Volume IV deals with the 'Middle Ages'. It starts with the expansion of Islam and closes with the discovery of the New World. Various events during this period led to a significant expansion in communications: the rapid spread of Islam and of Gengis Khan's Mongol Empire, as well as the Crusades and the development of trans-Saharan and maritime routes around Africa to the Indian Ocean, leading to multiplied exchanges between the peoples and cultures of Africa, Asia and Europe.