Heart. An organ whose importance was known throughout the history, but whose real function was not.
The one interesting fact about heart that springs to my mind is that one of the most important discoveries into how it actualy functions was done by a Czech anatomist Jan Evangelista Purkyně who discovered the specialized impluse-conveying neurons in 1837 (but I had to look that date up, since I have terrible memory for numbers). They bear his name “Purkynje fibers”.
I wonder whether kids in other nations around the world get to hear his name when learning about heart.