For once, this photo doesn't show the ninzioletto of the "Ponte dei Pugni," or "The Bridge of Fists," but rather the pavement. As you can see, in each corner of the bridge (located between Campo San Barnaba' and Campo Santa Margherita, the city's current hot spot for nightlife, in the sestiere of Dorsoduro) is a marble footprint. This bridge is one of only two of its kind in the city (the other is in the sestiere of Cannaregio).
What purpose would these footprints serve? Actually, they were the starting corners, like in a boxing ring, for a peculiar Venetian pasttime, "Battles of the Bridges." You can read more about them at this website and in a study of them written by historian Robert Davis entitled, The War of the Fists: Popular Culture and Public Violence in Late Renaissance Venice. My favorite tidbit of trivia from these tales is that, for a while, fistfighting became so synonymous in Venice with bridges that if two guys wanted to go at it in a campo (the Venetian equivalent of a piazza), they might apparently even build a makeshift bridge on dry land first, just to have a suitable place to whoop each other off of!