Almost everything we consider common knowledge today was once a total mystery. Around the second century AD, no one in the western world knew that the arteries, veins, and heart were filled with blood. Most thought they were filled with air. Here’s how one man disproved that theory.
It is my understanding that the word “artery” is derived from the Greek word for windpipe, “arteria”; so we are kind of holding on to the misconception.
One of my favourite stories about how digestion functions was learnt during a war (Napoleonic?). Anyone could be a doctor in those days, and lots learnt from sowing up soldiers… One soldier sustained a wound to the tummy, allowing a doctor to feed the soldier directly. But I don’t think the soldier made it 😉