Illustration of the nervous system from De humani corporis fabrica, attributed to Jan Van Calcar, a student of Titian.

During his lectureship at Padua, Vesalius was known for his hands-on approach to teaching anatomy. He pointed out and explained the anatomy of the cadaver as he was dissecting it, which combined the roles of lector, ostensor, and dissector in unorthodox ways. He challenged traditional etiquette and angered many of the older professors, which gave him a reputation as a revolutionary. These representations, however, obscure the context of Vesalius' discoveries. In fact, Vesalius furthered many of the theories and anatomical projects of Galenic medicine and even styled himself as a Second Galen.

One of the first major works Vesalius published, the Tabulae sex, fits very firmly within the Galenic tradition. It was published at the request of some of his students at Padua in 1538 to serve as a teaching aid. The illustrations were stylistically similar to Vesalius' later and more famous book De humani corporis fabrica. Yet the illustrations perpetuated many Galenic errors. In particular, the text maintains the existence of Galen's rete mirabile (a network of veins and arteries that only exists in some vertebrate animals) in humans, due perhaps to Vesalius' continued dissection of animals as well as humans.

Vesalius' extensive use of human dissections, alongside his early writings that supported many aspects of Galen's theories, indicate the tension that existed at the time between classical and Humanist concepts of anatomy. The phrase ad fontes can be seen as a return not to classical sources of knowledge, but to the true source of anatomical knowledge: the human body.