Pharynx

Through chewing, or mastication, the tongue manipulates food, the teeth grind it and the food is mixed with saliva. As a result, the food is reduced to a soft, flexible mass called a bolus that is easily swallowed. When food is first swallowed, it passes from the mouth into the pharynx.

The pharynx is a funnel-shaped tube that extends from the internal nares to the esophagus posteriorly and the larynx anteriorly. The pharynx is composed of skeletal muscle and lined by mucous membrane. Whereas the nasopharynx functions only in respiration, both the oropharynx and laryngopharynx have digestive as well as respiratory functions. Food that is swallowed passes from the mouth into the oropharynx and laryngopharynx before passing into the esophagus. Muscular contractions of the oropharynx and laryngopharynx help propel food into the esophagus.

Swallowing, or deglutition, is a mechanism that moves food from the mouth to the stomach. It is helped by saliva and mucus and involves the mouth, pharynx, and esophagus. Swallowing is divided into three stages: (1) the voluntary stage, in which the bolus is moved into the oropharynx; (2) the pharyngeal stage, the involuntary passage of the bolus through the pharynx into the esophagus; and (3) the esophageal stage, the involuntary passage of the bolus through the esophagus into the stomach.

TEETH

The teeth (dentes) are accessory structures of the digestive system located in sockets of the alveolar processes are covered mandible and maxillae. The alveolar processes are covered by the gingivae, or gums, which extend slightly into each socket forming the gingival sulcus. The sockets are lined by the periodontal ligament, which consists of dense fibrous connective tissue and is attached to the socket walls and the cemental surface of the roots. Thus, it anchors the teeth In position and also acts as a shock absorber to dissipate the forces of chewing.

A typical tooth consists of three principal portions. The crown is the exposed portion above the level of the gums. The root consists of one to three projections embedded in the socket. The neck is the constricted junction line of the crown and the root near the gumline.

Teeth are composed primarily of dentin, a calcified connective tissue that gives the tooth its basic shape and rigidity. The dentin encloses a cavity. The enlarged part of the cavity, the pulp cavity, lies in the crown and is filled with pulp, a connective tissue containing blood vessels, nerves, and lymphatic vessels. Narrow extensions of the pulp cavity run through the root of the tooth and are called root canals. Each root canal has an opening at its base, the apical foramen. Through the foramen enter blood vessels bearing nourishment, lymphatic vessels affording protection, and nerves providing sensation. The dentin of the crown is covered by enamel that consists primarily of calcium phosphate and calcium carbonate. Enamel is the hardest substance in the body and protects the tooth from the wear of chewing. It is also a barrier against acids that easily dissolve the dentin. The dentin of the root is covered by cementum, another bonelike substance, which attaches the root to the periodontal ligament.

Salivary Glands

Saliva is a fluid that is continuosly secreted by glands associated with the mouth. Ordinarily, just enough saliva is secreted to keep the mucous membranes of the mouth and pharynx moist, but when food enters the mouth secretion increases so the saliva can lubricate, dissolve, and begin the chemical breakdown of the food. The mucous membrane lining the cheeks contains many small glands, the buccal glands, that secrete small amounts of saliva. There are also minor salivary glands in the lips, tongue, and hard and soft palates that secrete small amounts of saliva. However, most saliva is secreted by the major salivary glands, accessory structures that lie outside the mouth and pour their contents into ducts that empty into the oral cavity. There are three pairs of salivary glands: parotid, submandibula, and sublingual glands.

The parotid glands are located inferior and anterior to the ears between the skin and the masseter muscle. They are compound tubuloacinar glands. Each secretes into the oral cavity vestibule via a duct, calles parotid (Stensen’s) duct, that pierces the buccinators muscle to open into the vestibule opposite the upper second molar tooth. The submandibular glands, which are compound acinar glands, are found beneath the base of the tongue in the posterior part of the floor of the mouth. Their ducts, the submadibular (Wharton’s) glands, run superficially under the mucosa on their side of the midline of the floor of the mouth and enter the oral cavity proper on either side of the lingual frenulum. The sublingual glands, also compound acinar glands, are superior to the submandibular glands, and their ducts, the lesser sublingual (Rivinus’s) ducts, open into the floor of the mouth in the oral cavity proper.

The parotid gland receives its blood supply from branches of the external carotid artery and is drained by vessels that are tributaries of the external jugular vein. The submandibular gland is supplied by branches of the facial artery and drained by tributaries of the facial vein. The sublingual gland is supplied by the sublingual branch of the lingual artery and the submental branch of the facial artery and is drained by tributaries of the sublingual and submental veins.

The salivary glands receive both sympathetic and parasympathetic innervations. The sympathetic fibers form plexuses on the blood vessels that supply the glands and serve as vasoconstrictors. The parotid gland receives sympathetic fibers from the plexus on the external carotid artery, whereas the submandibular and sublingual glands receive sympathetic fibers that contribute to the sympathetic plexus and accompany the facial artery to the glands. The parasympathetic fibers of the glands consist of secretomotor fibers to the glands.

The fluids secreted by the buccal glands, minor salivary glands, and the three pairs of major salivary glands constitute saliva. Amounts of saliva secreted daily vary considerably but range from 1000 to 1500 ml. Chemically, saliva is 99.5 percent water and 0.5 percent solutes. Among the solutes are salts, dissolved gases, and various organic substances, including urea and uric acid, serum albumin and globulin, mucin, the bacteriolytic enzyme aysozyme, and the digestive enzyme salivary amylase, are also present. Salivary amylase initiates the breakdown of starch. Saliva continues to be secreted heavily some time after food is swallowed. This flow of saliva washes out the mouth and dilutes and buffers the chemical remnants of irritating substances.

The Anatomy of Mouth (Oral Cavity)

MOUTH (ORAL CAVITY)

The mouth, also referred to as the oral or buccal cavity, is formed by the cheeks, hard and soft palates, and tongue. Forming the lateral walls of the oral cavity are the cheeks – muscular structures covered on the outside by skin and lined by nonkeratinized stratified squamos  epithelium. The anterior portions of the cheeks terminate in the superior and inferior lips.

The lips (labia) are fleshy folds surrounding the orifice of the mouth. They are covered on the outside by skin and on the inside by a mucous membrane. The transition zone where the two kinds of covering tissue meet is called the vermilion. This portion of the lips is nonkeratinized, and the color of the blood in the underlying blood vessels is visible through the transparent surface layer of the vermilion. The inner surface of each lip is attached to its corresponding gum by a midline fold of mucous membrane called the labial frenulum.

The orbicularis oris muscle and connective tissue lie between the external integumentary covering and the internal mucosal lining. During chewing, the cheeks and lips help keep food between the upper and lower teeth. They also assist in speech.

The vestibule of the oral cavity is bounded externally by the cheeks and lips and internally by the gums and teeth. The oral cavity proper extends from the vestibule to the fauces, the opening between the oral cavity and the pharynx or throat.

The hard palate, the anterior portion of the roof of the mouth, is formed by the maxillae and palatine bones, is covered by mucous membrane, and forms a bony partition between the oral and nasal cavities. The soft palate forms the posterior portion of the roof of the mouth. It is an arch-shaped muscular partition between the oropharynx and nasopharynx and is lined by mucous membrane.
Hanging from the free border of the soft palate is a conical muscular process called the uvula. On either side of the base of the uvula are two muscular folds that run down the lateral side of the soft palate. Anteriorly, the palatoglossal arch (anterior pillar) extends inferiorly, laterally, and anteriorly to the side of the base of the tongue. Posteriorly, the palatopharyngeal arch (posterior pillar) projects inferiorly,nlaterally, and posteriorly to the side of pharynx. The palatine tonsils are situated between the arches, and the lingual tonsil are situated between the arches, and the lingual tonsil is situated at the base of the tongue. At the posterior boder of the soft palate, the mouth opens into the oropharynx through the fauces.