The mouth (oral cavity, buccal cavity) is where food enters the digestive tract. The following features are found in the mouth: The vestibule is the narrow region between the cheeks and teeth and between the lips and teeth.
- The tongue defines the lower boundary of the mouth. It helps position the food during mastication (chewing) and gathers the chewed food into a ball, or bolus, in preparation for swallowing. The tongue is covered with papillae, small projections that help the tongue grip food. Many of the papillae bear taste buds.
- The palate defines the upper boundary of the mouth. The forward portion is the hard palate, hard because bone (maxillae and palatine) makes up this portion of it. Further back in the mouth, the soft palate consists of muscle and lacks any bone support. A conical muscular projection, the uvula, is suspended from the rear of the soft palate.
- Saliva contains water (99.5 percent), digestive enzymes, lysozyme (an enzyme that kills bacteria), proteins, antibodies (IgA), and various ions. Saliva lubricates the mouth, moistens food during chewing, protects the mouth against pathogens, and begins the chemical digestion of food. Chemical digestion is carried out by the digestive enzyme salivary amylase, which breaks down polysaccharides (starch and glycogen) into short chains of glucose, especially the disaccharide maltose (which consists of two glucose molecules). Saliva is produced by the following glands:
- Three pairs of salivary glands, the parotid, submandibular, and sublingual, lie outside the mouth. They deliver their secretions to the mouth via ducts.
- Buccal glands are located in the mucosa that lines the mouth.
- The teeth are embedded within sockets of the upper and lower jawbones (maxillae and mandible). Each tooth is surrounded by gum, or gingival, and held in its socket by a periodontal ligament. The 20 deciduous teeth (baby teeth or milk teeth) are eventually replaced by 32 permanent teeth. There are three types of teeth, based on shape and function:
- Incisors have a chisel-shaped edge suitable for biting off food.
- Canines (cuspids) are pointed fangs and are used for tearing food.
- Premolars (bicuspids) and molars have flat surfaces used for grinding and crushing.
A tooth has the following structural features:
- Dentin is a calcified tissue (like bone) that composes the bulk of the tooth.
- The crown is the portion of the tooth embedded in the bone.
- The neck is the region at the gum line where the crown and root meet.
- Enamel is the hard, nonliving material that covers the crown. Calcium compounds make the enamel the hardest substance in the body.
- Cementum is the bonelike substance that covers the root and binds it to the periodontal ligament.
- The pulp cavity is the central cavity inside the tooth. It contains blood vessels, nerves, and connective tissue (collectively called pulp).
The pharynx, or throat, receives the food from the mouth during swallowing. From the mouth, it moves back and down into the oropharynx and then descends into the laryngopharynx. The food then passes into the esophagus.
The esophagus is a 25-cm (10-inch) long tube that begins at the laryngopharynx and descends behind the trachea through the mediastinum (cavity between the lungs). It then passes through the diaphragm at an opening called the esophageal hiatus and connects to the stomach. Food is forced through the esophagus toward the stomach by peristalsis. Two sphincter muscles, the upper esophageal sphincter at the top of the esophagus and the cardiac sphincter (lower esophageal sphincter) at the bottom of the esophagus, control the movement of food into and out of the esophagus.