Awhile back I came across a panoramic photo of a beef slaughterhouse from Chicago's Jungle-era stockyards. Here's a look inside a pig slaughterhouse, with some explanation from "A Trip Through the Union Stockyards and Slaughterhouses".
The hogs are generally shipped in double-deck cars by farmers from all parts of the country to the Stock Yards. When they arrive they are fed and weighed and when sold are sent direct to the shackling pen of the slaughter houses through viaducts erected for that purpose. These viaducts are more than a mile in length.
The shackling pen is filled with hogs, and a boy in among them fastens a shackle upon one of the hind legs of the hog; a chain is hooked into a ring of the shackle and the hog is hoisted by machinery to a man who places the shackled hog upon a greased slanting rail; the end of the chain is then loosened and thrown back to the shackler again.
After stopping a few seconds to let the blood drain out the hog is allowed to slide down the greased rail until he reaches a boy who places a hook in the ring of the shackle, and then allows the hog to slide off the end of the rail. The sudden jerk caused by the drop of the hog causes the shackle to slip off of his leg and he falls into a pan of hot water. He is then kept rolling in the water by men with poles until he reaches the end of the pen where there is an apparatus for throwing him out upon a table.
The hair is then taken from his ears, after which he is fastened to the "scraper," a machine so arranged as to take almost all the hair off. After this he is scraped off by hand, of all the hair remaining in small bunches his head is then taken off.
From here he is sent to men who might be called "hog barbers," for they do nothing but shave hogs. When these men are through with him he looks very clean and white. Next his stomach is opened and the intestines taken out. This is done in short order by one man, and when through with him a swift stream; of water is forced all over him, giving the hog the last washing he is to have.