GLC
Head Chef
And as far as I know the traditional sausage casing is intestines.
Kind of. Natural casing is the intestinal sub-mucosa, the thin, smooth layer just below the heavily folded an slimy mucosa. So there's some good bit of processing to remove the outer muscle and fat layers. With beef casings, a layer of esophagus or bladder or one of the stomachs is also used. (Those big baloneys.) Intestines are processed using rollers or some other pressure device to crush and loosen everything that's not the tough sub-mucosa. (Again, little is wasted. Mucosa is used to make medical blood thinner.) What's left is collagen, the substance that makes skin, tendons, etc. tough enough to work and that makes heavily used muscle meat tough - that is, until it's cooked slow and moist and becomes gelatin, as we do with "cheap" cuts. And it's the collagen that's tough enough to survive the prep process and to contain the sausage filling under pressure. Likely it wasn't any great leap for early man to discover casing. They were already using skin, tendon, etc. by cleaning off the unwanted layers, and the toughness of intestines would have suggested that there was a useful layer to be had for use as bindings, and bladders and stomachs were very early used to contain water and to cook food prior to ceramics.