Plural of fistula; abnormal passages or tubes in the body, usually formed by disease or injury, that connect internal organs or the skin to cavities.
From Latin fistula meaning 'a pipe or reed,' because these abnormal passages resembled hollow tubes. The Latin plural -ae is used in medical terminology in English.
Medieval surgeons noticed that infected wounds sometimes formed hollow tunnel-like passages that leaked fluid, and since these looked like little pipes, they named them after the Latin word for 'reed'!
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