The protein collagen is responsible for the tough qualities of connective tissue. It is composed of coiled fibrous molecules, each one a sequence of amino acids held in a helix by hydrogen bonds and cross-links between the acids. The number of links increases as the animal ages; it is this, not an increase in the proportion of connective tissue generally, that makes the meat of older animals tougher than that of youngsters. The proportion of connective tissue varies according to the position of the meat in the animal’s body. The more there is present, the tougher the meat will be, and the cheaper the cut.