Insula Tiberina. -- The island in the Tiber seems to be the extremity of the ridge of which the Capitoline is a part. Owing perhaps to the harder character of its tufa, the river did not cut it away entirely, but divided and flowed on either side. The island thus formed is 269 meters long and its greatest width is 67 meters. According to tradition, its formation was due to the great quantity of grain which was cut from the estates of the Tarquins in the campus Martius after the expulsion of the kings, and thrown into the river just above this point. Whether the first bridge, built by the Romans, crossed the island or not, there is no allusion to any connection between it and the city until 291 B.C., and it formed no integral part of the city until sometime after that date. In the reorganization of Augustus, it was included in region XIV.
In the year 292 B.C., in consequence of a pestilence in Rome, an embassy was sent to Epidaurus to bring back the statue of the god Aesculapius. The embassy returned the next year, bringing, not the statue, but a serpent from Epidaurus, which abandoned the ship and swam to the island. A temple to Aesculapius was at once erected and the whole island consecrated as its temenos. It became therefore sacra, and did not pass into private possession. The island was also known as insula Aesculapii, insula serpentis Epidauri, and inter duos pontes. The temple was restored, probably about the time when the pons Fabricius was built, and its site is now occupied by the church of S. Bartolomeo. Some of the columns of the nave belonged probably to the temple or to the neighboring porticus.
Two other temples were afterwards erected within the original temenos of Aesculapius. The temple of Faunus, which was vowed in 196 and dedicated in 194 B.C. It was built with money received in fines, as is described as prostyle in form. The temple of Jupiter, which was vowed by L. Furius Purpureo in 200 B.C. and dedicated January 1, 194. It is probable that the cult here celebrated was that of Jupiter Veiovis, and that this temple stood in some relation to that of Jupiter Veiovis on the Capitoline.
Besides these temples, there was a shrine to the river god Tiberinus, to whon a sacrifice was offered on December 8, and an alter to Semo Sancus or Deus Fidius, which gave rise to the belief among the early Christians that Simon Magus was worshipped here.
As a result of the legend that the serpent had been brought by ship from Epidaurus, the island itself was made to resemble a ship. A stone platform was built round it, and upon it a wall was erected which in shape exactly reproduced the sides of a Roman ship. Before the great changes in the riverbed caused by the building of the new embankments, a considerable portion of the travertine stern could still be seen at the east end of the island, and even now a fragment of the wall may be seen under the staircase leading down from the morgue. An obelisk, fragments of which are in the museum at naples, is thought to have represented the mast. We have no information as to the time when this curious idea was carried out, but the remains of the wall point to the same period that the pons Fabricius, and it is quite possible that the erection of the two stone bridges was part of the same plan as the building of the ship.
Suetonius says that sick slaves were brought to the temple of Aesculapius and left there to be cured, and in general it appears that there was some attempt to reproduce the effect of the the great sanitarium at Epidaurus. (Platner)
Vincenzo Fasolo, "The Campo Marzio of G. B. Piranesi".