in the interior the pictorial effect of the black and white marble stripes on the walls and columns strikes the eye. Black and white are the colours of the civic coat of arms of Siena. The capitals of the columns in the west bays of the nave are sculpted with allegorical busts and animals. The horizontal moulding around the nave and the presbytery contains 172 plaster busts of popes dating from the 15th and 16th centuries starting with St. Peter and ending with Lucius III. The spandrels of the round arches below this cornice exhibit the busts of 36 emperors. The vaulted roof is decorated in blue with golden stars, replacing frescoes on the ceiling, while the formerets (half ribs) and the tiercerons (secondary ribs) are adorned with richly elaborated motifs.