Immobilization of Function

a type of change in organs during the historical development of animals. (The term was introduced by A. N. Severtsov.) In immobilization of function an organ that has had active motor function loses it; however, the organ is not reduced but preserved owing to the acquisition of another function, which, although passive, is useful to the organism. The most common instances of immobilization of function are in the fusion of previously movable parts of the skeleton. For example, the primitive upper jaw (palato-quad-rate cartilage) in the majority of fish is movably joined to the axial cranium, while in all terrestrial vertebrates the upper jaw fuses with the axial cranium, forming the floor of the cranium.