[Illustration: FIG. 591.--St. Mungo’s Cathedral. Transverse Section through Choir and Lower Church, looking East.] To accomplish this, with arches of about the same width as those of the side aisles, the wall had to be divided into two bays having a central pillar, which is not a pleasing feature. Above the main arcade rises the east wall of the cathedral, pierced with four graceful lancets (see Fig. 591).[65] This arrangement of a central mullion in the large upper window was almost rendered necessary in order to carry up the design of the single pillar in the centre of the main arcade, otherwise a central light, as at Salisbury, would doubtless have been preferable. Churches with an eastern ambulatory are occasionally to be met with in England, a fine example being the church of the Abbey of Dore, in Hertfordshire. This ambulatory aisle is the British equivalent for the aisle which is generally carried round foreign apses, and from which the apsidal chapels open. A similar plan to that of the eastern end of Glasgow Cathedral was reproduced at Rosslyn Collegiate Church two hundred years later, although with very different details and on a greatly reduced scale. Each bay of the choir (see Fig. 589) rises from clustered pillars having numerous attached shafts and mouldings (see Fig. 587), and caps elaborately carved with first pointed foliage. The triforium gallery contains two pointed openings, each embracing two smaller arches, and having trefoil and quatrefoil apertures in the spandrils. The clerestory consists of a double wall, with passage between. It has three pointed lights of equal height in the outer wall, and three corresponding openings in the interior, with boldly moulded shafts and arches. A small sharply pointed arch is introduced at each side of the main vaulting shafts. The caps on each floor are all of rounded form. The south doorway of the nave (Fig. 591) (now forming the principal entrance to the church) is, like that of the lower church, inserted between two buttresses; but it has no projecting porch, and finishes with a flat roof, above which the top of an ordinary traceried window is seen in the plane of the wall. To give importance to this entrance the flanking buttresses are finished at the top with niches for statues, and in connection with the corbels under these occur the only pieces of foliaged carving to be seen in the whole nave. The western doorway (see Fig. 569) contains two openings with a solid mullion between. This part of the structure has evidently been greatly repaired and altered when the western adjuncts above referred to were removed. The whole of the cathedral is vaulted except the central alleys of the nave and choir. Vaulting shafts are carried up in the nave (Fig. 592) from the string course above the main arcade, and in the choir (see Fig. 589) from the caps of the main piers to the wall head, as if the idea of vaulting the central area had been contemplated; but it is quite