The tongue contains numerous muscles overlaid by a squamous epithelium. It is important for preparing food prior to swallowing and speech. It also contains the highest number of taste receptors within the oral cavity.
Parts and Surfaces
It contains a root, attached to the hyoid bone posteriorly and mandible anteriorly. The body extends superiorly and anteriorly and ends at the apex. The dorsal surface of the tongue is divided by the sulcus terminalis, a V-shaped division, into a posterior and anterior part of differing embryological origins. The deep surface of the tongue is much smaller and runs from the apex of the tongue to the floor of the mouth.
Anterior and Posterior Parts
The anterior part of the tongue is divided by a median sulcus which extends to the apex. Beneath this, the lingual septum divides the tongue into right and left halves. The superior surface is rough due to the numerous papillae for taste reception. The inferior surface, which lies against the floor of the mouth, is smooth and vascular. This surface is connected to the floor of the mouth by a large frenulum.
The posterior part of the tongue forms the anterior wall of the oropharynx, and is smoother than the anterior part. Its walls are continuous laterally with the tonsillar fossa and inferiorly with the epiglottis, to which it is connected by two lateral and one median glossoepiglottic folds. This part of the tongue houses the lingual tonsil, a collection of lymphatic tissue which has a similar role to the other tonsils described.
Muscles of the Tongue
The muscles of the tongue are complex and divided into extrinsic (which extend outside the tongue and are involved in movements of the body) and intrinsic (which are within the tongue only and alter the shape of the organ).
The extrinsic muscles are:
- Genioglossus, which arises from the anterior end of the mylohyoid line and then fans out to attach to the entire central surface of the tongue to the hyoid bone. When contracting, the genioglossus may extend the apex of the tongue outside of the mouth, flatten the central part of the tongue or move it from side to side.
- Hyoglossus arises from entire anterior superior edge of the hyoid bone and greater cornu, and inserts into the tongue. It is quadrangular in shape and lateral to genioglossus.
- Chondroglossus arises from the lesser cornu of the hyoid bone, and inserts into the tongue between the fibres of genoiglossus and hyoglossus
- Styloglossus is the most lateral of the extrinsic muscles, and arises from the anterolateral aspect of the styloid process, and descends anteriorly and medially to enter the side of the tongue. It divides into longitudinal fibres and oblique fibres.
- Palatoglossus underlies the palatoglossal arch lateral to the tongue, and arises from the palatine aponeurosis. It inserts into the posterior part of the tongue. When active, the root of the tongue is elevated and the palatoglossal folds oppose, occluding the fauces.
The intrinsic muscles are the superior longitudinal (beneath the superior surface), the inferior longitudinal (above the inferior surface), the transverse and the vertical muscles. They are involved in altering the shape of the tongue.
Inferiorly with the floor of mouth and the hyoid bone
Laterally and anteriorly with the gingiva, teeth and oral vestibule
Posteriorly with the epiglottis and oropharynx
Superiorly with the oral cavity and palate
The tongue is often pink to red and appears to have a rough surface. It is divided into anterior and posterior parts by the terminal sulcus. The anterior tongue has a central midline groove which overlies the lingual septum. Large vallate papillae may be visible near the terminal sulcus. The posterior tongue has a cobbled appearance due to the numerous lymphoid follicles of the lingual tonsil.
The anterior tongue is lined with a keratinising stratified squamous epithelium, arranged into lingual papillae. There are multiple types of papillae, some of which contain taste buds. Taste buds are small cavities in which taste cells project microvilli in the spaces. The microvilla contain taste receptors.
The posterior tongue contains a non-keratinising stratified squamous epithelium and some underlying lymphoid tissue, similar to the palatine tonsils.
The tongue is supplied by the lingual artery, the third branch of the external carotid. This artery travels anteriorly between hyoglossus and the middle constrictor to enter the floor of the mouth, accompanied by the hypoglossal nerve and the lingual vein. At the anterior border of hyoglossus, it begins to ascend towards the apex of the tongue, between the genioglossus and inferior constrictor, when it divides into a deep lingual artery and sublingual artery. The branches of the lingual artery form numerous other anastamoses as well as a submucosal plexus of arterioles.
The branches of the lingual artery are:
- The dorsal lingual arteries, usually several, ascend medial to hyoglossus and supply the posterior part of the tongue. Unlike the other arteries, they do not anastamose.
- The sublingual artery arises at the anterior border of hyoglossus, at the point where the lingual artery begins its ascent. This artery passes anteriorly along the floor of the mouth between genioglossus and mylohyoid, sending branches to the sublingual gland, floor of mouth and the lingual, buccal and labial gingivae. It also anastamoses with the contralateral artery, sending a terminal branch into the midline of the posterior aspect of the mandible.
- The deep lingual arteries continue the course of the lingual artery, passing first superiorly and then anteriorly to the apex of the tongue, between the genioglossus and the inferior constrictor. It forms an anastamosis with the contralateral artery.
Two sets of vein drain the tongue. Dorsal veins drain the posterior and lateral parts of the tongue, merge with the veins accompanying the lingual artery and follow its course to the internal jugular vein. Deep lingual veins descend from the apex of the tongue and combine with sublingual veins. From here, they form a vena comitantes with the hypoglossal nerve, following it posteriorly to empty in the facial or internal jugular vein.
Lymphatic drainage from the tongue occurs in three routes – marginal, central and dorsal. The anterior tongue drains to either marginal or central routes, and the posterior tongue drains via the dorsal route. Central tongue tumours are more likely to spread to bilateral nodes. The apex and frenulum also drain readily to bilateral nodes.
The marginal regions drain lymph to multiple lymph node stations. These may include submental (IA), submandibular (IB) or jugulodigastric nodes (II). The posterior margin of the tongue will usually drain directly to jugulodigastric nodes.
Lymph from the central region of the tongue typically travels to jugulodigastric nodes, but some vessels may pierce the mylohyoid muscle to reach submandibular nodes.
Lymph from the posterior and dorsum of the tongue drains bilaterally into the superior deep cervical nodes (II).
The tongue is a complex structure, containing muscles, general sensory and special sensory innervation. The embryological origin of the anterior and posterior tongue also have a role to play in the complexity.
The muscles of the tongue, with the exception of palatoglossus in the palatoglossal arch, are supplied by the hypoglossal nerve (XII). Palatoglossus is supplied by the vagus nerve.
The lingual nerve (V3) is responsible for general sensory input from the anterior tongue, whereas the chorda tympani branch of the facial nerve (VII) conveys the special sense of taste from this region.
The posterior tongue sends general and special sensory input via the glossopharyngeal nerve (IX). A small area of the posterior tongue, just anterior to the epiglottis, is instead supplied by the internal laryngeal branch of the vagus nerve.
Routes of Cancer Spread
Tumours of the tongue may spread to the floor of mouth, mandible and erode through the overlying skin. Invasion of the mandible may lead to loss of teeth or anaesthesia of the lower lip due to involvement of the inferior alveolar nerve. Posterior tumours may invade into the epiglottis and cause swallowing difficulties, or alternatively into the fauces/tonsils and result in bleeding.
Tumours of the tongue may spread to submental, submandibular or superior deep cervical nodes based on their location. Bilateral spread is common from the midline tongue as well as from the posterior surface.
Tumours of the head and neck typically spread to lungs, bone and then liver.