Natural sausage casings are made from the sub-mucosa of the small intestine of meat animals, a layer of the intestine that consists mainly of naturally occurring collagen. In Western European cuisine and Chinese cuisine, most casings come from pigs, but elsewhere the intestines of sheep, goats, cattle and sometimes horses are also used.
From Wikipedia:
Natural sausage casings are made from the sub-mucosa of the small intestine of meat animals, a layer of the intestine that consists mainly of naturally occurring collagen. In Western European cuisine and Chinese cuisine, most casings come from pigs, but elsewhere the intestines of sheep, goats, cattle and sometimes horses are also used.
Breakfast sausage anyone?