Fig. 1.1
The oral cavity
The oral cavity is the initial site for the processing of food. It extends from the lips to the pharynx. The oral cavity is divided by the dental arches (formed by the teeth and alveoli) into two parts:
1.
Outer vestibule
2.
Inner oral cavity proper
The Oral Vestibule [1, 2]
The part of the oral cavity lying between the dental arches and the deep surfaces of the cheeks and lips is referred to as the oral vestibule. It is lined by mucous membrane. The parotid duct and the labial, buccal, and molar glands open into the oral vestibule. Anteriorly, it communicates exteriorly via the oral fissure and posteriorly with the oral cavity proper.
The Lips
The lips surround the oral fissure and are composed of the orbicularis oris muscle and submucosa (containing mucous labial glands, labial vessels, nerves, fatty tissues). They are lined externally by skin and internally by mucous membrane. Contraction of the orbicularis oris narrows the mouth and closes the lips like a sphincter, thus retaining the food bolus in the mouth [1, 2].
Oral Cavity Proper [2]
Boundaries
Roof: The hard and soft palates separate the oral cavity from the nasal cavities.
Floor: The floor is formed by the tongue and a muscular diaphragm formed by the muscles in the floor of the oral cavity.
Lateral walls: The lateral walls are formed by the cheeks.
Posteriorly: It opens into the pharynx at the oropharyngeal isthmus, which is bound superiorly by the soft palate, inferiorly by the tongue, and laterally on either sides by the faucial pillars formed by the palatoglossal (anterior arch) and palatopharyngeal (posterior arch) muscles.
Anteriorly: It communicates with the oral vestibule.
Cheeks
The cheeks form the lateral walls of the oral cavity and are continuous with the lips at the nasolabial sulcus. Each cheek is composed of skin, superficial fascia, parotid duct, mucous buccal and molar glands, vessels, nerves, lymphatics, fat, submucosa, and mucosa. The muscle of the cheek, the buccinator, arises from the pterygomandibular raphe and the posterior part of the maxilla and mandible opposite the molar teeth. The fibers of the buccinator together with fibers from the orbicularis oris insert into the lips [2]. The parotid duct pierces the buccinator muscle before opening into the oral cavity opposite the second upper molar tooth. This buccinator derives its nerve supply from the facial nerve. During mastication, the buccinators contract, press the cheeks against the teeth, and make the cheeks taut, thus preventing accumulation of food between the teeth and the cheeks [1–3].
Hard Palate [2]
The hard palate forms the partition between the oral and nasal cavities and is composed of the palatine processes of each maxilla anteriorly and the horizontal plates of each palatine bone posteriorly. Its posterior margin gives attachment to the soft palate. The alveolar arch lies anterior and lateral to the oral surface of the hard palate.
Soft Palate
The soft palate separates the nasopharynx from the oropharynx. It is lined by mucous membrane and consists of palatine aponeurosis (flattened tensor veli palatini tendon), taste buds, mucous glands, and muscles. The palatine aponeurosis splits in the midline to enclose the musculus uvulae, which forms the posterior free hanging midline projection, the uvula [1].
The various muscles involved with the mobility of the soft palate are listed in Table 1.1 [2].
Muscle | Origin | Insertion | Innervation | Function |
|---|---|---|---|---|
Tensor veli palatini | (A) Fibrous part of pharyngotympanic tube (B) Scaphoid fossa of sphenoid bone (C) Spine of sphenoid | Palatine aponeurosis | Mandibular nerve [V3] via the branch to the medial pterygoid muscle | (A) Tenses the soft palate (B) Opens the pharyngotympanic tube for pressure equalization |
Levator veli palatini Lies deep to the tensor palatini | Petrous part of the temporal bone anterior to opening for the carotid canal | Superior surface of the palatine aponeurosis | Vagus nerve [X] via the pharyngeal branch to the pharyngeal plexus | Only muscle to elevate the soft palate above the neutral position |
Palatopharyngeus A few fibers of the palatopharyngeus along with some fibers of the superior constrictor muscle form a ring around the posterior and lateral walls of the nasopharyngeal isthmus. On soft palate elevation, this muscle band forms a ridge called Passavant’s ridge | Superior surface of the palatine aponeurosis | Pharyngeal wall | Vagus nerve [X] via the pharyngeal branch to the pharyngeal plexus | (A) Depresses the soft palate (B) Moves the palatopharyngeal arch toward midline (C) Elevates the pharynx |
Palatoglossus | Inferior surface of the palatine aponeurosis | Lateral margin of the tongue | Vagus nerve [X] via the pharyngeal branch to the pharyngeal plexus | (A) Depresses the palate (B) Moves the palatoglossal arch toward midline (C) Elevates back of the tongue |
Musculus uvulae | Posterior nasal spine of the hard palate | Connective tissue of the uvula | Vagus nerve [X] via the pharyngeal branch to the pharyngeal plexus | (A) Elevates and retracts the uvula (B) Thickens the central region of the soft palate |
The soft palate on elevation comes into contact with Passavant’s ridge, closing the pharyngeal isthmus during swallowing. This separates the nasopharynx from the oropharynx, thus preventing nasal regurgitation. Depression of the soft palate closes the oropharyngeal isthmus.
Tongue
The tongue is a muscular structure forming the floor of the oral cavity. In addition to being an organ of taste, it plays an important role in the oral phase of swallowing. A median connective tissue septum divides the tongue into right and left halves, each half containing paired intrinsic and extrinsic muscles [1, 2]. Figure 1.2 shows some of the various tongue muscles.
Fig. 1.2
Muscles of the tongue
Intrinsic muscles (originate and insert within the tongue):
Superior longitudinal
Inferior longitudinal
Transverse
Vertical
Extrinsic muscles (originate outside the tongue and insert within the substance of the tongue):
Genioglossus
Hyoglossus
Styloglossus
Palatoglossus
The base of the tongue is attached to the epiglottis by the median and lateral glossoepiglottic folds. The vallecula lies between the median and lateral glossoepiglottic folds on either side. The tongue is attached to the mandible by the genioglossus, to the hyoid bone by the hyoglossus, to the styloid process by the styloglossus, and to the palate by the palatoglossus. The extrinsic muscles protrude, retract, depress, and elevate the tongue, while the intrinsic muscles alter the shape of the tongue by lengthening, shortening, curling, and uncurling its apex and sides and flattening and rounding its surface [1, 2]. They thus help in performing finer movements of the tongue during speech, mastication, and swallowing.
Innervation of the Tongue
All the tongue muscles are innervated by the hypoglossal nerve, except the palatoglossus which is innervated by the vagus nerve via its pharyngeal branch to the pharyngeal plexus [1, 2]. Sensory fibers from the anterior portion of the tongue are carried by the lingual nerve, a branch of the trigeminal nerve. Special sensory taste fibers from the anterior two-thirds of the tongue leave the tongue and travel along with the lingual nerve before traveling via the chorda tympani nerve, a branch of the facial nerve. Special sensory taste fibers from the posterior one-third of the tongue are supplied by the glossopharyngeal nerve.
Muscles of Mastication
The muscles of mastication are responsible for the movements of the mandible during swallowing and speech. The various muscles are listed in Table 1.2 [2].
Muscle | Origin | Insertion | Innervation | Function |
|---|---|---|---|---|
Masseter | Superficial layer: maxillary process of the zygomatic bone, anterior two-thirds of the zygomatic process of the maxilla Deep layer: medial aspect of the zygomatic arch and the posterior part of its inferior margin | Superficial fibers: angle of the mandible and lower part of the lateral surface of the ramus of the mandible Deep fibers: central and upper parts of the ramus of the mandible and the coronoid process | Masseteric nerve from the anterior trunk of the mandibular nerve [V3] | Elevation of the mandible |
Temporalis | Bony surface of the temporal fossa superior to the inferior temporal line, attached laterally to the deep surface of the temporalis fascia | Fibers converge inferiorly to form a tendon passing between the zygomatic arch and the infratemporal crest of the greater wing of the sphenoid to insert into margins and deep surface of the coronoid process and the anterior border of the ramus of the mandible | Deep temporal nerves from the anterior trunk of the mandibular nerve [V3] | Elevation and retraction of the mandible; side-to-side movements of the mandible |
Lateral pterygoid | Upper head: roof of the infratemporal fossa Lower head: lateral surface of the lateral pterygoid plate | Pterygoid fovea of the neck of the mandible; capsule of the temporomandibular joint near the internal attachment of the capsule to the articular disk | Nerve to the lateral pterygoid from the anterior trunk of the mandibular nerve [V3] | Protrusion and side-to –side movements of the mandible; pulls the articular disk and head of the mandible forward onto the articular tubercle |
Medial pterygoid | Superficial head: tuberosity and pyramidal process of the maxilla Deep head: medial surface of the lateral pterygoid plate; pyramidal process of the palatine bone | Medial surface of angle and the ramus of the mandible | Nerve to the medial pterygoid from the mandibular nerve [V3] | Elevation and side-to-side movements of the mandible; assists the lateral pterygoid in mandibular protrusion |
Unilateral contraction of the pterygoid muscles leads to contralateral movement of the mandible. Coordinated movements on both sides result in proper chewing of the food bolus [2].
Teeth and Dentition [1, 2]
The teeth play an important role in mastication. The teeth are embedded in a horseshoe-shaped bony ridge called alveolar process in the maxilla and mandible. Loss of teeth results in resorption of the alveolar bone and disappearance of the alveolar arches. The teeth and adjacent alveolar regions are surrounded by the gingivae (gums). Two sets of teeth develop in humans, deciduous teeth and permanent teeth. The deciduous teeth erupt from the gingivae between 6 months and 2 years of age [1]. Premolars and third molars are absent in children. The 20 deciduous teeth consist of two incisors, one canine, and two molar teeth in each half of the upper and lower jaws. These are replaced by the permanent incisor, canine, and premolar teeth. The jaws elongate forward to accommodate the permanent molar teeth, which erupt posterior to the deciduous molars [1, 2]. Replacement of the deciduous teeth by permanent teeth begins at about 6 years of age and continues into adulthood [2]. A fully dentured adult jaw consists of 32 teeth. There are 16 teeth each in the upper and lower jaw – the central incisor, the lateral incisor, the canine, the first premolar, the second premolar, the first molar, the second molar, and the third molar.
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