The mouth is for eating and talking through, and its structure is adapted accordingly. It also serves as an emergency airway in dyspnoea but its structure has nothing to do with this function; it merely provides a bigger airhole than the narrow nostrils. The tongue is for grasping food, for moving it during mastication, and for helping to swallow it. The delicate movements of the tongue turn laryngeal noise into articulate speech. In addition its mucous membrane is highly sensitive, even more than fingertips, and it also possesses the sense of taste.
The mouth extends from the lips to the palatoglossal arches (anterior pillars of the fauces). It is enclosed by the lips and cheeks; the slit-like space between lips/cheeks and teeth/gingivae (gums) is the vestibule of the mouth. The space inside the teeth and gums is the mouth (oral) cavity proper. The floor is the mylohyoid muscle, and the roof is the hard palate. Rising from the floor of the mouth, the tongue occupies much of the oral cavity.
The lips and cheeks are covered with hairy skin, except for the red margin of the lips, which is devoid of hair, and has a rich capillary blood supply, hence the colour. The red margin is highly sensitive and is represented by a large area in the sensory cortex. It is the main exploratory sensory area in babies, before they learn to use their hands for stereognosis.
The oral cavity is lined with stratified squamous epithelium, which is keratinized on the gums, hard palate and much of the dorsum of the tongue, but not elsewhere. The mucous membrane is adherent on lips and cheek to the face muscles, on tongue to the muscles thereof, and on the hard palate to the periosteum of the bone. It is therefore seldom caught between the teeth when chewing.
On the mucous membrane of the cheek the parotid duct opens opposite the second upper molar tooth (Fig. 6.17). Nearby are the tiny openings of the ducts of the molar glands which lie on the outer surface of the buccinator. There are many other mucous glands (buccal and labial) scattered in the mucous membrane of the vestibule.
Nerve supply. Much of the mucous membrane of the inside of the cheeks and lips is supplied by the buccal branch of the mandibular nerve, with contributions from the mental branch of the inferior alveolar (also mandibular) and the infraorbital branch of the maxillary nerve; the last two also supply the red margin of the lower and upper lips respectively.
The gingivae (gums) are firmly attached to the periosteum of the alveolar bone and extend to surround the necks of the teeth. They consist of dense vascular fibrous tissue covered by epithelium. At the gingival crest, the epithelium dips down to line a sulcus, at the floor of which the epithelium is attached to the surface of the tooth. The change from alveolar mucosa (continuous with that of the cheek) to gingival mucosa is marked by an abrupt change of colour, from red shiny alveolar to pink opaque gingival.
The upper gums are supplied by the superior alveolar, greater palatine and nasopalatine nerves (maxillary), while the lower receive their innervation from the inferior alveolar, buccal and lingual nerves (mandibular). The buccal nerve does not usually innervate the upper gums.
Teeth
The bulk of a tooth consists of dentine, a hard avascular calcified tissue penetrated by minute canals, the dentinal tubules. The part of the tooth that projects into the mouth is the crown which is covered by enamel, the hardest tissue in the body, and the part held in the jaw is the root which is covered by cementum, a calcified tissue rather like bone. The junction between enamel and cementum is the cervical margin or neck. Because enamel and cementum meet, dentine is not normally exposed on the surface. Inside the dentine is the pulp cavity. The cavity is filled by dental pulp, loose connective tissue, with nerves (below), blood vessels (see p. 363) and lymphatics (see p. 331), all of which gain access to the pulp through the apical foramen. The pulp is covered with a single layer of tall columnar cells, the odontoblasts, lying in contact with the inner surface of the dentine. Throughout life they retain the power to produce dentine within the pulp cavity if the surface of the dentine is breached.
The odontoblasts give off fine cytoplasmic processes that occupy the dentinal tubules.
The tooth is slung in its bony socket by the periodontal ligament, consisting of collagen fibres passing obliquely from the alveolar bone towards the apex of the tooth. It is really the modified periosteum of the alveolar bone and is radiolucent; it shows as a clear interval between tooth and bone shadows in a radiograph.
Permanent dentition
The human adult has from the midline 2 incisors, 1 canine, 2 premolars and 3 molars; that is, 8 teeth in each half-jaw, or 32 teeth in all. The shape of a tooth is adapted to its function. The incisors are for biting and cutting, the canines for holding and tearing, the premolars and molars for chewing and grinding. In clinical dentistry it is common to refer to teeth by number (1 to 8 starting from the midline) rather than by name.
The teeth can be distinguished from one another by the characteristics of their roots and crowns. The upper molars have three roots each; two are lateral and one is medial. The lower molars have two roots each, one anterior and one posterior. All the other teeth have a single root, except for the first upper premolar which usually has a bifid root.
The incisor crowns are chisel-shaped. Upper and lower incisors do not meet edge to edge, but by a sliding overlap, like the blades of a pair of scissors. The canine crowns are pyramidal or conical, The premolar (bicuspid teeth) crowns have two cusps (lingual and buccal). Upper molars have four, lower molars five, cusps on their crowns.
Nerve supply
The term nerve supply of a tooth really means the nerve supply of the pulp; some fine nerve filaments may enter some dentinal tubules, but most of the dentine and all the enamel and cementum have no innervation. The pulp and periodontal ligament share the same nerve.
The upper teeth are supplied by the superior alveolar nerves, anterior, middle and posterior, which form a plexus above the apices of the teeth. The middle nerve may be absent.
In the lower jaw the molars and premolars are supplied by the main trunk of the inferior alveolar nerve, whose terminal incisor branch supplies the canine and incisors, overlapping to the opposite central incisor.
Dental anaesthesia
The alveolar bone of the maxilla is relatively porous, so anaesthetic solution deposited in the gingivae opposite the apex of a tooth root will readily penetrate the bone to anaesthetize the tooth for dental procedures. Infiltration of the buccal aspect of the jaw will allow painless drilling of the tooth, but for extraction the palatal aspect must be infiltrated as well.
For the teeth of the lower jaw infiltration anaesthesia is usually effective only for the incisors. The other mandibular teeth are embedded in bone that is denser and does not allow sufficient penetration of the anaesthetic agent. For these teeth, inferior alveolar nerve block is required; for extraction it is necessary to include block of the nearby lingual and buccal nerves as well in order to anaesthetize the adjacent soft tissues.
For infiltration anaesthesia on the buccal (outer) aspect of the jaw, the needle is inserted opposite the appropriate tooth just below or into the buccal fold (where the mucosa is reflected between jaw and cheek), with the tip of the needle directed to the level of the apex of the tooth. On the palatal side, the point of insertion of the needle is midway between the gingival margin and the midline of the palate.
For inferior alveolar and lingual nerve block, the needle is inserted orally through the buccinator above the level of the occlusal surface of the molar teeth and in front of the pterygomandibular raphe, which raises a visible and palpable ridge in the opened mouth; the needle passes behind the (palpable) deep tendinous band of the temporalis muscle (see p. 357). The line of approach is from the premolar teeth of the opposite side, and a small injection is made 0.5 cm from the mucosal surface, when the needle is above the lingual nerve; the main injection is made another 1 cm deeper above the lingula, where the inferior alveolar nerve enters the mandibular foramen, which is situated midway between the anterior and posterior borders of the mandibular ramus. Entry of the anaesthetic agent within the parotid fascia around the deep part of the parotid, and its diffusion through the gland substance, may cause a transient facial paralysis.
Tooth position
The teeth of the upper jaw lie in a continuous curve, like a horseshoe. In the alveolar bone the outer (buccal) plate is thinner than the inner (palatal) plate. In the lower jaw the curve of the anterior teeth straightens out in the molar region. In the alveolar bone of the mandible the labial (outer) plate is thinner than the lingual (inner) plate over incisors, canines and premolars, but in the posterior molar region the lingual plate is thinner than the buccal; the lingual nerve lies here beside the third molar tooth and is at risk when the tooth is extracted.
The attachment of mylohyoid is below the apices of most of the mandibular teeth—an apical abscess thus points in the mouth. The apices of the second and third molars lie below the mylohyoid line and an apical abscess bursting through the inner plate points in the neck.
Deciduous dentition
The deciduous, or milk, teeth begin to erupt at about the sixth month and are completely erupted at the end of the second year. They consist of 5 teeth in each half-jaw, 20 in all. There are 2 incisors, 1 canine and 2 molars. They are shed as the permanent teeth erupt. The deciduous molars are replaced by the permanent premolars, not by permanent molars which have no counterpart in the deciduous
dentition.
Development and eruption of teeth
Teeth are derived by budding of the epithelium (ectoderm) lining the mouth. The buds of ectoderm produce only the enamel; they evoke a reaction in the surrounding mesoderm, which differentiates to produce the dentine and cementum under the influence of neural crest cells.
In the mouth cavity (stomodeum) of the 5-week embryo (12 mm long) an ingrowth of ectoderm occurs over the site of the future gums. A curved sheet of ectoderm grows into the adjacent mesoderm, tilting medially. This is the primary dental lamina. From its outer surface a series of buds grow into the mesoderm, one for each deciduous tooth. At a later stage a similar series of buds grow (more medially) from the depths of the primary dental lamina, one bud for each permanent tooth. When these epithelial buds are well formed the primary dental lamina becomes absorbed. Remnants of this epithelium may later grow into cysts or tumours.
The developed tooth erupts by a combination of elongation of the root and absorption of the overlying bone. The elongating root remains ensheathed in an upgrowth of alveolar bone.
The approximate normal times of eruption are:
Deciduous teeth
6 months Lower central incisors
7 months Upper central incisors
8–9 months Lateral incisors
1 year First molars
18 months Canines
2 years Second molars.
Permanent teeth
6 years First permanent molars
7 years Central incisors
8 years Lateral incisors
9 years First premolars
10 years Second premolars
11 years Canines
12 years Second permanent molars
17–21 years Third permanent molars (wisdom teeth).
A lower tooth usually precedes its opposite number in the upper jaw. The first permanent molar (the 6-year molar) erupts before any deciduous teeth have been shed. The second permanent molar does not erupt until 12 years of age. In the intervening period the five deciduous teeth in each half-jaw are replaced. The order of replacement is first the incisors, central and lateral, then the milk molars, first and second and, last of all, the long-rooted canine.
Hard palate
The palate is the roof of the mouth. Between the teeth it lies on a basis of bone, the hard palate.
Behind the teeth and hard palate the soft palate projects down.
The hard palate is made up of the palatal process of the maxilla and the horizontal plate of the palatine bone, meeting at a cruciform suture formed of intermaxillary, interpalatine and palatomaxillary sutures. In the midline at the front of the hard palate lies the incisive fossa, into which open the incisive canals, each ascending into its half of the nasal cavity. The greater palatine foramen lies medial to the last molar tooth; just behind it the lesser palatine foramina perforate the palatine bone.
The mucous membrane of the front of the hard palate is strongly united with the periosteum and the attachment of the periosteum to the bone is secured by multiple fibrous tissue pegs (Sharpey's fibres) that leave a finely pitted bone surface on the dried skull. This fixation is for mastication; the moving bolus does not displace the mucous membrane. There are transverse masticatory ridges in this part of the mucoperiosteum. From a little papilla overlying the incisive fossa a narrow low ridge, the palatine raphe, runs anteroposteriorly; the submucosa is absent here. Over the horizontal plate of the palatine bone mucous membrane and periosteum are separated by a mass of mucous gland tissue;
Sharpey's fibres are few here, and the bone surface is smooth. From the hard palate the mucous membrane curves down to the undersurface of the soft palate. The stratified squamous epithelium is keratinized on the hard palate, and non-keratinized on the soft palate.
Blood supply is by the greater palatine artery (from the third part of the maxillary artery), which emerges from the greater palatine foramen and passes forwards around the palate (lateral to the nerve) to enter the incisive canal and pass up into the nose. Veins accompany the artery back to the pterygoid plexus. Other veins pass back to the supratonsillar region and join the pharyngeal plexus.
Lymph drainage is to retropharyngeal and deep cervical lymph nodes.
Nerve supply is by the greater palatine nerve (from the maxillary via the pterygopalatine ganglion) as far forward as the incisive fossa. The anterior part of the palate, behind the incisor teeth (the area of the premaxilla) is supplied by the two nasopalatine nerves, from the same source.
Tongue
The tongue is essentially a mass of skeletal muscle covered by mucous membrane, and with a midline fibrous septum separating the two muscular halves. It has a dorsum, tip, inferior surface and root. The anterior two-thirds, or oral part, of the dorsum faces upwards towards the hard palate, and the posterior one-third, or pharyngeal part, faces backwards towards the oropharynx. The stratified squamous epithelium is keratinized on the oral part and non-keratinized on the pharyngeal part. The tip is the most anterior and mobile part and merges into the inferior surface. The mucous membrane of the inferior surface is thin and smooth, similar to that of the floor of the mouth and cheek.
The oral anterior two-thirds of the dorsum of the tongue is covered by mucous membrane into which the underlying muscles are inserted. The surface is roughened by the presence of three types of papillae: filiform, fungiform and vallate. The filiform papillae are minute conical projections that give rise to the velvety appearance of the tongue. Fungiform papillae are visible as discrete pink
pinheads, more numerous towards the edges of the tongue; each bears a few taste buds (there are none on filiform papillae). The vallate papillae are about a dozen in number and are arranged in the form of a V with the apex pointing backwards, just in front of an ill-defined shallow groove, the sulcus terminalis, which marks the junction of the oral and pharyngeal parts of the tongue. Each is a cylindrical projection surrounded by a circular sulcus and a raised outer wall (Fig. 6.33). There are many taste buds and serous glands in the sulcus that surrounds each vallate papilla. There are no other glands on the dorsum of the anterior two-thirds of the tongue. The vallate papillae are far back on the oral surface and so not in contact with the food being chewed, but food juices and saliva reach them and so flavours are transmitted to them.
Figure 6.33 Dorsum of the tongue.
There are scattered mucous and serous glands under the tip and sides. On the undersurface behind the tip there is a rather large mixed gland, the anterior lingual gland, on each side of the midline. From each gland small ducts open on the undersurface of the tongue. A retention cyst of this gland is the probable cause of the clinical condition known as a ranula.
The posterior third of the dorsum of the tongue slopes downwards from the sulcus terminalis as the anterior wall of the oropharynx. At the apex of the sulcus is a small depression, the foramen caecum, the remains of the upper end of the thyroglossal duct (see p. 26). There are no papillae behind the sulcus. The smooth mucous membrane has a nodular appearance from the presence of underlying masses of mucous and serous glands and aggregations of lymphoid follicles. The latter constitute the
‘lingual tonsil’, part of Waldeyer's ring (see p. 387). Between the tongue and epiglottis there is a midline flange of mucous membrane, the median glossoepiglottic fold, on each side of which is a depression, the vallecula (see p. 387), bounded laterally by a similar mucosal fold, the lateral glossoepiglottic fold, extending from the side of the epiglottis to the wall of the pharynx; as the latter fold is not attached to the tongue it would be better named pharyngoepiglottic.
When the tip of the tongue is raised to the roof of the mouth, the inferior surface of the tongue can be inspected. A small midline septum of mucous membrane (lingual frenulum) unites it to the floor of the mouth. Lateral to this the deep lingual vein can usually be seen through the mucosa (the lingual artery and nerve that are near it are not visible), and farther laterally still is another fold of mucosa, the fimbriated fold. Foliate papillae are a series of vertical parallel folds of mucous membrane on the
sides of the posterior part of the tongue (Fig. 6.33). They are the site of numerous taste buds.
The palatoglossal arches (anterior pillars of the fauces) are ridges of mucous membrane raised up by the palatoglossus muscles. They extend from the undersurface of the front of the soft palate to the sides of the tongue in line with the vallate papillae. The whole constitutes the oropharyngeal isthmus. In front of it is the mouth, behind it is the pharynx; and it is narrower than either. It is closed by depression of the palate and elevation of the dorsum of the tongue, and narrowed by contraction of the palatoglossus muscles.
Muscles
The muscles of the tongue are divided into intrinsic and extrinsic groups; the intrinsic muscles are wholly within the tongue and not attached to bone, while the extrinsic muscles have a bony attachment. There are four muscles in each group in each half of the tongue, with a midline fibrous septum dividing the organ into two symmetrical halves. The muscles of the intrinsic group are the superior and inferior longitudinal, transverse and vertical, and the extrinsic group comprises genioglossus (which is the largest of all the muscles and makes up the bulk of the tongue), hyoglossus, styloglossus and palatoglossus.
Genioglossus arises from the superior mental spine (genial tubercle) of the mandible (see Fig. 8.5B, p. 510), whence the fibres radiate backwards in a fan-shaped manner to be inserted into the mucous membrane of the tongue, with the lowest fibres passing down to the hyoid body.
Hyoglossus arises from the length of the greater horn of the hyoid bone and from the lateral part of its body. It extends upwards as a quadrilateral sheet, its upper border interdigitating at right angles with the fibres of styloglossus, and is attached to the side of the tongue. Superficial (lateral) to the muscle from above downwards lie the lingual nerve, submandibular duct, and the hypoglossal nerve with its accompanying veins, while passing deep to its posterior border from above downwards are the glossopharyngeal nerve, stylohyoid ligament and the lingual artery with its accompanying veins lying on the anterior fibres of the middle constrictor (Fig. 6.24).
Styloglossus arises from the front of the lower part of the styloid process and the upper part of the stylohyoid ligament. It passes forwards below the superior constrictor to be inserted into the side of the tongue, interdigitating with the upper fibres of hyoglossus.
Palatoglossus descends from the undersurface of the palatine aponeurosis to the side of the tongue, forming with its fellow of the opposite side the palatoglossal arch. It is described further with the soft palate (see p. 389).
Blood supply
The tongue is supplied by the lingual artery (see p. 342), which runs above the greater horn of the hyoid bone deep to hyoglossus and passes forwards to the tip. Beneath hyoglossus it gives off dorsal lingual branches into the posterior part. At the anterior border of hyoglossus it gives a branch to the sublingual gland and the floor of the mouth. There are small contributions from the tonsillar branch of the facial artery and from the ascending pharyngeal artery. The fibrous septum dividing the two halves of the tongue prevents any significant anastomosis of blood vessels across the midline. Accordingly the septum is a less vascular site for surgical division of the tongue for hemiglossectomy or