Fistulae are produced by contusions and tubercles, and they are also
occasioned by rowing, on horseback, when blood accumulates in the
nates near the anus. For, having become putrid, it spreads to the
soft parts (the breech being of a humid nature, and the flesh in which
it spreads being soft), until the tubercle break and corrupt below
at the anus. When this happens, a fistula is formed, having an ichorous
discharge, and faeces pass by it, with flatus and much and abomination.
It is produced, then, by contusions when any of the parts about the
anus are bruised by a blow, or a fall, or a wound, or by riding, or
rowing, or any such cause. For blood is collected, and it, becoming
corrupted, suppurates; and the from the the same accidents happen,
as have been described in the case of tubercles.