Flemish anatomist who founded the sixteenth century heritage of careful observation characterized by "refinement of observation." Vesalius changed the organization of the medical school classroom, bringing the students close to the operating table. He demonstrated that, in many instances, Galen and Mondino de' Luzzi were incorrect (the heart, for instance, has four chambers). He conducted his own dissections, and worked from the outside in so as not to damage the cadaver while cutting into it. Vesalius also wrote the first anatomically accurate medical textbook, De Humani Corporis Fabrica (1543), which was complete with precise illustrations. Vesalius's careful observation, emphasis on the active participation of medical students in dissection lectures, and anatomically accurate textbooks revolutionized the practice of medicine. Through Vesalius's efforts, medicine was now on the road to its modern implementation, although major modifications and leaps of understanding were, of course, necessary to make its practice actually safe for the patient.