ORGANIZATION OF THE ORAL AND NASAL CAVITIES
The oral and nasal cavities lie near the body midline, inferior and medial to the orbital cavities, anterior to the pharynx and medial to the infratemporal fossa. They are separated from one another by the palate. Each cavity has an entrance and an exit. The oral cavity opens at the mouth and is bounded at its sides by teeth and cheeks. It is bounded posteriorly by two pairs of folds of mucous membrane which cover two important palatal muscles, the palatoglossus and palatopharyngeus (Fig. 1 ). The opening between these pillars is the exit from the oral cavity, the fauces. The nasal cavity opens anteriorly at the nostrils or Hares. It is divided by a septum into left and right halves and these end posteriorly at the choanae.
The nasal cavity is divided into left and right halves by the nasal septum. The septum is formed by the perpendicular plate of the ethmoid bone above, the vomer bone posteriorly and by an extensive cartilage anteriorly (Fig. 2). Each nasal cavity has a roof and a lateral wall. The roof is formed by the cribriforrn plate of the ethmoid bone, through which olfactory nerves enter the nasal cavity. The lateral walls are formed from several different bones, but their most striking features' their three conchae or turbinates, are formed primarily from the ethmoid bone and a separate bone known as the inferior nasal concha. The conchae are large, medially-directed bony elements which are covered with highly vascular mucous membranes. Air is thought to be heated and humidified as it is drawn past the conchae during breathing. Beneath each shelf-like concha is a meatus. The meatuses serve as the openings of communications between the nasal cavity and the paranasal sinuses
Blood Supply and Innervation of the Nasal Cavity
The nasal cavity receives its vascular supply from three sources (Fig. 4). The predominant supply is from benches of the sphenopalatine artery. Afterleaving the sphenopalatine foramen, the sphenopalatine artery divides into a lateral nasal branch, supplying the conchae, a posterior septal branch to the septum, and greater and lesser palatine arteries, which supply the hard and soft palates, respectively. In addition, th nasal cavity receives arterial blood from anterior ethmoidal branches of the ophthalmic artery that course medially out of the orbit, and branches of the super labial artery which enter from the face. Venous blood leaves the region in veins that course alongside the arteries. These drain into the facial, ophthalmic, and sphenopalatine veins. The latter vein form part of the pterygoid plexus of veins in the infratemporal fossa. The lyphatic drainage of this region is not well-known. Lymphatic vessels from the nasal region end in deep cervical lymph nodes.
The lateral walls of the oral cavity are formed by the buccinator muscles, which attach the opening of the mouth to the opening of the oral pharynx through a band of connective tissue called the ptervgomandibular rapine. This band runs between the medial pterygoid plate of the spheroid bone and the internal surface of the mandible (Fig. 7), near the lower third molar tooth. Two prominent muscles lie just medial to the rapine and tend to dominate the posterior border of the oral cavity. The palatopharyngeus and palatoglossus (Fig. 1 ) course from the tongue and pharynx to the palate and are covered by mucous membranes. Between the muscles lie the palatine tonsils or tonsils, which are actually just large lymph nodes. The two muscles are sometimes called the tonsillar pillars.
On the floor of the oral cavity is the tongue, a large collection of muscles covered by mucous membrane. The dorsal surface of the tongue is divided into an anterior 2/3 or oral part and a posterior 1/3 or pharyngeal part by a prominent sulcus terminalis (Fig. 1). The oral part of the tongue has a velvety appearance, since its mucous membrane is covered by small projections called papillae. Most of these papillae are pointed filiform papillae. Numerous, mushroom-shaped fungiform papillae are dispersed among the filiform papillae. The fungiform papillae are notable for containing taste buds at their base. These taste buds receive sensory innervation from axons coursing with the chords tympani nerve, a branch of the facial nerve. The pharyngeal part of the tongue lies just behind a row of very large vallate papillae which form the sulcus terminalis (Fig. 1). The vallate papillae also contain taste buds. These taste buds receive their innervation from the glossopharyngeal nerve. The posterior 1/3 of the tongue contains no real papillae but is composed almost entirely of masses of lymphoid tissue known as the lingual tonsils.
The muscles of the tongue can be divided into extrinsic and intrinsic muscles. Extrinsic muscles serve to move the tongue about in the oral cavity. Intrinsic muscles act to change the shape of the tongue. There are four extrinsic tongue muscles: genioglossus, hyoglossus, styloglossus, and palatoglossus (Fig. 8 ). The genioglossus attaches to the anterior extreme of the mandible and acts to pull the tongue forward or protrude it. The hyoglossus attaches to the hyoid bone. It pulls the tongue down and posterior or retracts and depresses. The stvloglossus attaches to the styloid process. It retracts and elevates the tongue. The palatoglossus attaches to the palate. It has a stronger effect in depressing the palate than any effect on the tongue. The intrinsic tongue muscles tend to be arranged as longitudinal, transverse, and radial fibers. All tongue muscles are innervated by the hypoglossal or 12th cranial nerve.
Floor of the Mouth
The mylohyoid muscles are attached to the inner surfaces of the mandible and the hyoid bone, but are more extensively attached to each other at a midline rapine. The two muscles thus form a muscular sling which marks the floor of the oral cavity (Fig. 9). They are innervated by branches of the lingual nerve, the mylohyoid nerve. The sublingual salivary glands lie immediately on top of the mylohyoid muscle and along the inner surface of the mandible. These glands open into the sublingual region as several small, short ducts. The submandibular salivary gland lies posterior to the sublingual gland and is wrapped around the posterior margin of the mylohyoid muscle. It is found in the digastric triangle of the neck. Its long duct runs anteriorly on the medial surface of the sublingual gland to open into the sublingual region. The lingual nerve lies between the two salivary glands, just lateral to the hyoglossus muscle. It passes beneath the submandibular duct to enter the tongue from beneath (Fig. 8). The lingual nerve provides the main source of general sensation to the anterior 2/3 of the tongue, but also carries with it axons derived from the facial nerve which course in the chorda tympani. Chorda tympani fibers provide special sensation to the anterior 2/3 of the tongue and pre-ganglionic parasympathetic innervation to the submandibular ganglion. Both salivary glands are innervated by parasympathetic postganglionic axons derived from cell bodies in the submandibular ganglion. This ganglion lies on the surface of the submandibular gland. Postganglionic axons course either directly to the submandibular gland or they rejoin the lingual nerve to supply the sublingual gland. These innervation patterns are summarized in figure 6.
The palate forms the roof of the oral cavity. It consists of hard and soft portions. The hard palate consists of the palatine process of the maxilla, the palatine bone, and part of the medial pterygoid plate of the spheroid bone (Fig. 2). It is covered with mucous membrane and its posterior extreme is continuous with a fibromuscular extension, the soft palate or uvula. The soft palate contains five small muscles which serve to change its shape relative to the oral and nasal cavities: palatoglossus, palatopharyngeus, tensor palati, levator palati, and musculus uvulae. The palatoglossus and palatopharyngeus were described above. They function primarily to raise the tongue and oral pharynx to approximate the palate - i.e. to control the opening of the fauces. They are innervated by pharyngeal branches of the vagus nerve. The tensor palati attaches to the medial pterygoid plate and courses around its hook or hamulus to attach to the sides of the soft palate (Fig. 10). It functions to tense the soft palate and it is innervated by the mandibular nerve. The more posterior and medial levator palati attaches to the superior surface of the soft palate. It elevates the soft palate to help close oh the choanae. The musculus uvulae is an intrinsic muscle of the soft palate that acts to stiffen the uvula.