Galen although having made some very good observations had unfortunately until this time been considered unassailable and none of his claims had ever been checked in all this time. Before Vesalius the Galen texts would be preached and a dissection of an animal would follow, Vesalius however took a far more hand-on approach and taught by dissection believing that direct observation was the best way to teach, like Leonardo Da Vinci he also kept notebooks filled with illustrations and diagrams. After some students started copying his work he published these. This produced an attack response from one of his former lecturers.
Sometime later a judge became interested in Vesalius's work and allowed him access to the bodies of criminals for dissection. Because of this Vesalius was able to make the first anatomical correct drawings of that kind (Leonardo's having not been published yet). He also discovered that Galen's research had been mainly based on the dissection of Barbary Apes and began to disprove Galen's observations with proof he had discovered while dissecting the bodies of criminals.