Cities emptied out following the disintegration of the Roman empire in the west. In the high middle ages urbanization, however, gathered momentum. From the eleventh to the fifteenth centuries new patterns of economic relations began to reconfigure the structure of European societies as feudalism gave way to the rise of social systems based upon free exchange. The welfare of the needy was taken up by devout secular philanthropists with the approval of the Church. Philanthropists and political rulers began to found hospitals in the late medieval period with the sanction of the Pope. With the decline of the Roman empire, the classical tradition of learning survived in the Byzantine empire based at Constantinople. From the Greco-Romantradition of medicine was passed on to Arab physicians through Syriac translations prepared by Nestorian and Monophysite Christians, sectarians who were driven out of Byzantium because of their heresies and settled in Persia.