Surviving medical texts are a testament to the work of Muslim physicians and their desire to understand and heal the human body. Physicians practicing in the Islamic world drew on the works of early physicians such as Galen and Dioscorides (see image 18), which contained information about the healing properties of plants. Physicians also drew upon pre-Islamic "folk" practices. By the later eighth century, the Abbasid court's interest in medical and scientific knowledge led to the creation of the famous House of Wisdom (Bait al-Hikma) in Baghdad, in which scientific texts were translated, studied, and preserved. Through these efforts, physicians had access to an extensive body of medical writings—some in their original language and others translated into Arabic. By the end of the ninth century, concepts such as Galen's theory of the four humors (black bile, yellow bile, phlegm, and blood) had been completely absorbed into Arab medical theory and practice.
Once this extensive corpus of medical writings became widely available, the need for systematization became more important. Al-Razi (known in the west as Rhazes), a ninth-century medical pioneer from Iran and the first to write about measles and smallpox, took on the monumental task of compiling the corpus of Islamic medical knowledge into one source—the formidable Comprehensive Book of Medicine. Scientists from the Islamic world were also responsible for many original innovations in the field of science and medicine. For instance, one of the world's most important early physicists, Ibn Al-Haytham, wrote a famous and influential treatise on how the human eye works, which still forms the basis for modern optical theory.
By the beginning of the thirteenth century, Islamic medical sources (including both original writings and translations of classical treatises) began to make their way to the West, where they were eventually incorporated into European medical theory and practice.