Well There Is Offal and There Are Meat Scraps
Offal(brains, head, tounge, trotters,muscle and organ meat) is something one person won't touch, while another consider's good eats. Where it cannot be sold and or there is a surplus, offal often goes into pet food.
Meat scraps, say the left overs from a roast or whatever, are often put into sausage, but then again so is offal, you just have to know (or ask), what you are getting.
Mr. Armour (of Chicago meatpacking fame), got the idea to use "every part of an animal except the tail", and set his factories to work turning what was once discarded parts into everything from canned meats, pet food, sausages, to hair for brushes, buttons, gelatin, and god only knows what else.
Local butchers often were doing this already, that is one could purchase rendered fat, sheep, cow or pig heads, brains and or any other part of a slaughtered animal from just down the corner.
Once while at school for a "science day" our teacher brought in "offal". IIRC my group got brains, and soon as the thing was "dumped" onto a desk (covered with a cloth, IIRC), I was out like a light. Later when walking home one of my school mates couldn't understand what my problem was. Her family had been eating brains for long as she was around. That was it for me, and put me off my dinner that night! *LOL*