Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

6. Oral Cavity

6.1 Oral Cavity, Overview

A Lips and labial creases

Anterior view. The upper and lower lips meet at the angle of the mouth. The oral fissure opens into the oral cavity. Changes in the lips noted on visual inspection may yield important diagnostic clues: Blue lips (cyanosis) suggest a disease of the heart, lung, or both, while deep nasolabial creases may reflect chronic diseases of the digestive tract.

В Oral cavity

Anterior view. The dental arches with the alveolar processes of the maxilla and mandible subdivide the oral cavity into several parts (see also C):

 Oral vestibule: the part of the oral cavity bounded on one side by the teeth and on the other side by the lips or cheeks.

 Oral cavity proper: the cavity of the mouth in the strict sense (within the dental arches, bounded posteriorly by the palatoglossal arch).

 Fauces: the throat (boundary with the pharynx: palatopharyngeal arch).

The fauces communicate with the pharynx through the faucial isthmus. The oral cavity is lined with non keratinized, stratified squamous epithelium that is moistened by secretions from the salivary glands (see p. 113). Squamous cell carcinomas of the oral cavity are particularly common in smokers and heavy drinkers.

C Organization and boundaries of the oral cavity

Midsagittal section, left lateral view. The muscles of the oral floor and the adjacent tongue together constitute the inferior boundary of the oral cavity proper. The roof of the oral cavity is formed by the hard palate in its anterior two-thirds and by the soft palate (velum) in its posterior third (seeF). The uvula hangs from the soft palate between the oral cavity and pharynx. The keratinized stratified squamous epithelium of the skin blends with the nonkeratinized stratified squamous epithelium of the oral cavity at the vermilion border of the lip. The oral cavity is located below the nasal cavity and anterior to the pharynx. The midportion of the pharynx, called the oropharynx, is the area in which the airway and foodway intersect (b).

D Neurovascular structures of the hard palate

Inferior view. The arteries and nerves of the hard palate (skeletal anatomy is shown on p.28) pass downward through the incisive foramen and the greater and lesser palatine foramina into the oral cavity. The nerves are terminal branches of the trigeminal nerve’s maxillary division (CN V2), and the arteries arise from the territory of the maxillary artery (neither are shown here)

E Sensory innervation of the palatal mucosa, upper lip, cheeks, and gingiva

Inferior view.

Note that the region shown in the drawing receives sensory innervation from different branches of the trigeminal nerve (buccal nerve from the mandibular division (CN V3, all other branches from the maxillary division, CN V2).

F Muscles of the soft palate

Inferiorview. The soft palateforms the posterior boundary of the oral cavity, separating it from the oropharynx. The muscles are attached at the midline to the palatine aponeurosis, which forms the connective tissue foundation of the soft palate. The tensor veli palatini, levator veli palatini, and musculus uvulae can be identified in this dissection. While the tensor veli palatini tightens the soft palate, simultaneously opening the inlet to the pharyngotympanic (auditory) tube (see p. 145), the levator veli palatini raises the soft palate to a horizontal position. Both of these muscles, but not the musculus uvulae, also contribute structurally to the lateral pharyngeal wall.

6.2 Tongue:

Muscles and Mucosa

A Surface anatomy of the lingual mucosa

Superior view. While the motor properties of the tongue are functionally important during mastication, swallowing, and speaking, its equally important sensory functions include taste and fine tactile discrimination. The tongue is endowed with a very powerful muscular body (see Ca). The upper surface (dorsum) of the tongue is covered by a highly specialized mucosal coat and consists, from front to back, of an apex (tip), body, and root.

The V-shaped furrow on the dorsal surface (the sulcus terminalis) further divides the tongue into an anterior (oral, presulcal) part and a posterior (pharyngeal, postsulcal) part. The anterior part comprises the anterior two-thirds of the tongue, and the posterior part comprises the posterior third. At the tip of the “V" is the foramen cecum (vestige of embryological migration of the thyroid gland). This subdivision is a result of embryological development and explains why each part has a different nerve supply (see p. 107). The mucosa of the anterior part is composed of numerous papillae (see B), and the connective tissue between the mucosal surface and musculature contains many small salivary glands. The physician should be familiar with them, as they may give rise to tumors (usually malignant).

The taste buds are bordered by serous glands (see Bb-e) that are known also as von Ebner glands; they produce a watery secretion that keeps the taste buds clean.

В The papillae of the tongue

a Sectional block diagram of the lingual papillae, b-e Types of papillae. The papillae are divided into four morphologically distinct types:

b Vallate papillae: encircled by a wall and containing abundant taste buds.

c Fungiform papillae: mushroom-shaped, located at the sides of the tongue (they have mechanical receptors, thermal receptors, and taste buds).

d Filiform papillae: thread-shaped papillae that are sensitive to tactile stimuli.

e Foliate papillae: located on the posterior sides of the tongue, containing numerous taste buds.

C Muscles of the tongue

a Left lateral view, b anterior view of a coronal section.

There are two sets of lingual muscles: extrinsic and intrinsic. The extrinsic muscles are attached to specific bony sites outside the tongue, while the intrinsic muscles have no attachments with skeletal structures. The extrinsic lingual muscles include the:

 genioglossus,

 hyoglossus,

 palatoglossus,

 styloglossus.

The intrinsic lingual muscles include the:

 superior longitudinal muscle,

 inferior longitudinal muscle,

 transverse muscle,

 vertical muscle.

The extrinsic muscles move the tongue as a whole, while the intrinsic muscles alter its shape. Except for the palatoglossus, which is supplied by the vagus nerve (CN X), all of the lingual muscles are innervated by the hypoglossal nerve (CN XII).

D Unilateral hypoglossal nerve palsy

Active protrusion of the tongue with an intact hypoglossal nerve (a) and with a unilateral hypoglossal nerve lesion (b).

When the hypoglossal nerve is damaged on one side, the genioglossus muscle is paralyzed on the affected side. As a result, the healthy (innervated) genioglossus on the opposite side dominates the tongue across the midline toward the affected side. When the tongue is protruded, therefore, it deviates toward the paralyzed side.

6.3 Tongue:

Neurovascular Structures and Lymphatic Drainage

A Nerves and vessels of the tongue

a Left lateral view, b view of the inferior surface of the tongue

The tongue is supplied by the lingual artery (from the maxillary artery), which divides into its terminal branches, the deep lingual artery and the sublingual artery. The lingual vein usually runs parallel to the artery and drains into the internal jugularvein. The lingual mucosa receives its somatosensory innervation (sensitivity to thermal and tactile stimuli) from the lingual nerve, which is a branch of the trigeminal nerve’s mandibular division (CN V3). The lingual nerve transmits fibers from the chorda tympani of the facial nerve (CNVII), among them the afferent taste fibers for the anterior two-thirds of the tongue. The chorda tympani also contains presynaptic, parasympathetic visceromotor axons which synapse in the submandibular ganglion, whose neurons in turn innervate the submandibular and sublingual glands (see p. 81 for further details). The palatoglossus receives its somatomotor innervation from the vagus nerve (CNX), the other lingual muscles from the hypoglossal nerve (CN XII).

В Somatosensory innervation (left side) and taste innervation (right side) of the tongue

Anterior view. The tongue receives its somatosensory innervation (e.g., touch, pain, thermal sensation) from three cranial nerve branches:

 Lingual nerve (branch of mandibular nerve CN V3)

 Glossopharyngeal nerve (CN IX)

 Vagus nerve (CN X)

Three cranial nerves also convey the taste fibers: CNVII (facial nerve, chorda tympani), CN IX(glossopharyngeal nerve), and CN X (vagus nerve). Thus, a disturbance of taste sensation involving the anterior two-thirds of the tongue indicates the presence of a facial nerve lesion, whereas a disturbance of tactile, pain, or thermal sensation indicates a trigeminal nerve lesion (see also pp.75 and 81).

C Lymphatic drainage of the tongue and oral floor

Left lateral view (a) and anterior view (b).

The lymphatic drainage of the tongue and oral floor is mediated by submental and submandibular groups of lymph nodes that ultimately drain into the lymph nodes along the internal jugular vein (ajugular

lymph nodes). Because the lymph nodes receive drainage from both the ipsilateral and contralateral sides (b), tumor cells may become widely disseminated in this region (for example, metastatic squamous cell carcinoma, especially on the lateral border of the tongue, frequently metastasizes to the opposite side).

6.4 Oral Floor

A Muscles of the oral floor

Superior view (a) and left lateral view (b).

The oral floor is formed by a muscular sheet that stretches between the two rami of the mandible. This sheet consists of four muscles, all of which are located above the hyoid bone and are thus collectively known as the suprahyoid muscles:

1. Mylohyoid: The muscle fibers from each side fuse in a median raphe (covered superiorly by the geniohyoid).

2. Geniohyoid: strengthens the central portion of the oral floor.

3. Digastric: The anterior belly of the digastric is located in the oral floor region; its posterior belly arises from the mastoid process.

4. Stylohyoid: arises from the styloid process. Its tendon is perforated by the intermediate tendon of the digastric.

All four muscles participate in active opening of the mouth. They also elevate the hyoid bone and move it forward during swallowing.

В Innervation of the oral floor muscles

a Left lateral view (right half of the mandible viewed from the medial side), b Sagittal section through the right petrous bone at the level of the mastoid process and mastoid air cells, viewed from the medial side, c Left lateral view.

The muscles of the oral floor have a complex nerve supply (different branchial arch derivatives) with contributions from three different nerves:

a The derivatives of the mandibular arch (mylohyoid, anterior belly of the digastric) are supplied by the mylohyoid nerve, a branch of the mandibular division (CN V3).

b The derivatives of the second branchial arch (posterior belly of the digastric, stylohyoid) are supplied by the facial nerve, c The geniohyoid (and the thyrohyoid) muscles are supplied by the ventral rami of Cl spinal nerve, which travel with the hypoglossal nerve.

6.5 Oral Cavity:

Pharynx and Tonsils

A Waldeyer’s ring

Posterior view of the opened pharynx. All the components of Waldeyer’s ring can be seen in this view. Waldeyer’s ring is composed of immunocompetent lymphatic tissue (tonsils and lymph follicles). The tonsils are “immunological sentinels” surrounding the passageways from the mouth and nasal cavity to the pharynx. The lymph follicles are distributed over all of the epithelium, showing marked regional variations. Waldeyer’s ring consists of the following structures:

 The unpaired pharyngeal tonsil on the roof of the pharynx

 The paired palatine tonsils

 The lingual tonsil

 The paired tubal tonsils (tonsillae tubariae), which may be thought of as lateral extensions of the pharyngeal tonsil

 The paired lateral bands

В Palatine tonsils: location and abnormal enlargement

Anterior view of the oral cavity.

a The palatine tonsils occupy a shallow recess on each side, the tonsillar fossa, which is located between the anterior and posterior pillars (palatoglossal arch and palatopharyngeal arch).

b and c The palatine tonsil is examined clinically by placing a tongue depressor on the anterior pillar and displacing the tonsil from its fossa while a second instrument depresses the tongue. Severe enlargement of the palatine tonsil (due to viral or bacterial infection, as in tonsillitis) may significantly narrow the outlet of the oral cavity, causing difficulty in swallowing (dysphagia).

C Pharyngeal tonsil: location and abnormal enlargement

Sagittal section through the roof of the pharynx.

a Located on the roof of the pharynx, the unpaired pharyngeal tonsil can be examined by means of posterior rhinoscopy (see p. 119). It is particularly well developed in (small) children and begins to regress at 6 or 7 years of age.

b An enlarged pharyngeal tonsil is very common in preschool-age children. (Chronic recurrent nasopharyngeal infections at this age often evoke a heightened immune response in the lymphatic tissue, causing “adenoids” or “polyps.”) The enlarged pharyngeal tonsil blocks the choanae, obstructing the nasal airway and forcing the child to breathe through the mouth. Since the mouth is then constantly open during respiration at rest, an experienced examiner can quickly diagnose the adenoidal condition by visual inspection.

D Histology of the lymphatic tissue of the oral cavity and pharynx

Because of the close anatomical relationship between the epithelium and lymphatic tissue, the lymphatic tissue of Waldeyer’s ring is also designated lymphoepithelial tissue.

a Lymphoepithelial tissue. Lymphatic tissue, both organized and diffusely distributed, is found in the lamina propria of all mucous membranes and is known as mucosa-associated lymphatic tissue (MALT). The epithelium acquires a looser texture, with abundant lymphocytes and macrophages. Besides the well-defined tonsils, smaller collections of lymph follicles may be found in the lateral bands (salpingopharyngeal folds). They extend almost vertically from the lateral wall to the posterior wall of the oropharynx and nasopharynx.

b Structure of the pharyngeal tonsil. The mucosal surface of the pharyngeal tonsil is raised into ridges that greatly increase its surface area. The ridges and intervening crypts are lined by ciliated respiratory epithelium.

c Structure of the palatine tonsil. The surface area of the palatine tonsil is increased by deep depressions in the mucosal surface (creating an active surface area as large as 300 cm2). The mucosa is covered by non keratinized stratified squamous epithelium.

6.6 Salivary Glands

A Major salivary glands

Lateral view (a) and superior view (b).

Three large, paired sets of glands are distinguished:

1. Parotid glands

2. Submandibular glands

3. Sublingual glands

The parotid gland is a purely serous gland (watery secretions). The submandibular gland is a mixed seromucous gland, and the sublingual gland is a predominantly mucous-secreting (mucoserous) gland. The glands produce approximately 0.5-2 liters of saliva per day. Their excretory ducts open into the oral cavity. The excretory duct of the parotid gland (the parotid duct) crosses over the masse- ter muscle, pierces the buccinator, and opens in the oral vestibule opposite the second upper molar. The excretory duct of the submandibular gland (submandibular duct) opens on the sublingual papilla behind the lower incisor teeth. The sublingual gland has many smaller excretory ducts that open on the sublingual fold, or into the submandibular duct. The saliva keeps the oral mucosa moist, and it contains the starch-splitting enzyme amylase and the bactericidal enzyme lysozyme. The presynaptic parasympathetic fibers (not shown here) for autonomic control of the salivary glands arise from the superior and inferior salivatory nuclei and are distributed to the glands in various nerves (see pp. 78,81, and 84), where they synapse with clusters of local ganglion cells, or in the submandibular ganglion (p. 106). Sympathetic fibers are distributed to the ducts along vascular pathways.

В Minor salivary glands

In addition to the three major paired glands, 700-1000 minor glands also secrete saliva into the oral cavity. They produce only 5-8% of the total output, but this amount suffices to keep the mouth moist when the major salivary glands are not functioning.

C Bimanual examination of the salivary glands

The two salivary glands of the mandible, the submandibular gland and sublingual gland, and the adjacent lymph nodes are grouped around the mobile oral floor, and so they must be palpated against resistance. This is done with bimanual examination.

D Spread of malignant parotid tumors along anatomical pathways

Malignant tumors of the parotid gland may directly invade surrounding structures (open arrows); they may also spread via regional lymph nodes (solid arrows), or spread systemically (metastasize) through the vascular system.

E Intraglandular course of the facial nerve in the parotid gland

The facial nerve divides into branches within the parotid gland (the parotid plexus separates the gland into a superficial part and deep part) and is vulnerable during the surgical removal of parotid tumors. To preserve the facial nerve during parotidectomy, it is first necessary to locate and identify the facial nerve trunk. The best landmarkfor locating the nerve trunk is the tip of the cartilaginous auditory canal.