dangerous rival to Della Rovere dynastic ambitions in the Marche, Sixtus placed Venice under interdict in 1483. He also lined the coffers of the state by unscrupulously selling high offices and privileges. In ecclesiastical affairs, Sixtus promoted the dogma of the Immaculate Conception, which had been confirmed at the Council of Basle in 1439, and he designated 8 December as its feastday. In 1476, he issued the apostolic constitution Cum Praeexcelsa, establishing a Mass and Office for the feast. He formally annulled the decrees of the Council of Constance in 1478. Slavery The two papal bulls issued by Pope Nicholas V, Dum Diversas of 1452 and Romanus Pontifex of 1455, had effectively given the Portuguese the rights to acquire slaves along the African Coast by force or trade. Those concessions were confirmed by Sixtus in his own bull, Aeterni regis, of 21 June 1481. Arguably the "ideology of conquest" expounded in those texts became the means by which commerce and conversion were facilitated. In November 1476, Isabel and Fernando ordered an investigation into rights of conquest in the Canary Islands, and in the spring of 1478, they sent Juan Rejon with sixty soldiers and thirty cavalry to the Grand Canary, where the natives retreated inland. Sixtus's earlier threats to excommunicate all captains or pirates who enslaved Christians in the bull Regimini Gregis of 1476 could have been intended to emphasise the need to convert the natives of the Canary Islands and Guinea and establish a clear difference in status between those who had converted and those who resisted. The ecclesiastical penalties were directed towards those who were enslaving the recent converts. Princely patronage As a civic patron in Rome, even the anti-papal chronicler Stefano Infessura agreed that Sixtus should be admired. The dedicatory inscription in the fresco by Melozzo da Forlì in the Vatican Palace records: "You gave your city temples, streets, squares, fortifications, bridges and restored the Acqua Vergine as far as the Trevi..." In addition to restoring the aqueduct that provided Rome an alternative to the river water, which had made the city famously unhealthy, he restored or rebuilt over 30 of Rome's dilapidated churches such as San Vitale (1475) and Santa Maria del Popolo, and he added seven new ones. The Sistine Chapel was sponsored by Sixtus IV, as was the Ponte Sisto, the Sistine Bridge (the first new bridge across the Tiber since Antiquity), and the