The protein used in pet food comes from a variety of sources. When cattle, swine, chickens, lambs, or other animals are slaughtered, the choice cuts such as lean muscle tissue are trimmed away from the carcass for human consumption. However, about 50% of every food-producing animal does not get used in human foods. Whatever remains of the carcass -- bones, blood, intestines, lungs, ligaments, and almost all the other parts not generally consumed by humans -- is used in pet food, animal feed, and other products. These "other parts" are known as "by-products," "meat-and-bone-meal," or similar names on pet food labels.