The pharynx connects the nasal and oral cavities with the larynx and esophagus (see fig. 17.7). It can be divided into the following parts:
1. The nasopharynx is located superior to the soft palate. It communicates with the nasal cavity and provides a passageway for air during breathing. The auditory tubes, which connect the pharynx with the middle ears, open through the walls of the nasopharynx (see chapter 12, pp. 470-471).
2. The oropharynx is posterior to the mouth. It opens posterior to the soft palate into the nasopharynx and projects downward to the upper border of the epiglottis. This portion is a passageway for food moving downward from the mouth and for air moving to and from the nasal cavity.
3. The laryngopharynx is located just inferior to the oropharynx. It extends from the upper border of the epiglottis downward to the lower border of the cricoid cartilage of the larynx and is a passageway to the esophagus.
Shier-Butler-Lewis: V. Absorption and 17. Digestive System © The McGraw-Hill
Human Anatomy and Excretion Companies, 2001
Physiology, Ninth Edition
Sticky foods, such as caramel, lodge between the teeth and in the crevices of molars, feeding bacteria such as Actinomyces, Streptococcus mutans, and Lactobacillus. These microbes metabolize carbohydrates in the food, producing acid by-products that destroy tooth enamel and dentin (fig. 17A). The bacteria also produce sticky substances that hold them in place.
If a person eats candy bars but does not brush the teeth soon afterward, the actions of the acid-forming bacteria will produce decay, called dental caries. Unless a dentist cleans and fills the resulting cavity that forms where enamel is destroyed, the damage will spread to the underlying dentin. The tooth becomes very sensitive.
Dental caries can be prevented in several ways:
into the enamel's chemical structure, strengthening it.
4. Apply a sealant to children and adolescents' teeth that have crevices that might hold onto
decay-causing bacteria. The sealant is a coating that keeps acids from eating away at tooth enamel.
One dental researcher took an unconventional approach to preventing dental caries that, understandably, was never commercialized. He invented a mouthwash consisting of mutant bacteria that would replace Streptococcus mutans but would not decay enamel. Consumer acceptance of a mutant bacterial brew was an obstacle! ■
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