Three-dimensional architecture of the intrinsic tongue muscles, particularly the longitudinal muscle, by the chemical-maceration method

Muscle bundles of the transverse and vertical muscles of the tongue become flat when they enter the longitudinal muscle layers of the tongue, where they form a tunnel-like structure that surrounds the longitudinal muscle of the tongue. However, the three-dimensional architecture of longitudinal muscle fibers of the tongue has not been clarified. In the present study, we evaluated the function of the intrinsic muscles of the tongue by studying the three-dimensional architecture of the longitudinal muscle. Muscle bundles of the longitudinal muscle of the anterior part of a rabbit's tongue were exposed by the chemical-maceration and modified chemical-maceration methods and examined by scanning electron microscopy. In the longitudinal muscle of the tongue, muscle bundles running in the anteroposterior direction were arranged at regular intervals. These muscle bundles bifurcated or ramified at a sharp angle at each level from the superficial layer to the deep layer and joined or fused with adjacent muscle bundles. In addition, these ramified muscle bundles ran obliquely into shallower or deeper layers of the muscle, as well as in the same plane. Consequently, the longitudinal muscle of the tongue as a whole had a three-dimensional mesh-like structure. The transverse and vertical muscles of the tongue entered this mesh-like structure of muscle bundles of the longitudinal muscle as flat muscle bundles. The transverse and vertical muscles showed no ramification in the center of the tongue, where there is no longitudinal muscle. These results suggest that the three intrinsic muscles of the tongue are interlaced with one another and are bound tightly in the longitudinal muscle. This structure may enable the dorsum of the tongue to harden for pressing food during mastication and shifting the food posteriorly for swallowing.