CHAPTER 30 Oral cavity
The mouth or oral cavity extends from the lips and cheeks externally to the anterior pillars of the fauces internally, where it continues into the oropharynx (Fig. 30.1). The mouth can be subdivided into the vestibule external to the teeth and the oral cavity proper internal to the teeth. The palate forms the roof of the mouth and separates the oral and nasal cavities. The floor of the mouth is formed by the mylohyoid muscles and is occupied mainly by the tongue. The lateral walls of the mouth are defined by the cheeks and retromolar regions. Three pairs of major salivary glands (parotid, submandibular and sublingual) and numerous minor salivary glands (labial, buccal, palatal, lingual) open into the mouth. The muscles in the oral cavity are associated with the lips, cheeks, floor of the mouth and tongue. The muscles of the lips and cheeks are described with the face in Chapter 29. The muscles of the soft palate are described with the pharynx in Chapter 33.
CHEEKS
The external features of the cheeks are described in Chapter 29. Internally, the mucosa of the cheek is tightly adherent to buccinator and is thus stretched when the mouth is opened and wrinkled when closed. Ectopic sebaceous glands may be evident as yellow patches (Fordyce’s spots). Their numbers increase in puberty and in later life.
LIPS
The external features of the lips are described in Chapter 29. The central part of the lips contains orbicularis oris. Internally, the labial mucosa is smooth and shiny and shows small elevations caused by underlying mucous glands.
The position and activity of the lips are important in controlling the degree of protrusion of the incisors. With normal (competent) lips, the tips of the maxillary incisors lie below the upper border of the lower lip, and this arrangement helps to maintain the ‘normal’ inclination of the incisors. When the lips are incompetent, the maxillary incisors may not be so controlled and the lower lip may even lie behind them, thus producing an exaggerated proclination of these teeth. A tight, or overactive, lip musculature may be associated with retroclined maxillary incisors. The lips are kept moist both by tongue deposition of saliva and by numerous minor salivary glands within them. These glands are liable to trauma by the teeth, particularly in the lower lip: this can produce a mucocele as a result of either extravasation of saliva into the submucosal tissues or retention of saliva within the gland or its duct.
ORAL VESTIBULE
The oral vestibule is a slit-like space between the lips or cheeks on one side and the teeth on the other. When the teeth occlude, the vestibule is a closed space that only communicates with the oral cavity proper in the retromolar regions behind the last molar tooth on each side. Where the mucosa that covers the alveolus of the jaw is reflected onto the lips and cheeks, a trough or sulcus is formed which is called the fornix vestibuli. A variable number of sickle-shaped folds containing loose connective tissue run across the fornix vestibuli. In the midline these are the upper and lower labial frena (or frenula). Other folds may traverse the fornix near the canines or premolars. The folds in the lower fornix are said to be more pronounced than those in the upper fornix (Fig. 30.2).
ORAL MUCOSA
MASTICATORY MUCOSA AND THE GINGIVAE
Vascular supply and lymphatic drainage
The lymph vessels of the labial and buccal gingivae of the maxillary and mandibular teeth unite to drain into the submandibular nodes, though in the labial region of the mandibular incisors they may drain into the submental lymph nodes. The lingual and palatal gingivae drain into the jugulodigastric group of nodes, either directly or indirectly through the submandibular nodes.
Innervation
The nerves supplying the gingivae in the upper jaw come from the maxillary nerve via its greater palatine, nasopalatine, and anterior, middle and posterior superior alveolar branches (see Table 30.2). Surgical division of the nasopalatine nerve, for example during the removal of an ectopic canine tooth, causes no obvious sensory deficit in the anterior part of the palate, which suggests that the territory of the greater palatine nerve reaches as far forwards as the gingivae lingual to the incisor teeth or the nerve has large regenerative potential. The mandibular nerve innervates the gingivae in the lower jaw by its inferior alveolar, lingual and buccal branches.
OROPHARYNGEAL ISTHMUS
The oropharyngeal isthmus lies between the soft palate and the dorsum of the tongue, and is bounded on both sides by the palatoglossal arches. Each palatoglossal arch runs downwards, laterally and forwards, from the soft palate to the side of the tongue and consists of palatoglossus and its covering mucous membrane (Fig. 30.3). The approximation of the arches shuts off the mouth from the oropharynx, and is essential to deglutition (see Ch. 33).
FLOOR OF THE MOUTH
The floor of the mouth is a small horseshoe-shaped region situated beneath the movable part of the tongue and above the muscular diaphragm formed by the mylohyoid muscles (Fig. 30.4, see Fig. 30.7). A fold of tissue, the lingual frenulum, extends onto the inferior surface of the tongue from near the base of the tongue. It occasionally extends across the floor of the mouth to be attached onto the mandibular alveolus, known colloquially as a ‘tongue tie’; historically, this has been removed to aid speech, but the evidence for this is scanty. The submandibular salivary ducts open into the mouth at the sublingual papilla (caruncle), which is a large centrally positioned protuberance at the base of the tongue.
The sublingual folds lie on either side of the sublingual papilla and cover the underlying submandibular ducts and sublingual salivary glands. The blood supply of the floor of the mouth is described with the blood supply of the tongue (see p. 505). The main muscle forming the floor of the mouth is mylohyoid, with geniohyoid lying immediately above it.
Mylohyoid
Mylohyoid lies superior to the anterior belly of digastric and, with its contralateral fellow, forms a muscular floor for the oral cavity. It is a flat, triangular sheet attached to the whole length of the mylohyoid line of the mandible (Fig. 30.4A, see Fig. 30.7). The mylohyoid line is of variable length, sometimes ending before the lower third molar (wisdom) tooth. The posterior fibres of mylohyoid pass medially and slightly downwards to the front of the body of the hyoid bone near its lower border. The middle and anterior fibres from each side decussate in a median fibrous raphe that stretches from the symphysis menti to the hyoid bone. The median raphe is sometimes absent, in which case the two muscles form a continuous sheet, or it may be fused with the anterior belly of digastric. In about one-third of subjects there is a hiatus in the muscle through which a process of the sublingual gland protrudes.
Geniohyoid
Geniohyoid is a narrow muscle which lies above the medial part of mylohyoid (see Figs 30.6, 30.10). It arises from the inferior mental spine (genial tubercle) on the back of the symphysis menti, and runs backwards and slightly downwards to attach to the anterior surface of the body of the hyoid bone. The paired muscles are contiguous and may occasionally fuse with each other or with genioglossus.
Fig. 30.6 Muscles of the tongue and pharynx. Palatoglossus is not shown here, but is depicted in Fig. 33.4.
PALATE
HARD PALATE
The hard palate is formed by the palatine processes of the maxillae and the horizontal plates of the palatine bones (see Fig. 30.14A). The hard palate is bounded in front and at the sides by the tooth-bearing alveolus of the upper jaw and is continuous posteriorly with the soft palate. It is covered by a thick mucosa bound tightly to the underlying periosteum. In its more lateral regions it also possesses a submucosa containing the main neurovascular bundle. The mucosa is covered by keratinized stratified squamous epithelium which shows regional variations and may be ortho- or parakeratinized.
The submucosa in the posterior half of the hard palate contains minor mucous-type salivary glands (Fig. 30.3). They secrete via numerous small ducts which often drain into a larger duct that opens bilaterally at the paired palatine foveae. These depressions, sometimes a few millimetres deep, flank the midline raphe at the posterior border of the hard palate. They provide a useful landmark for the extent of an upper denture; if not observed during construction of the denture they cause the denture to become unstable when the soft palate moves during deglutition and mastication. The upper surface of the hard palate is the floor of the nasal cavity and is covered by ciliated respiratory epithelium.
Vascular supply and lymphatic drainage of the hard palate
The veins of the hard palate accompany the arteries and drain largely to the pterygoid plexus.
Innervation of the hard palate
The sensory nerves of the hard palate are the greater palatine and nasopalatine branches of the maxillary nerve, which all pass through the pterygopalatine ganglion. The greater palatine nerve descends through the greater palatine canal, emerges on the hard palate from the greater palatine foramen, runs forwards in a groove on the inferior surface of the bony palate almost to the incisor teeth and supplies the gums and the mucosa and glands of the hard palate (Fig. 30.3). It also communicates with the terminal filaments of the nasopalatine nerve. As it leaves the greater palatine canal, it supplies palatine branches to both surfaces of the soft palate. The lesser (middle and posterior) palatine nerves, which are much smaller, descend through the greater palatine canal and emerge through the lesser palatine foramina in the tubercle of the palatine bone to supply the uvula, tonsil and soft palate. The nasopalatine nerves enter the palate at the incisive foramen and are branches of the maxillary nerve which pass through the pterygopalatine ganglion to supply the anterior part of the hard palate behind the incisor teeth.
TONGUE
ORAL (PRESULCAL) PART
The presulcal part of the tongue is located in the floor of the oral cavity. It has an apex touching the incisor teeth, a margin in contact with the gums and teeth, and a superior surface (dorsum) related to the hard and soft palates. On each side, in front of the palatoglossal arch, there are four or five vertical folds, the foliate papillae, which represent vestiges of larger papillae found in many other mammals. The dorsal mucosa has a longitudinal median sulcus and is covered by filiform, fungiform and circumvallate papillae (Fig. 30.5). The mucosa on the inferior (ventral) surface is smooth, purplish and reflected onto the oral floor and gums: it is connected to the oral floor anteriorly by the lingual frenulum. The deep lingual vein, which is visible, lies lateral to the frenulum on either side. The plica fimbriata (fimbriated fold), a fringed mucosal ridge directed anteromedially towards the apex of the tongue, lies lateral to the vein. This part of the tongue develops from the lingual swellings of the mandibular arch and from the tuberculum impar, and this embryological derivation explains its sensory innervation.
MUSCLES OF THE TONGUE
Genioglossus
Genioglossus is triangular in sagittal section, lying near and parallel to the midline. It arises from a short tendon attached to the superior genial tubercle behind the mandibular symphysis, above the origin of geniohyoid. From this point it fans out backwards and upwards (Fig. 30.6). The inferior fibres of genioglossus are attached by a thin aponeurosis to the upper anterior surface of the hyoid body near the midline (a few fasciculi passing between hyoglossus and chondroglossus to blend with the middle constrictor of the pharynx). Intermediate fibres pass backwards into the posterior part of the tongue, and superior fibres ascend forwards to enter the whole length of the ventral surface of the tongue from root to apex, intermingling with the intrinsic muscles. The muscles of opposite sides are separated posteriorly by the lingual septum. Anteriorly they are variably blended by decussation of fasciculi across the midline. The attachment of the genioglossi to the genial tubercles prevents the tongue from sinking back and obstructing respiration, therefore anaesthetists pull the mandible forward to obtain the full benefit of this connection.
Hyoglossus
Hyoglossus is thin and quadrilateral, and arises from the whole length of the greater cornu and the front of the body of the hyoid bone (Fig. 30.6). It passes vertically up to enter the side of the tongue between styloglossus laterally and the inferior longitudinal muscle medially. Fibres arising from the body of the hyoid overlap those from the greater cornu.
Styloglossus
Styloglossus is the shortest and smallest of the three styloid muscles (Fig. 30.6). It arises from the anterolateral aspect of the styloid process near its apex, and from the styloid end of the stylomandibular ligament. Passing downwards and forwards, it divides at the side of the tongue into a longitudinal part, which enters the tongue dorsolaterally to blend with the inferior longitudinal muscle in front of hyoglossus, and an oblique part, overlapping hyoglossus and decussating with it.
Stylohyoid ligament
The stylohyoid ligament is a fibrous cord which extends from the tip of the styloid process to the lesser cornu of the hyoid bone (Fig. 30.6). It gives attachment to some fibres of styloglossus and the middle constrictor of the pharynx and is closely related to the lateral wall of the oropharynx. Below it is overlapped by hyoglossus. The ligament is derived embryologically from the second branchial arch. It may be partially calcified.
Palatoglossus
Palatoglossus is closely associated with the soft palate in function and innervation, and is described with the other palatal muscles (see Ch. 33).
VASCULAR SUPPLY AND LYMPHATIC DRAINAGE OF THE TONGUE
Lingual artery
The tongue and the floor of the mouth are supplied chiefly by the lingual artery, which arises from the anterior surface of the external carotid artery. It passes between hyoglossus and the middle constrictor of the pharynx to reach the floor of the mouth accompanied by the lingual veins and the glossopharyngeal nerve. At the anterior border of hyoglossus, the lingual artery bends sharply upwards (Fig. 30.8). It is covered by the mucosa of the tongue and lies between genioglossus medially and the inferior longitudinal muscle laterally. Near the tip of the tongue, it anastomoses with its contralateral fellow; this contribution is important in maintaining the blood supply to the tongue in any surgical resection of the tongue. The branches of the lingual artery form a rich anastomotic network, which supplies the musculature of the tongue, and a very dense submucosal plexus. Named branches of the lingual artery in the floor of the mouth are the dorsal lingual, sublingual and deep lingual arteries.
Lymphatic drainage
The mucosa of the pharyngeal part of the dorsal surface of the tongue contains many lymphoid follicles aggregated into dome-shaped groups, the lingual tonsils. Each group is arranged around a central deep crypt, or invagination, which opens onto the surface epithelium. The ducts of mucous glands open into the bases of the crypts. Small isolated follicles also occur beneath the lingual mucosa. The lymphatic drainage of the tongue can be divided into three main regions, marginal, central and dorsal. The anterior region of the tongue drains into marginal and central vessels, and the posterior part of the tongue behind the circumvallate papillae drains into the dorsal lymph vessels. The more central regions may drain bilaterally, and this must be borne in mind when planning to remove malignant tumours of the tongue that are approaching the midline. If the tumour has a propensity for lymphatic spread, both cervical chains may be involved.
Marginal vessels
Vessels from the lateral margin of the tongue cross the sublingual gland, pierce mylohyoid and end in the submandibular nodes. Others end in the jugulodigastric or jugulo-omohyoid nodes. Vessels from the posterior part of the lingual margin traverse the pharyngeal wall to the jugulodigastric lymph nodes (Fig. 30.9).
INNERVATION OF THE TONGUE
The muscles of the tongue, with the exception of palatoglossus, are supplied by the hypoglossal nerve. Palatoglossus is supplied via the pharyngeal plexus (see Ch. 33). The pathways for proprioception associated with the tongue musculature are unknown, but presumably may involve the lingual, glossopharyngeal or hypoglossal nerves, and the cervical spinal nerves which communicate with the hypoglossal nerve.
Lingual nerve
The lingual nerve arises from the posterior trunk of the mandibular nerve in the infratemporal fossa (see Fig. 31.13A) where it is joined by the chorda tympani branch of the facial nerve and often by a branch of the inferior alveolar nerve. It then passes below the mandibular attachment of the superior pharyngeal constrictor and pterygomandibular raphe, closely applied to the periosteum of the medial surface of the mandible, until it lies opposite the distal (posterior) root of the third molar tooth, where it is covered only by the gingival mucoperiosteum. At this point it can contact the lingual cortical plate and may be at the level of, or higher than, the alveolar bone crest. It next passes medial to the mandibular attachment of mylohyoid, which carries it progressively away from the mandible, and separates it from the alveolar bone covering the mesial root of the third molar tooth, and then passes downward and forward on the deep surface of mylohyoid (i.e. the surface nearer the mucosa covering the floor of the mouth), crossing the lingual sulcus beneath the mucosa. In this position it lies on the deep portion of the submandibular gland which bulges over the top of the posterior border of mylohyoid. It passes below the submandibular duct which crosses it from medial to lateral, and curves upward, forward and medially to enter the tongue (Figs 30.8, 30.10). Within the tongue the lingual nerve lies first on styloglossus and then the lateral surface of hyoglossus and genioglossus, before dividing into terminal branches that supply the overlying lingual mucosa. The lingual nerve is connected to the submandibular ganglion (see Fig. 31.15) by two or three branches, and also forms connecting loops with twigs of the hypoglossal nerve at the anterior margin of hyoglossus.
The lingual nerve is at risk during surgical removal of (impacted) lower third molars, and, after such operations, up to 0.8% of patients may develop lingual sensory disturbance, which may persist in 0.3% (Robinson & Smith 1996). The nerve is also at risk during operations to remove the submandibular salivary gland, because the duct must be dissected from the lingual nerve, and because its connection to the submandibular ganglion pulls it into the operating field.