Structural proteins are those proteins with the primary purpose of producing the essential structural components of the cell. Measured in terms of their total mass, the largest class is the structural one. Structural proteins are fibrous proteins. The most familiar of the fibrous proteins are probably the keratins, which form the protective covering of all land vertebrates: skin, fur, hair, wool, claws, nails, hooves, horns, scales, beaks and feathers. Equally widespread, if less visible, are the actin and myosin proteins of muscle tissue. Another group of fibrous structural proteins are the silks and insect fibers. In addition, there are the collagens of tendons and hideswhich form connective ligaments within the body and give extra support to the skin where needed.