Figure 1

Hypothetical mechanisms of tongue base retraction (TBR) in isolation. (a) Anatomy of the macaque cranium, mandible, hyoid, and tongue (pink). The oral volume (blue) is a chamber composed of rigid hard palate, hyoid, mandible, and tooth rows, and floored by variably stiff suprahyoid muscles such as mylohyoid (red). The oral tongue is enclosed within this chamber when the jaws are closed, whereas the tongue base is free to expand posteriorly. Each of the following mechanisms is shown in isolation; however, these mechanisms are not mutually exclusive and tongue base retraction could occur through a combination of the three. (b) Illustration of the extrinsic muscle shortening hypothesis. The hyoglossus (light blue), palatoglossus (dark pink), and styloglossus (green) muscles all have posteriorly-oriented trajectories throughout swallowing and could each theoretically cause TBR. (c) Illustration of the intrinsic muscle shortening hypothesis, showing a primate tongue in coronal cross-section. Colors for extrinsic tongue muscles correspond to those in b. Assuming that the tongue behaves as a classic muscular hydrostat, intrinsic tongue muscles (shades of gray) – specifically, the transverse fibers – shorten to produce narrowing (blue). If regional volume is conserved, then this narrowing produces simultaneous lengthening (red) and deepening (green) of the tongue. (d) Illustration of the hydraulic hypothesis. Hyoid protraction by geniohyoid (blue) and elevation of the mouth floor by co-contraction of mylohyoid (red) and the digastric bellies (anterior, yellow; posterior, purple) produce a net anterosuperior movement of the hyoid and mouth floor that encroach into the oral volume containing the tongue. When the tongue is incompressible within the confines of the oral volume, its base must displace posteriorly–TBR.