The passage intervening between the rectangular enclosure and the octagon is divided into two stories, thus providing the church with an ambulatory below and a gallery above.
The domed octagon which forms the core of the building stands at a distance of some 181⁄2 feet from the rectangle within which it is placed. It measures 531⁄2 feet by 501⁄2 feet. The eight piers at its angles rise to a height of 331⁄2 feet from the floor to the springing of the dome arches. The archways thus formed, except the bema arch, are filled in with two pairs of columns in two stories set on the outer plane of the piers. The lower colonnade is surmounted, after the classic fashion, by a horizontal entablature profusely carved while the upper columns are bound by arches, thus making seven sides of the octagon a beautiful open screen of fourteen columns and as many triple arcades, resplendent with marbles of various hues and rich with carved work. The mass of the piers is relieved by their polygonal form, a fluted cymatium along their summit, and a repeating design of a flower between two broad leaves below the entablature. Though the flower points upwards it has been mistaken for a cluster of grapes.[108] At the four diagonal points the sides of the octagon are semicircular, forming exhedrae, an arrangement which gives variety to the lines of the figure, widens the central area, secures more frontage for the gallery, and helps to buttress the dome. The same feature appears in S. Sophia, whereas in San Vitale all the sides of the octagon, excepting the eastern side, are semicircular. The extension of the interior area of a building (square or octagonal) by means of niches at the angles or in the sides, or both at the angles and in the sides, was a common practice.[109]
There is considerable difference in the size of the piers and the dome arches. The eastern piers stand farther apart than their companions, and consequently the arch over them, the triumphal arch of the sanctuary, is wider and loftier than the other arches. The bays to the north-east and the south-east are also wider than the bays at the opposite angles. The apse is semicircular within, and shows three sides on the exterior. As in S. Sophia and S. Irene, there is no prothesis or diaconicon.
The pairs of columns, both below and above, are alternately verd antique and red Synnada marble, resting on bases of the blue-veined white marble from the island of Marmora. The capitals on the lower order are of the beautiful type known as the 'melon capital,' a form found also in San Vitale at Ravenna and in the porch of S. Theodore in Constantinople (p. [ 246]). The neckings are worked with the capitals, and enriched by 'egg-and-dart' pointing upwards. In the centre of the capitals was carved the monogram of Justinian or that of Theodora. Most of the monograms have been effaced, but the name of the empress still appears on the capital of the western column in the south bay, while that of Justinian is found on the first capital in the south-western bay; on both capitals in the north-western bay, accompanied by the title Basileus; and, partially, on the last capital in the north-eastern bay.
In the soffit of the architrave are sunk panels of various patterns, the six-armed cross occurring twice. The beadings of the fasciae are enriched with the designs commonly known as 'rope,' 'bead-and-reel,' 'egg-and-dart,' and again 'bead-and-reel.'
The frieze is in two heights. The lower portion is a semicircular pulvinar adorned with acanthus leaves, deeply undercut; the upper portion is occupied by a long inscription in raised ornamental letters to the honour of Justinian, Theodora, and S. Sergius. The cornice is decorated with dentils, 'bead-and-reel,' projecting consols, 'egg-and-dart,' and leaves of acanthus.
The inscription (Fig. [20]) may be rendered thus: Other sovereigns, indeed, have honoured dead men whose labour was useless. But our sceptred Justinian, fostering piety, honours with a splendid abode the servant of Christ, Creator of all things, Sergius; whom nor the burning breath of fire, nor the sword, nor other constraints of trials disturbed; but who endured for the sake of God Christ to be slain, gaining by his blood heaven as his home. May he in all things guard the rule of the ever-vigilant sovereign, and increase the power of the God-crowned Theodora whose mind is bright with piety, whose toil ever is unsparing efforts to nourish the destitute.
The inscription is not mere flattery to the founders of the church. Justinian and Theodora were devout after the fashion of their day, and took a deep interest in the poor. The empress erected an asylum for fallen women, hostels for strangers, hospitals for the sick, and homes for the destitute. 'On the splendid piece of tapestry embroidered in gold which formed the altar cloth of S. Sophia, she was represented with Justinian as visiting hospitals and churches.'[110]
