Myth or reality: Cactus can save your life

Does anyone know the answer to this old saw? Is it really true what you’ve seen in a hundred old westerns, where a parched cowboy lives to fight another day by drinking from a cactus? Can you really cut one open and drink its juice to stay alive if you’re lost in the desert and out of water? I’ve heard that the juice–if there is any–is too viscous to quench a thirst. (What brought this to mind was that I was watching a survival show, and the host created drinking water from his urine by building a solar still out of a sheet of plastic, a cup, a stick, and a few rocks.