After preliminary maceration, moistening, and formation into a bolus by the actions of the structures of the oral cavity and salivary glands, food passes rapidly through the pharynx to the esophagus. The rapid passage of food through the pharynx keeps it clear for the passage of air. The food passes more slowly through the gastrointestinal tract, and during its transit through the stomach and small intestine, the major alterations associated with digestion, solubilization, and absorption occur. Absorption occurs chiefly through the wall of the small intestine. Undigested food and other substances within the alimentary canal, such as mucus, bacteria, desquamated cells, and bile pigments, are excreted as feces.
The alimentary mucosa is the surface across which most substances enter the body.
The alimentary mucosa performs numerous functions in its role as an interface between the body and the environment. These functions include the following:  Secretion. The lining of the alimentary canal secretes, at specific sites, digestive enzymes, hydrochloric acid, mucin, and antibodies.
 Absorption. The epithelium of the mucosa absorbs metabolic substrates (e.g., the breakdown products of digestion) as well as vitamins, water, electrolytes, recyclable materials such as bile components and cholesterol, and other substances essential to the functions of the body.
 Barrier. The mucosa serves as a barrier to prevent the entry of noxious substances, antigens, and pathogenic  organisms. Immunologic protection. Lymphatic tissue within the mucosa serves as the body’s first line of immune defense.
The functions listed above are discussed at the beginning of the next chapter. The digestive system is considered in three chapters that deal, respectively, with the oral cavity and pharynx (this chapter); the esophagus and gastrointestinal tract (Chapter 17); and the liver, gallbladder, and pancreas (Chapter 18).
The oral cavity consists of the mouth and its structures, which include the tongue, teeth and their supporting structures (periodontium), major and minor salivary glands, and tonsils.
The oral cavity is divided into a vestibule and the oral cavity proper. The vestibule is the space between the lips, cheeks, and teeth. The oral cavity proper lies behind the teeth and is bounded by the hard and soft palates superiorly, the tongue and the floor of the mouth inferiorly, and the entrance to the oropharynx posteriorly.
Each of the three major salivary glands are paired structures; they include the following:  Parotid gland, the largest of the three glands, located in the infratemporal region of the head. Its excretory duct, the parotid (Stensen’s) duct, opens at the parotid papilla, a small elevation on the mucosal surface of the cheek opposite the second upper molar tooth.
 Submandibular gland, located in the submandibular triangle of the neck. Its excretory duct, the submandibular (Wharton’s) duct, opens at a small fleshy prominence (the sublingual caruncle) on each side of the lingual frenulum on the floor of the oral cavity.
 Sublingual gland, lying inferior to the tongue within the sublingual folds at the floor of the oral cavity. It has a number of small excretory ducts; some enter the submandibular duct, and others enter individually into the oral cavity.
The parotid and submandibular glands have relatively long ducts that extend from the secretory portion of the gland to the oral cavity. The sublingual ducts are relatively short.
The minor salivary glands are located in the submucosa of the oral cavity. They empty directly into the cavity via short ducts and are named for their location (i.e., buccal, labial, lingual, and palatine).