The north/south "Middle" is not a usage I've heard. There is an old custom to refer to the region between North and South (basically MD, WV, KY, and MO, roughly the slave states that did not secede in 1861) as the Border states. But this may be fading; though I've seen it used in many older books, even to refer to much more recent periods than the Civil War, I also now find there are many people today who do not know the term.