Galen was a Physician just like Vesalius. He was main man in anatomy for a 1000 years. The main difference between them was Galen was not allowed to dissect humans for religious reasons. Galen lived much earlier than Vesalius. Galen was called a founding father of medicine and anatomy. He had the first theories of the how the Human body works and what makes is is made up of. The only thing Galen was missing was dissecting a human body. He often dissected pigs and cats. With the dissection of the animals, he made theories of how the human body worked. Later he was proved wrong on most of his theories by Vesalius. Galen's book has been standard for 100's of years. Vesalius's mission was to prove Galen wrong.
The first thing Vesalius proved wrong was the lower jaw. The lower jaw was one piece of bone, Galen said it was two. Galen said their were holes in the septum of the heart. Vesalius proved that wrong because there were no holes. Galen also said the liver had five lobes. Vesalius also proved this wrong by saying there was just two. Vesalius blames that Galen was only able to dissect animals. Many people were mad at Vesalius and did not believe him because he dared to challenge Galen. In 1543 he published: De Humani Corporis Fabrica, which means the fabric of the human body in English.