Back in the day folks ate the whole animal. So do we, technically, but the odd bits tend to be disguised better.
Pig brains, scrapple and head cheese are 3 different things.
Brains - no explanation needed. My mother used to scramble them with eggs.
Scrapple is leftovers from butchering - bones, meat scraps, some times liver heart etc. boiled with onions salt & pepper. The bones are picked out, the meat & onions chopped fine then put back into the boiling liquid with cornmeal and cooked until it's solid. The result is put into pans, chilled then sliced and fried. It's quite good. I still make it, but I cheat - I use breakfast sausage, as I don't butcher (kind of difficult if you don't live on a farm)
Head cheese is a pig head - brains removed, that's been simmered until the meat falls off the bone. The meat is packed in pans or sausage casing with the liquid and chilled. The skin and ears make the broth very gelatinous and give the sausage it's consistency. I like it, as do folks I've served it to. I just call it "terraine du tete".