In the central chamber there are four niches in which formerly stood the urns and tombstones of the illustrious persons buried here. A large sarcophagus of porphyry found here was used for the tomb of Pope Innocent II. in the Lateran, and the lid may still be seen in the Baptisterium of St. Peter’s, where it is used as a font. The chamber was lighted and ventilated by square passages cut through the stone in a slanting direction, and the rain water was carried off by other channels, which conveyed it into drains at the foot of the building. It does not appear to be certainly known whether other chambers may not exist in the interior which have not been yet discovered. Piranesi gives a number of additional chambers besides the two above mentioned, but his representation is probably conjectural. After the burial of Nerva no more room was left in the Mausoleum of Augustus for the interment of the imperial ashes. Trajan’s remains were deposited under his column in the forum bearing his name, but Hadrian gladly seized the opportunity of adding another to the many colossal structures he had already reared. The mausoleum was begun at the same time with the Ælian bridge in the year A.D. 135. The bricks of which part of the building consists have stamps of various years of Hadrian’s reign, and show that the greater part of the building was erected by him, though Antoninus Pius probably completed it. Hadrian’s son Ælius, who died before his father, was the first Cæsar whose ashes were placed in this tomb. After him, Hadrian himself was buried here, and then the Emperor Antoninus Pius, and his wife the elder Faustina, three of their sons, Fulvius Antoninus, M. Galerius Aurelius Antoninus, and L. Aurelius Verus, the colleague of M. Aurelius in the empire, and a daughter Aurelia Fadilla. No record has been preserved of the burial of M. Aurelius, but it seems probable that his ashes were deposited here, as the Mausoleum of Hadrian continued to be the tomb of the Antonines till the time of Severus, who built a third imperial monument, the Septizonium, on the Appian Road.[106] Four children of M. Aurelius were buried here, who died during their father’s life, named Aurelius Antoninus, T. Ælius Aurelius, and Domitia Faustina, and also his miserable son and successor the Emperor Commodus. The inscriptions recording all these burials, were copied by the anonymous writer of Einsiedlen in the ninth century, when they were apparently still legible upon the south wall of the square basement. The inscriptions recording the names Hadrian and M. Aurelius may have been placed upon the upper part of the tomb, like those on the Plautian Tomb and the Tomb of Cæcilia Metella, and may therefore either have escaped the notice of the above-mentioned anonymous traveller, or have been stripped off with the marble casing of the exterior.