THE MOUTH AND ASSOCIATED STRUCTURES
The mouth (see Figure 11.5) opens into the oral cavity, or buccal cavity. Anatomically, the mouth extends from inside the lips to the fauces, a constriction or nar-rowed area that can be seen in the back of the mouth. Beyond this, is the pharynx. The roof of the mouth is formed by the palate. The anterior two-thirds, the hard palate, is hard, containing bone. The posteriorone-third, the soft palate, has muscle and no bone and is, therefore, soft. The conical downward projection from the soft palate is known as the uvula. The soft palate and the uvula project backward from the hard palate and separate the pharynx into the oropharynx andnasopharynx. They prevent food from entering the nose during swallowing by closing the opening be-tween the nasopharynx and oropharynx.
The fauces, or the posterior region of the mouth, has two mucosal arches on each side. Going from the palate toward the tongue is the palatoglossal arch, and behind it, going toward the pharynx, is the palatopharyngeal arch. When a person gags, themuscles in the two arches contract and narrow the opening between the mouth and the pharynx, a protective mechanism that prevents unwanted ob-jects from entering the digestive tract.
Between the two arches in the side, lie the palatinetonsils. The tonsils are part of the lymphoid system,and they help filter bacteria and toxins that may enter through the mucosa (inner lining) of the mouth.
The teeth (see Figure 11.6) are important for break-ing the food into small pieces. This process is known as mastication. The teeth arise from alveoli, small sockets in the mandibular and maxillary bones. The tooth is surrounded at the base by the gums, or gingivae.the bulk of each tooth is a bone-like material called dentin, which covers a cavity at the center of thetooth called the pulp cavity. Blood vessels and nerves pass through a canal (root canal) at the base (root) of the tooth to enter the pulp cavity. Collagen fibers (peridontal ligament), from the root of the tooth to the bone, hold the tooth in place. The ligament is re-inforced to the bone by cementum. The crown of the tooth is the portion visible above the gums. The dentin of the crown is covered by a layer of enamel, which is a crystalline form of calcium phosphate.
The teeth are modified according to function; four types have been identified. The blade-like incisors, found in the center of the mouth, cut, clip, and nip. The cuspids, orcanines—conical, with a sharp ridge and pointed tip—slash and tear. The bicuspids, or premolars, with flattened crowns and prominentridges, help crush, mash, and grind food. The poste-riorly located molars, larger versions of the bicuspids, crush and grind. In an adult, there are 3 pairs of molars, 2 pairs of bicuspids, 1 pair of cuspids, and 2 pairs of incisors in each jaw.
The cheeks, or lateral walls of the oral cavity, are comprised of the buccinator muscles and pads of fat. Anteriorly, the cheek is continuous with the lip, or labia. The space between the teeth and the cheeks isthe vestibule.
The tongue is muscular and has its own functions. It positions the food on the teeth, initiates swallowing, has taste buds that help taste food, and plays a key role in speech. The muscles of the tongue are con-trolled by the hypoglossal nerve (cranial nerve XII). Sensations such as touch, pain, and pressure are car-ried to the brain by the trigeminal nerve (cranial nerve V). The special sensation of taste is carried by the facial nerve, the glossopharyngeal nerve, and the vagus nerve. The facial nerve carries sensations from the anterior two-thirds of the tongue, and the glos-sopharyngeal carries it from the posterior one-third. The vagus nerve carries taste sensations from other areas of the mouth, such as the palate and pharynx.
The surface of the tongue appears fuzzy and has minute projections called papillae. Most of the taste buds—the sensory organs of taste—are located on the papillae. The taste buds have connections with nerve-endings that carry the sensation of taste to the brain. In humans, there are four basic tastes: sweet, sour, bitter, and salt. Bitter substances are best tasted on the back of the tongue; sour along the edges; sweet at the tip; and salt on the dorsum, anteriorly. Taste is sensed when the substances dissolved in the oral flu-ids come in contact with the taste buds.