The pharyngeal surface or posterior one-third of the tongue curves inferiorly and becomes oriented more in the vertical plane. The oral and pharyngeal surfaces are separated by a V-shaped terminal sulcus of the tongue. This terminal sulcus forms the inferior margin of the oropharyngeal isthmus between the oral and pharyngeal cavities. At the apex of the V-shaped sulcus is a small depression (the foramen cecum of the tongue), which marks the site in the embryo where the epithelium invaginated to form the thyroid gland. In some people a thyroglossal duct persists and connects the foramen cecum on the tongue with the thyroid gland in the neck.
The superior surface of the oral part of the tongue is covered by hundreds of papillae (Fig. 8.254B):
Filiform papillae are small cone-shaped projections of the mucosa that end in one or more points.
Fungiform papillae are rounder in shape and larger than the filiform papillae, and tend to be concentrated along the margins of the tongue.
The largest of the papillae are the vallate papillae, which are blunt-ended cylindrical papillae invaginations in the tongue’s surface—there are only about 8 to 12 vallate papillae in a single V-shaped line immediately anterior to the terminal sulcus of the tongue.
Foliate papillae are linear folds of mucosa on the sides of the tongue near the terminal sulcus of tongue.
The papillae in general increase the area of contact between the surface of the tongue and the contents of the oral cavity. All except the filiform papillae have taste buds on their surfaces.
Inferior surface of tongue
The undersurface of the oral part of the tongue lacks papillae, but does have a number of linear mucosal folds (see Fig. 8.265). A single median fold (the frenulum of the tongue) is continuous with the mucosa covering the floor of the oral cavity, and overlies the lower margin of a midline sagittal septum, which internally separates the right and left sides of the tongue. On each side of the frenulum is a lingual vein, and lateral to each vein is a rough fimbriated fold.
The mucosa covering the pharyngeal surface of the tongue is irregular in contour because of the many small nodules of lymphoid tissue in the submucosa. These nodules are collectively the lingual tonsil.
There are no papillae on the pharyngeal surface.
The bulk of the tongue is composed of muscle (Fig. 8.254 and Table 8.21).
The tongue is completely divided into left and right halves by a median sagittal septum composed of connective tissue. This means that all muscles of the tongue are paired. There are intrinsic and extrinsic lingual muscles.
Except for the palatoglossus, which is innervated by the vagus nerve [X], all muscles of the tongue are innervated by the hypoglossal nerve [XII].
The intrinsic muscles of the tongue (Fig. 8.255) originate and insert within the substance of the tongue. They are divided into superior longitudinal, inferior longitudinal, transverse, and vertical muscles, and they alter the shape of the tongue by: lengthening and shortening it, curling and uncurling its apex and edges, and flattening and rounding its surface.
Working in pairs or one side at a time the intrinsic muscles of the tongue contribute to precision movements of the tongue required for speech, eating, and swallowing.
Extrinsic muscles of the tongue (Fig. 8.255 and Table 8.21) originate from structures outside the tongue and insert into the tongue. There are four major extrinsic muscles on each side, the genioglossus, hyoglossus, styloglossus, and palatoglossus. These muscles protrude, retract, depress, and elevate the tongue.