naturally the pious builders did not wish to pull down these relics of the former church, so that a certain amount of contraction had to be effected in order that these towers should form part of the new plan. This arrangement also fitted in with the determination to build a chapel of the martyred St. Thomas at the end of the church, on the site of the former Trinity Chapel. For the Trinity Chapel had been much narrower than the new choir, but this contraction enabled the rebuilders to preserve its dimensions.