History Channel's: How Sex Changed the World: Had a section about Leonardo da Vinci and his insights into human sexuality.
During the 1500's it was believed that a "Ghostly wind" emanated from the liver and testicles and filled up the penis with air to cause an erection. Dissection was illegal in the 1500's but Leonardo's managed to dissect many bodies anyways - probably by hiring grave robbers or by using the bodies of criminals.
Ghostly wind that emanated from the liver and filled up the hollow penis with air. What they thought at that time.
1500's Dissection was illegal. Inquisition. Grave robbers.
The penis is filled with blood. Pelvic bone is the structure that pushes the penis. Sexual biology.
"Da Vinci was never really capable of fully documenting his discoveries because of the overriding mores and bans on human dissection, particularly the sexual dissections."