Leonardo da Vinci based his drawings on empirical evidence that he gained through the dissection of animal and human cadavers.
Leonardo da Vinci was in a hospital in Florence speaking to a 100 year old man in the winter of 1507 when the gentleman gently passed away. This inspired Leonardo to return to his work as a scientist and dissect the cadaver of this centenarian. Da Vinci, to that point, had already dissected ten cadavers, but had gotten away from the practice to focus on other endeavors. What is remarkable about Leonardo’s work in anatomy is that he was never formally trained to dissect human tissue, and by all accounts, showed a remarkable expertise in the practice. Da Vinci also used animal corpses to examine tissue and organs. By dissecting an ox, he made the discovery that the heart has four chambers over 500 years before it was published as scientific fact.
Throughout the course of his career, by his own recollection, he worked on over 30 human corpses.