Morphine, like many prescription drugs, may require larger doses for pain control when used for extended periods of time. Over time, the body can develop a tolerance for the medication and will require more medication to experience the same level of effectiveness.
By Kimberly Munro Fri, Mar 18, 2011. The first installment of our Ancient Peru series relates what archaeology has revealed about its first cities and civilizations. The expansive, culturally rich civilizations that existed in the Americas at the time of European contact came as a great surprise to the early Spanish explorers. Most notable were the Aztec empire in Central Mexico and the Incan empire in Peru, both of which controlled great expanses of land and millions of people.
History of the Aztecs and the History of the Maya and History of Mesoamerican civilization. There are wiki entries for Mayan Civilization, Aztec, Inca Empire, Mexica, Mesoamerica (nice timeline included here; very good article), Andean civilizations. All of these entries are good starting places for further research.