1: The Circulatory System

You don't have to be a doctor to know how important the heart is...but back in ancient Greece, you could be a doctor and STILL have no idea how important the heart is.

Back then, doctors like second-century Greek physician Galen believed (no kidding) that the liver (not the heart) circulated blood (along with some bile and phlegm), while the heart (really) circulated "vital spirit"(whatever that is).

How could they be so wrong? It gets worse.

Galen hypothesized that the blood moved in a back-and-forth motion and was consumed by the organs as fuel. What's more, these ideas stuck around for a very long time. How long?

It wasn't until 1628 that English physician William Harvey let us in on our heart's big secret. His "An Anatomical Study of the Motion of the Heart and of the Blood in Animals" took a while to catch on, but a few hundred years later, it seems beyond common sense -- perhaps the ultimate compliment for a scientific idea.