THE MECHANISM OF TONGUE PROJECTION IN CHAMELEONS 11. ROLE OF SHAPE CHANGE IN A MUSCULAR HYDROSTAT

In this paper we investigate the interaction between the accelerator muscle (the muscle that powers tongue projection) and the entoglossal process (the tongue's skeletal support) that occurs during tongue projection in chamaeleonid lizards. Previous work has shown that there is a delay of about 185 ms between the onset of accelerator muscle activity and the onset of tongue projection. In conjunction with anatomical observations, in vitro preparations of the accelerator muscle mounted on isolated entoglossal and surrogate processes were stimulated tetanically, and the resulting movements were recorded on video at 200fields s-'. Three results indicate that morphological features of the entoglossus and the accelerator muscle delay the onset of tongue projection following the onset of accelerator contrac-tion: (1) the entoglossus is parallel-sided along the posterior 90 % of its shaft, only tapering at the very tip, (2) the sphincter-like portion of the accelerator muscle, which effects tongue projection, makes up the posterior 63 % of the muscle and does not contact the tapered region of the entoglossus at rest, and (3) accelerator muscles mounted on the entoglossus undergo longitudinal extension and latera