Why the chameleon has spiral-shaped muscle fibres in its tongue

The intralingual accelerator muscle is the primary actuator for the remarkable ballistic tongue projection of the chameleon. At rest, this muscle envelopes the elongated entoglossal process, a cylindrically shaped bone with a tapering distal end. During tongue projection, the accelerator muscle elongates and slides forward along the entoglossal process until the entire muscle extends beyond the distal end of the process. The accelerator muscle fibres are arranged in transverse planes (small deviations are possible), and form (hitherto unexplained) spiral-shaped arcs from the peripheral to the internal boundary. To initiate tongue projection, the muscle fibres probably generate a high intramuscular pressure. The resulting negative pressure gradient (from base to tip) causes the muscle to elongate and to accelerate forward. Effective forward sliding is made possible by a lubricant and a relatively low normal stress exerted on the proximal cylindrical part of the entoglossal process. A relatively high normal stress is, however, probably required for an effective acceleration of muscle tissue over the tapered end of the process. For optimal performance, the fast extension movement should occur without significant (energy absorbing) torsional motion of the tongue. In addition, the tongue extension movement is aided by a close packing of the muscles fibres (required for a high power density) and a uniform strain and work output in every cross-section of the muscle. A quantitative model of the accelerator muscle was developed that predicts internal muscle fibre arrangements based on the functional requirements above and the physical principle of mechanical stability. The curved shapes and orientations of the muscle fibres typically found in the accelerator muscle were accurately predicted by the model. Furthermore, the model predicts that the reduction of the entoglossal radius towards the tip (and thus the internal radius of the muscle) tends to increase the normal stress on the entoglossal bone