I'm not that good with French, but verse IV appears to be about guards pissing in the next cell. La Fontaine, I think, was the title of Marcel Duchamp's urinal

I'm not that good with French, but verse IV appears to be about guards pissing in the next cell. La Fontaine, I think, was the title of Marcel Duchamp's urinal installation; I suspect it might be a common euphemism?