Because of its role, there are many blood vessels in the spleen, making it very prone to bleeding if it is damaged. It (along with the liver) is one of the more common sites of serious internal bleeding after physical trauma.
Galen and Hippocrates vital humors theory
Granted I was reading about this last night, but I can't remember it to the full extent now.
[1] Black bile is actually the source of the term "melancholy" — from the Greek melankholia, from melan- ("black") and kholē ("bile")!
>“The gall, placed in the concave of the liver, extracts choler to it: the spleen, melancholy; which is situate on the left side, over against the liver, a spongy matter, that draws this black choler to it by a secret virtue [emphasis mine], and feeds upon it, conveying the rest to the bottom of the stomach, to stir up appetite, or else to the guts as excrement.”
~ Robert Burton, Anatomy of Melancholy
Not exactly "Renaissance".
(I'm also French, but my litterature teacher had explained that "spleen" was an organ in English)
... and in German it's related to craziness. "Einen Spleen haben" literally "Having a spleen" means someone is quirky or eccentric.
What's kind of funny is that the French word for this organ, "rate", is used in the idiom "Se dilater la rate", which means laughing a lot.
So the same organ's name is related to both sadness and laughter, depending on the language.
I now realize that I still don't know what a spleen is or what it does in the body; time to fire up Wikipedia!