The McRib contains a mixture of tripe, heart, and scalded stomach, which is then mixed with salt and water to extract proteins from the muscle. The proteins bind all the pork trimmings together so that it can be re-molded into any specific shape — in this case, a fake slab of ribs. http://finance.yahoo.com/news/11-amazing-facts-about-the-mcdonald-s-mcrib-170212930.html
My college roommate (who was gorgeous) always went ape shit when the mc rib came back. I swear she'd eat one everyday. I only go apeshit for the Shamrock shakes.