The upper end of the digestive tract is bounded anteriorly by the teeth and lips, posteriorly by the oral pharynx, laterally by the teeth and cheeks, superiorly by the hard and soft palate, and inferiorly by the tongue and floor of the mouth.

A. Wall Structure: The mucosa includes the lining epithelium and the underlying lamina propria. Nonkeratinized stratified squamous epithelium (mucous membrane) covers all internal surfaces of the oral cavity and pharynx except the teeth. The lamina propria is a vascular connective tissue with papillae like those of the dermis. The papillae contain capillaries that nourish the epithelium. The oral cavity has no muscularis mucosae. The submucosa is a more fibrous connective tissue than the lamina propria; it contains many blood vessels and small salivary glands. The oral cavity lacks a standard muscularis externa. Skeletal muscle underlies the submucosa in the lips, cheeks, tongue, floor of the mouth, oral pharynx, soft palate, and its downward extension, the uvula. Bone underlies the thin submucosa of the hard palate and gums (gingiva).

B. Lips: Here, there is a transition from nonkeratinized mucous membrane to the keratinized stratified squamous epithelium of the skin. The thin keratinized layer covering the lips' ver million border allows the reddish color of blood in vessels of the lamina propria to show through. Hair follicles, keratin, and additional pigment help distinguish the outer lip surface from the inner in tissue sections.

C. Tongue: This is a mass of skeletal muscle covered by a mucosa. The mucosa is bound tightly to the muscle by the lamina propria, which penetrates between the bundles of muscle fibers. There is little or no submucosa. The muscle is arranged in bundles of many sizes; these are separated by connective tissue and cross each other in 3 planes. This gives the tongue the flexibility required for speech, positioning food, chewing, and swallowing. The mucosa differs on the dorsal (upper) and ventral (lower) surfaces. The ventral surface has a thin nonkeratinized stratified squamous epithelium underlain by a lamina propria. The epithelium covering the dorsal surface is partly keratinized. The anterior two-thirds of the dorsal surface is separated from the posterior third by a V-shaped groove. Behind this, the epithelium invaginates to form the crypts of the lingual tonsils. Cryptless patches of lymphoid tissue in the lamina propria cause surface bulges in this region. The anterior two-thirds of the dorsal surface has many papillae-projections of the mucosal surface. There are 4 types of papillae.

1. Filiform papillae are the most numerous. They are sharp, often partly keratinized, conical projections that lack taste buds.
2. Fungiform papillae resemble mushrooms. Each has taste buds on its expanded upper surface but not on its narrow stalk. Fungiform papillae occur singly and are scattered among the filiform papillae.
3. Foliate papillae are poorly developed in humans. They occur in rows separated by furrows into which serous glands in the lamina propria drain. The furrow walls (sides of the papillae) harbor many taste buds.
4. Circumvallate papillae are the largest and least numerous, with only 7-12 occurring near the V-shaped groove at the back of the tongue. Each is surrounded by a ringlike ridge of mucosa from which it is separated by a circular furrow, whose walls contain taste buds on both sides. As with the foliates, ducts from serous (von Ebner's) glands empty into the furrow and periodically wash the chemical stimuli from the taste buds, allowing new tastes to be sensed

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