Slavery, cinematic or otherwise, is a tricky proposition. Unlike the Holocaust and projects such as the recent ``Band of Brothers,'' there's nothing heroic about American involvement, nothing to latch on to in patriotic terms. Quite the opposite, actually. Revisiting the brutality of bondage often means dealing with the notion that our ancestral bloodlines very likely played a part in this cruel and unusually punishing institution, whether as victimizer or victim. Nothing is more uncomforting and unnerving than that. This explains why ``The Middle Passage,'' airing Saturday night on HBO, would never appear on network television - and not just because its ghastly scene of a tortured African slave with his sliced fingers, blood dripping into the cracks of a rat-infested ship's floor, is too graphic for prime time. It's the subject - and the unfortunate but inescapable fact that viewers will watch most anything ``except'' a show about slavery.