Andreas Vesalius' great contribution was in the field of anatomy.
Before Vesalius, doctors relied on the works of Galen and other ancient writers. However, Galen had only dissected the bodies of animals, which were different from humans.
In 1537, aged just 22, Vesalius became professor of medicine at Padua University. He insisted that his medical students should perform dissections to find out how the human body worked.
A local judge took an interest in Vesalius' work. He allowed Vesalius to use the bodies of executed criminals for dissection. Vesalius was now able make repeated dissections of humans.
In 1543 Vesalius published his book, The Fabric of the Human Body. He employed artists to make accurate drawings of the human body. These gave doctors more detailed knowledge of human anatomy.
Vesalius had proved that some of Galen’s ideas on anatomy were wrong, eg Galen claimed that the lower jaw was made up of two bones, not one. He encouraged others to investigate for themselves and not just accept traditional teachings.