Ponte San Rocco on Rio de la Frescada, looking towards the sweet Calle del Scaleter, Dorsoduro. Scaleteri were vendors of candy and biscuits. Pasticceria "Tonolo" is a stone throw away at the intersection of Crosera and Calle San Pantalon. The device shown in the photo is one of the many scattered all over Venice. They seem two have had two different functions: as "gobbe antibandito" (antibandit bump) they were built to prevent criminals from lurking in dark corners, and as "pissotte" to prevent urination, especially near sacred places. The one shown below can be found near Chiesa de la Fava.  Thanks to Yvonne for her lead on the subject. You can read more about it in the wonderful venessia.com