> *It was erroneously believed that tears rose "from the heart" and
> traveled in a duct to the eye. Leonardo actually includes such a duct in
> his cross section of the eye.*
>
> Where would Leonardo have picked up such information? From medical or
> religious tradition?
>
Maybe just from language or metaphor. We still talk about emotions being
heart-felt, or our hearts breaking when we mean we're unhappy. Today, the
language is figurative, but apparently was taken more literally at some
point. Also, may have some edge of truth. People can get heart attacks from
emotional shock.
If you mean how did the idea get started that the heart was the seat of
emotion, maybe just the idea that the heart was necessary to life. My
understanding is that early peoples, say the Egyptians, had no idea of the
function of the brain, and it was removed and discarded during mummification
(not put into the canopic jars that held the other organs). It's hard to
tell what a brain does just by looking at it. It's easier to see that blood
flows through the heart, that it's a pump of sorts.
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