THE TONGUE. LINGUA (Fig. 95).--The =tongue= is a muscular organ covered with mucous membrane; in life it is very mobile. It is an elongated organ, flat above, broadest in the middle, and very slightly narrowed at each end. It extends from the incisor teeth to the isthmus faucium and nearly fills the mouth cavity. The caudal third of the tongue forms the floor of the mouth cavity, so that the tongue has here no ventral surface, but is directly continuous with underlying organs. It is in this region that the extrinsic muscles of the tongue (except the genioglossus) enter it. The cranial two-thirds of the tongue is partly or entirely free from the floor of the mouth, the cranial one-third being completely free and movable. In about the middle third the ventral surface of the tongue is held to the floor of the mouth by the fold known as the =frenulum linguæ=. The frenulum contains parts of the two genioglossus muscles, which enter the tongue through it. The ventral surface and lateral borders of the tongue are smooth, soft, and free from papillæ. The dorsal surface is raised into papillæ of various kinds, and has a slight median longitudinal furrow. The caudal part of the dorsal surface is softer, redder, and marked with papillæ of a different kind from those of the rest of the tongue. From the caudal end a small median vertical fold, the =frenulum= (or =plica=) =glossoepiglottica= passes from the dorsal surface of the tongue to the cranial surface of the epiglottis. [Illustration: FIG. 95.--TONGUE, EPIGLOTTIS, AND OPENING OF LARYNX. _a_, filiform papillæ; _b_, fungiform papillæ; _b′_, very large papillæ at the sides of the tongue; _c_, circumvallate papillæ; _d_, tonsils; _e_, epiglottis; _f_, plica aryepiglottica; _g_, arytenoid cartilages (covered with mucosa); _h_, glottis; _i_, false vocal cords; _j_, true vocal cords.] The =papillæ= of the tongue are of three kinds. 1. The very numerous =filiform papillæ= (_a_) (=papillæ filiformes=); many of them are horny and tooth-like, with points turned caudad. These are most numerous at the middle of the free end of the tongue. 2. The =fungiform papillæ= (_b_) (=papillæ fungiformes=) are found scattered over the surface of the middle of the tongue caudad of the large filiform papillæ. They are enlarged at their free ends. There is a prominent row of very large ones (_b′_) opposite the circumvallate papillæ at the borders of the tongue. 3. The =circumvallate papillæ= (_c_) (=papillæ vallatæ=) are blunt and each is surrounded by a trench which is bounded in turn by a raised wall. They are in two rows of two or three each, which converge near the base of the tongue so as to form a V with the apex directed caudad. MUSCLES OF THE TONGUE (Fig. 96).--=M. genioglossus= (_f_) passes from the symphysis of the lower jaw into the tongue and lies beneath (dorsad of) the geniohyoid (_g_).