Structural correlates of function in the "opercularis" muscle of amphibians.

This study characterizes the fine structure of the "opercularis" muscles of selected frogs and salamanders (Genera: Hyla; Desmognathus; Ambystoma). The "opercularis" muscle originates on the shoulder girdle and inserts on the opercular plate in the fenestra ovalis of the otic capsule. Each of the three genera used exhibits one of the major gross dispositions of this muscle found in amphibians. In each case the "opercularis" muscle contains large numbers of tonic fibers: 80% in Hyla; 90% in Desmognathus; 45% in Ambystoma. These fibers correspond to the class-5 tonic fibers of Smith and Ovalle (1973). The remained of the fibers in the "opercularis" correspond to those in the class-3 "phasic" of Smith and Ovalle. The muscle from which the "opercularis" is derived (levator scapulae in Hyla, cucullaris in Desmognathus) is comprised of fibers which correspond to the class-2 phasic fibers of Smith and Ovalle. The fiber composition of the "opercularis" indicates that it is constructed to sustain contraction over long periods of time. This composition is supportive of the functional role in audition proposed for the muscle by Lombard and Straughan (1974). Evidence is presented that indicates that fiber size may be body size dependent and thus is an inappropriate criterion of fiber type identification.