http://en.wikipedia.org/wiki/Casing_%28sausage%29

Sausage casing is the material that contains and encloses the filling of a sausage. Casings may be either natural or artificial.

Natural casings of sausages 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 come mainly from sheep and pigs intestines, though beef intestine is also used.

Artificial casings are made of collagen, cellulose, or even plastic. Not all casings are edible.

Moisture and heat make casings softer and more porous.