Chapter 1. Anatomy of the oral cavity and the jaws
Oral vestibule 1
Floor of the mouth 2
Maxillary bones 2
Maxillary sinus 2
Bones contributing to the hard palate 2
The mouth (oral cavity) is concerned with the ingestion (and selection) of food, with its subsequent mastication and swallowing, and with speech and breathing. It extends from the lips and cheeks externally to the pillars of the fauces at the oropharyngeal isthmus internally. The palate forms the roof of the mouth and the tongue occupies the floor of the mouth. The lateral walls of the oral cavity are defined by the cheeks and retromolar regions. The jaws are the tooth-bearing bones, comprising two maxillary bones forming the upper jaw and a single bone, the mandible, forming the lower jaw.
Using correct and appropriate anatomical and dental terminologies, you must:
• be able to describe, accurately and in detail, all the visible features present in the mouth
• be able to describe the anatomical features of the bones (mandible and maxilla) that comprise the jaws.
The lips (labia) are covered externally by skin and internally by mucous membrane. The red portion of the lip (the vermilion) is a human characteristic. Laterally, the upper lip is separated from the cheeks by nasolabial grooves. The labiomarginal sulci delineate the lower lip from the cheeks. A labiomental groove separates the lower lip from the chin. In the midline of the upper lip runs the philtrum. The corners of the lips are termed the labial commissures. Lips that are lightly closed at rest are described as being ‘competent’; ‘incompetent’ lips describe a situation where, with the facial muscles relaxed, a lip seal is not produced. The position of the lips is important in controlling protrusion of the incisors. With ‘competent’ lips, the tips of the maxillary incisors lie below the upper border of the lower lip and thus maintain the ‘normal’ inclination of the incisors. With ‘incompetent’ lips, the maxillary incisors may not be so controlled and the lower lip may even lie behind them, thus producing an exaggerated proclination.
The cheeks (buccae) extend intra-orally from the labial commissures to the ridge of mucosa overlying the ascending ramus of the mandible. Its mucosa is non-keratinized and is tightly adherent to the buccinator muscle. The parotid duct drains into the cheek opposite the maxillary second molar. A hyperkeratinized line called the linea alba may be seen at a position related to the occlusal plane. In the retromolar region, in front of the pillars of the fauces, a fold of mucosa containing the pterygomandibular raphe extends from the upper to the lower alveolus.
The oral vestibule is the space between the lips and cheeks and the teeth and alveolus. The mucosa covering the alveolus is reflected onto the lips and cheeks to form a trough or sulcus called the vestibular fornix. Here, the mucosa may show sickle-shaped folds called frena that contain loose connective tissue. The gums (gingivae) are described on pages 238–239.
The palate forms the roof of the mouth and separates the oral and nasal cavities. The immovable hard palate lies anteriorly and the movable soft palate posteriorly. The skeleton of the hard palate is bony while that of the soft palate is fibrous. The hard palate is covered by a keratinized mucosa, which is firmly bound to underlying bone. The incisive papilla immediately behind the maxillary central incisors covers the nasopalatine nerves as they emerge from the incisive fossa. In the midline, and extending posteriorly from the incisive papilla, is the palatine raphe. Here, the oral mucosa is attached directly to bone without a submucous layer. Palatine rugae are elevated ridges that radiate from the incisive papilla and the anterior part of the palatine raphe. At the junction of the palate and the alveolus lies a submucosa in which run the greater palatine nerves and vessels. The shape and size of the dome of the palate vary considerably. The boundary between the soft palate and the hard palate may be distinguished by a change in colour, the soft palate being a darker red with a yellowish tint. Extending laterally from the free border of the soft palate are the palatoglossal and palatopharyngeal folds (the palatoglossal fold being anterior). These folds cover the palatoglossus and palatopharyngeus muscles and between them lies the palatine tonsil. This tonsil is a collection of lymphoid material. At the free edge of the soft palate, in the midline, is the palatal uvula. The oropharyngeal isthmus is where the oral cavity and the oropharynx meet (at the pillars of the fauces).
Floor of the mouth
The floor of the mouth proper is a horseshoe-shaped region above the mylohyoid muscles and beneath the movable part of the tongue. It is covered by a lining of non-keratinized mucosa. In the midline, near the base of the tongue, the lingual frenum extends on to the inferior surface of the tongue. The sublingual papilla, onto which the submandibular salivary ducts open, is centrally positioned at the base of the tongue. On either side of this papilla are the sublingual folds, beneath which lie the submandibular ducts and sublingual salivary glands. The floor of the mouth is occupied by the tongue and this is described on pages 51–53.
The maxillary bone consists of a body, frontal, zygomatic, alveolar and palatine processes, and an orbital plate. The body of the maxilla forms the skeleton of the anterior part of the cheek. The posterior convexity of the body is the maxillary tuberosity and it has several small foramina for the passage of the posterior superior alveolar nerves. In the midline, the alveolar processes of the two maxillae meet at the intermaxillary suture, whence they diverge to form the opening into the nasal fossae. At the lower border of this nasal aperture, in the midline, lies the anterior nasal spine. The frontal process extends above the nasal aperture towards the bridge of the nose. The orbital plate forms the floor of the orbit and below the orbital rim lies the infra-orbital foramen (through which the infra-orbital branch of the maxillary nerve emerges). The zygomatic process extends upwards to articulate with the zygoma. From the entire lower surface of the body arises the alveolar process supporting the maxillary teeth. The structure of the alveolus is described on pages 221–225. The medial aspect of the maxilla forms the lateral wall of the nose. The maxillary palatine process extends horizontally from the medial surface of the maxilla where the body meets the alveolar process.
The maxillary sinus (antrum) is situated in the body of the maxilla. It is pyramidal in shape. The base (medial wall) forms part of the lateral wall of the nose. The apex extends into the zygomatic process of the maxilla. The roof of the sinus is part of the floor of the orbit, and the floor of the sinus is formed by the alveolar process and part of the palatine process of the maxilla. The anterior wall of the sinus is the facial surface of the maxilla and the posterior wall is the infratemporal surface of the maxilla. Running in the roof of the sinus are the infra-orbital nerve and vessels. The anterior superior alveolar nerve and vessels run in the anterior wall of the sinus. The posterior superior alveolar nerve and vessels pass through canals in the posterior surface of the sinus. The medial wall of the maxillary sinus contains the opening (ostium) of the sinus that leads into the middle meatus of the nose. As this opening lies well above the floor of the sinus, its position is unfavourable for drainage. The roots of the cheek teeth are related to the floor of the maxillary sinus. The maxillary air sinus is lined by respiratory epithelium (a ciliated columnar epithelium), with numerous goblet cells. The sinus is innervated by the infra-orbital nerve and the superior alveolar branches of the maxillary nerve.
Bones contributing to the hard palate
The four major bones contributing to the hard palate are the palatine processes of the maxillae and the horizontal plates of the palatine bones. The junction between the palatine processes is the median palatine suture. Anteriorly, behind the central incisors, is the incisive fossa through which pass the nasopalatine nerves. The posterior edges of the palatine processes articulate with the horizontal plates of the two palatine bones to form the transverse palatine suture. Laterally, this junction is incomplete, forming the greater palatine foramina, through which pass the greater palatine nerves and vessels. Behind the greater palatine foramina lie the lesser palatine foramina, through which pass the lesser palatine nerves and vessels. The junction of the two palatine bones in the midline completes the median palatine suture. At the posterior border of the horizontal palatine plates is the posterior nasal spine.
The mandible consists of a horizontal body and two vertical rami. The body of the mandible carries the mandibular teeth and their associated alveolar processes. The mental protuberance constitutes the chin. Above the mental protuberance lies a shallow depression termed the incisive fossa. Midway in the height of the body of the mandible, related to the premolar teeth, is the mental foramen, through which pass the mental branches of the inferior alveolar nerve. The inferior margin of the body meets the posterior margin of the ramus at the angle of the mandible. This is the site of insertion of the masseter muscle and stylomandibular ligament. The alveolus forms the superior margin of the mandibular body. The junction of the alveolus and ramus is demarcated by an external oblique ridge, which runs across the body of the mandible towards the mental foramen. The coronoid and condylar processes are at the superior border of the ramus. The coronoid process provides attachment for the temporalis muscle. The condylar process has a neck supporting an articular surface, which fits into the mandibular fossa of the temporal bone to form the temporomandibular joint. Close to the midline, on the inferior surface of the mandibular body, lie the digastric fossae, into which are inserted the anterior bellies of the digastric muscles. Above the fossae are the genial spines that serve as attachments for the geniohyoid muscles and the genioglossus muscles. Passing upwards and backwards across the medial surface of the body of the mandible is the internal oblique ridge. From this ridge, the mylohyoid muscle takes origin. Around the angle of the mandible, the bone is roughened for the attachment of the medial pterygoid muscle. In the centre of the medial surface of the ramus lies the mandibular foramen, through which the inferior alveolar nerve and artery pass into the mandibular canal. A bony process, the lingula, extends from the anterosuperior surface of the foramen. The mylohyoid groove may be seen running down from the postero-inferior surface of the foramen. The mandibular canal begins at the mandibular foramen and extends to the region of the premolar teeth, where it bifurcates into the mental and incisive canals.
Which of the following statements are true and which are false?
a. The oral cavity is demarcated from the oropharynx by the palatoglossal folds.
b. When the teeth are in occlusion, the vestibule of the mouth communicates with the oral cavity proper in the retromolar region.
c. The linea alba in the cheek is a hyperkeratinized line representing the site of the occlusal plane.
d. The parotid gland usually drains opposite the maxillary second molar tooth.