Collagen is an inert, rigid protein found predominantly in skin, ligaments, bones and teeth. Its most distinctive attribute, essential to a transmitter of mechanical force, is inelasticity. Its fundamental structural unit is tropo-collagen, a molecular rod about 2600A in length and 15A in diameter and 300,000 molecular weight. In tendons these macromolecules, grouped as collagen fibrils, run parallel to the axis. In skin the fibrils are interlaced and branched.