What is Sausage Casing?
Sausage casing or just casing is the material that encloses the filling of a sausage. Casings are typically divided into two categories, natural and artificial. 
Natural casings are made from the submucosa, a layer of the intestine that consists mainly of collagen. The fat and the inner mucosa lining are removed. Natural casings tend to be brittle once cooked and tend to "snap" when the sausage is bitten. They may also rupture during the cooking process; often, this indicates that the cooking was done too rapidly. Natural casings may be hardened and rendered less permeable through drying and smoking processes. Natural casings are generally made from porcine, bovine or ovine intestines. 
Artificial casings are made of cellulose or even plastic and may not be edible.