Three of the great triumphal arches which were among Rome's unique contributions to architecture stand today: the Arch of Severus at the west end of the Forum, the Arch of Titus at its eastern end, and the Arch of Constantine beyond it. Strangely enough, little is recorded in ancient literature about these three; more is known of those which have disappeared. The three which remain, however, have kept their identity through the centuries because of clearly readable inscriptions.
By the Middle Ages such arches had become outstanding 'marvels' from the imperial past. The Mirabilia defined them as 'Arches Triumphal . . . the which were made for an Emperor returning from a triumph, and whereunder they were led with worship by the senators, and his victory was graven thereon for a remembrance to posterity.' This definition gives a vivid picture of the pageantry of imperial Rome and was undoubtedly in the minds of those who cleared the way for the triumphal procession of Charles V in 1536 from the Arch of Titus through the Forum and the Arch of Severus. It is not, however, completely accurate. The permanent arches probably replaced temporary structures which were erected for the actual procession of a general or emperor after a successful war. In some cases the finished arch was so constructed that no procession could pass under it. That of Severus, for instance, was approached by steps; no road led under it in ancient times; the road did not run beneath but beside the Arch of Constantine.
The Arch of Titus does span the Sacred Way, but though it commemorates the capture of Jerusalem in A.D. 70, the victor never saw it completed; it was erected some time after his death in 81. It is not mentioned in ancient literature, but a late Roman relief shows it with the title ARCUS IN SACRA VIA SUMMA, 'Arch at the summit of the Sacred Way'.
Its medieval name 'Arch of the Seven Lamps', or, as the Mirabilia puts it, 'Arch of the Seven Lamps of Titus and Vespasian', comes from the famous relief on one of its inner faces showing the seven-branched candlestick and other spoils from the temple at Jerusalem carried in triumph through Rome. A later version adds 'where is Moses his candlestick having seven branches, with the Ark'. The oblong object carried in this relief is not, however, the Ark but the golden table of the shewbread used in the temple ritual.
In the Middle Ages the Arch of Titus was included in the fortress of the Frangipani family and had a room built into its upper story. Much of this structure was removed in the fifteenth century but some still appears in Van Heemskerck's drawing. When the supporting buildings at the sides were taken down in 1821, the centre was found to be so weakened that the architect Valadier rebuilt the lost ends in travertine. The original inscription still remains on the eastern side, which appears in the drawing; Valadier copied it for the side facing the Forum.
East of the Arch of Titus and close by the Colosseum stands the Arch of Constantine, which recalls, perhaps more than any other monument in Rome except the Senate House, the victory of Christianity. It was erected about A.D. 315 in honour of Constantine's victory over his rival Maxentius at the Milvian Bridge near Rome in 312--the victory which, according to tradition, led to the emperor's recognition of Christianity the next year. It was just before this battle, so the story goes, that he beheld the vision of the cross with the motto 'In this sign thou shalt conquer'. The inscription extols not only the emperor's greatness of mind but also 'the inspiration of the Deity' (unnamed), but since deity had, by this time, become an imperial attribute, nothing concerning the emperor's religion then can be gathered from this statement. Like the Arch of Titus, that of Constantine passes unnoticed in existing classical literature.
Though harmonious in its general conception, Constantine's arch is largely made up of fragments from earlier monuments. Among these older parts are the round medallions on the two long sides; all the reliefs of the upper story and of the middle passage; and the free-standing figures, now considerably restored, which are above the columns. Of Constantine's time, and of much cruder workmanship, are the narrow bands of relief above the side arches and on the ends, those on the bases of the columns, and the delightfully designed round medallions on the ends showing the setting moon and the rising sun.
In the Middle Ages and Renaissance this was sometimes popularly called the arcus Thracius or Arco de Trasio from the free-standing figures of the Thracian or Dacian prisoners above the columns, which Constantine borrowed from some monument of Trajan's. The Mirabilia, however, gives it the old name, 'Arch of Constantine by the Amphitheatre', keeping for both it and the Colosseum the terms used by Romans of antiquity.