Bocche dei Leoni
'Mouths of Truth'
These were the terrible Lions’ Mouths… these were the throats down which went the anonymous accusation thrust in secretly in the dead of night by an enemy, that doomed many an innocent man to walk the Bridge of Sighs and descend into the dungeon which none entered and hoped to see the sun again.Mark Twain, Innocents Abroad
No fan of Venice, Mark Twain's imagination went a bit crazy. Although the bocche dei leone (they were also called the 'mouths of truth') were indeed slots set up by the Republic for complaints against fellow citizens or the State, no unsigned accusations without the support of two witnesses were even considered by the Council of Ten (and accusers knew they themselves would be given the punishment of the crimes if they reported falsely).
For all that, the procedure had such an evil reputation that when someone joked to Montesquieu that he was being watched by the Ten, he immediately packed his bags and left town.
Napoleon ordered them taken down to prove that French law was now the law of the land, but six bocche di leone have survived in the city. Seek them out on the Palazzo Ducale (there are two), in the Torcello museum, San Martino, Santa Maria della Visitazione, and the gate of the Law School of the University of Venice (although this last one may be a copy).