Connective tissue includes some living cells, but consists mainly of molecules that the cells secrete into the large spaces between them. The most important of these molecules for the cook are the protein filaments that run throughout the tissue and reinforce it. One, a protein called elastin for its stretchiness, is the main component of blood vessel walls and ligaments, and is especially tough; its cross-links cannot be broken by the heat of cooking. Fortunately there isn’t much of it in most muscle tissue.