By default, Galen was able to view much of the abdominal cavity. His study on pigs and apes, however, gave him more detailed information about the organs and provided the basis for his medical tracts. Around 100 of these tracts survive and fill 22 volumes of modern text. His two great anatomical works are "On anatomical procedure" and "On the uses of the parts of the body of man". The information in these tracts became the foundation of authority for all medical writers and physicians for the next 1300 years until they were challenged by Vesalius and Harvey in the 16th century. It was through his experiments that Galen was able to overturn many long-held beliefs, such as the theory that the arteries contained air which carried it to all parts of the body from the heart and the lungs. This belief was based originally on the arteries of dead animals, which appeared to be empty. Galen was able to demonstrate that living arteries contain blood, but his error, which became the established medical orthodoxy for centuries, was to assume that the blood goes back and forth from the heart in an ebb-and-flow motion. From the 3rd century BCE until the 12th century, human anatomy was mainly learned through books and animal dissection. While it was claimed by 19th century polemicists that human dissection became restricted after Boniface VIII passed a papal bull that forbade the dismemberment and boiling of corpses for funerary purposes and this is still repeated in some generalist works, this claim has been debunked as a myth by modern historians of science. For many decades human dissection was thought unnecessary when all the knowledge about a human body could be read about from early authors such as Galen. In the 12th century, as universities were being established in Italy, Emperor Frederick II made it mandatory for students of medicine to take courses on human anatomy and surgery. Students who had the opportunity to watch Vesalius in dissection at times had the opportunity to interact with the animal corpse. At the risk of letting their eagerness to participate become a distraction to their professors, medical students preferred this interactive teaching style at the time. In the universities the lectern would sit elevated before the audience and instruct someone else in the dissection of the body, but in his early years Mondino de Luzzi performed the dissection himself making him one of the first and few to use a hands on approach to teaching human anatomy. Specifically in 1315, Mondino de' Liuzzi is credited with having "performed the first human dissection recorded for Western Europe."