Receiving Region
Receiving Region
The first region of the alimentary canal consists of devices for feeding and swallowing. These include mouthparts (for example, mandibles, jaws, teeth, radula, bills), buccal cavity and muscular pharynx. Most metazoans other than suspension feeders have salivary glands (buccal glands) that produce lubricating secretions containing mucus to assist swallowing (Figure 34-9). Salivary glands often have other specialized functions such as secretion of toxic enzymes for quieting struggling prey and secretion of salivary enzymes to begin digestion. The salivary secretion of the leech, for example, is a complex mixture containing an anesthetic substance (making its bite nearly painless) and several en-zymes that prevent blood coagulation and increase blood flow by dilating veins and dissolving the tissue cement that binds cells together.
Salivary amylase is a carbohydratesplitting enzyme that begins hydrolysis of plant and animal starches. It is found only in certain herbivorous molluscs, some insects, and in primate mammals, including humans. Starches are long polymers of glucose. Salivary amylase does not completely hydrolyze starch, but breaks it down mostly into twoglucose fragments called maltose. Some free glucose and longer fragments of starch are also produced. When the food mass (bolus) is swallowed, salivary amylase continues to act for some time, digesting perhaps half of the starch before the enzyme is inactivated by the acidic environment of the stomach. Further starch digestion resumes beyond the stomach in the intestine.
The tongue is a vertebrate innovation, usually attached to the floor of the mouth, that assists in food manipulation and swallowing. It may be used for other purposes, however, such as food capture (for example, chameleons, woodpeckers, anteaters) or as an olfactory sensor (many lizards and snakes).
In humans, swallowing begins with the tongue pushing moistened food toward the pharynx. The nasal cavity closes reflexively by raising the soft palate. As the food slides into the pharynx, the epiglottis tips down over the trachea, nearly closing it (Figure 34-10). Some particles of food may enter the opening of the trachea but contraction of laryngeal muscles prevents it from going farther. Once food is in the esophagus, peristaltic contraction of esophageal muscles forces it smoothly toward the stomach.
The first region of the alimentary canal consists of devices for feeding and swallowing. These include mouthparts (for example, mandibles, jaws, teeth, radula, bills), buccal cavity and muscular pharynx. Most metazoans other than suspension feeders have salivary glands (buccal glands) that produce lubricating secretions containing mucus to assist swallowing (Figure 34-9). Salivary glands often have other specialized functions such as secretion of toxic enzymes for quieting struggling prey and secretion of salivary enzymes to begin digestion. The salivary secretion of the leech, for example, is a complex mixture containing an anesthetic substance (making its bite nearly painless) and several en-zymes that prevent blood coagulation and increase blood flow by dilating veins and dissolving the tissue cement that binds cells together.
Salivary amylase is a carbohydratesplitting enzyme that begins hydrolysis of plant and animal starches. It is found only in certain herbivorous molluscs, some insects, and in primate mammals, including humans. Starches are long polymers of glucose. Salivary amylase does not completely hydrolyze starch, but breaks it down mostly into twoglucose fragments called maltose. Some free glucose and longer fragments of starch are also produced. When the food mass (bolus) is swallowed, salivary amylase continues to act for some time, digesting perhaps half of the starch before the enzyme is inactivated by the acidic environment of the stomach. Further starch digestion resumes beyond the stomach in the intestine.
The tongue is a vertebrate innovation, usually attached to the floor of the mouth, that assists in food manipulation and swallowing. It may be used for other purposes, however, such as food capture (for example, chameleons, woodpeckers, anteaters) or as an olfactory sensor (many lizards and snakes).
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| Figure 34-10 Oral cavity and throat in humans in sagittal section (A), and sequence of swallowing (B to D). |
In humans, swallowing begins with the tongue pushing moistened food toward the pharynx. The nasal cavity closes reflexively by raising the soft palate. As the food slides into the pharynx, the epiglottis tips down over the trachea, nearly closing it (Figure 34-10). Some particles of food may enter the opening of the trachea but contraction of laryngeal muscles prevents it from going farther. Once food is in the esophagus, peristaltic contraction of esophageal muscles forces it smoothly toward the stomach.
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