Fig. 2. Schematic representation of the longitudinal deformation of the tongue body and the interaction with the tongue ensheathing. The extrinsic muscles are only partly drawn. Abbreviations: dsh, distal tongue ensheathing; ggl, m. genioglossus; hgl, m. hyoglossus; psh, proximal tongue ensheathing; sh, tongue sheet. (A) Detail from Fig. 1A. The tongue sheet is a tubular structure that inserts the tongue body at the proximal and distal ensheathing, thus forming a loose second `skin' around the tongue body. (B) In the retracted `rest' position, the tubular sheet distally folds inward, resulting in a double sheathing along the tongue tip. The outer layer of the tongue sheet is fixed to the connective tissue of the mouth floor (as indicated by the thin vertical lines). (C) While the tongue protrudes, the inner sheet unfolds outward as the posterior tongue part, i.e. between the proximal and distal ensheathing, elongates and the distal tongue part is revealed. Thus, the tongue sheet forms an almost frictionless bearing for tongue protrusion.