Andreas Vesalius contributed to modern science by questioning the existing view of human anatomy. This view was established by Galen, an ancient Greek doctor, who wrote many books and articles on the human body. Galen's understanding of human anatomy, however, was not based on dissections of the human body. In fact, Galen often dissected animals (whose anatomy is very different from humans') and made a number of mistakes as a result.
In contrast, Vesalius believed human dissections were crucial to understand human anatomy. As a young physician, Galen was given access to the bodies of criminals to dissect and use in his studies. In 1543, he published his most influential work, De Humani Corporis Fabrica (On the Fabric of the Human Body), in which he presented his view of human anatomy based completely on his own observations.
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