The casing is an animal product made by scraping the large and small intestines of livestock. It is mainly used as a coat for filling sausages and enemas; sheep casings can also be made into catgut for making tennis racket lines, bow strings and surgical sutures. The casings are divided into pig casings, sheep casings and cattle casings according to animals. According to the parts, there are large casings and small casings. The intestine is divided into four layers: mucosa, submucosa, muscularis and serosa from the inside out. According to the different requirements of animal species and processed products, the treatment of the upper four layers is also different. Good quality casings should be tough and translucent. Now emerging is collagen edible casings.