Collagen a tough protein which is one of the main structural materials of the bodies of animals. Tendons and cartilage are mostly collagen; it provides most of the strength of skin; bone is a composite of collagen and the hard mineral calcium phosphate. Collagen is also found in many other tissues, including muscle.
The ‘gristle’ in meat is collagen. It is not only hard to chew but impossible for a human to digest. However, it may be boiled down to make gelatin, which is digestible although of low nutritional quality.