Mouth, Pharynx, and Teeth
Mouth, Pharynx, and Teeth
The digestive system functions to receive, store, and break down food to absorb nutrients. Both physical and chemical digestion processes begin in the mouth. The presence of food in the mouth causes the salivary glands to deliver saliva into the mouth. Often even the smell or anticipation of food will cause activation of the salivary glands. Digestive enzymes in saliva begin to moisten, soften, and dissolve the food before passing it to the next stage of the digestive system. Saliva also contains mucin, which protects the mouth from abrasion, and buffers, which prevent dental cavities by neutralizing acid in the mouth. The tongue is used to taste and manipulate food during chewing.
Fish, amphibians, and reptiles principally use teeth to grip prey and prevent it from escaping until it can be swallowed whole. Birds have no teeth, but many have bills with serrated edges for shredding. Often the upper bill is hooked for seizing and tearing apart prey. Other bill shapes are designed for seed eating and fishing. Because snakes have no limbs, their mechanism of feeding has been modified. Rapid strikes, venom, and constriction are used to immobilize prey. Backward-curving fangs also help snakes to seize and hold prey. Snakes tend to eat large prey items, and they are able to unhinge their jaws to swallow them. For some animals, the physical reduction of food is necessary to release nutrients from indigestible components and to increase the surface contact between food and digestive juices. Physical reduction of food in the mouth is accomplished by the chewing, rasping, and grinding of teeth. The function and type of teeth depend upon the specialized food habits of the animal.
Mammals exhibit true mastication, meaning that their teeth are involved in chewing as opposed to just tearing or crushing. Mammals typically have four types of teeth. The incisors at the front of the mouth are used for biting, cutting, and stripping; the canines are used for seizing, piercing, and tearing. Further back in the mouth, premolars and molars are used for grinding and crushing. Carnivores have well-developed canines for seizing prey and tearing meat. Herbivores have reduced canines, but well-developed molars for grinding. Their teeth help them break apart the tough cellulose walls of plants to access nutrients. Rodents have self-sharpening incisors that grow throughout life and must be worn away by gnawing to keep them from growing too long.
The teeth of some vertebrates have evolved to serve functions other than feeding. For example, elephant tusks are modified upper incisors used for defense, attack, and rooting for food. Male wild boars have modified canines that are used as weapons in male-to-male combat. Elk use specialized teeth to make mating and territorial calls.
After food is physically and chemically reduced in the mouth, it passes to the next stage of digestion. Food is swallowed when the tongue pushes the food to the back of the mouth and into the pharynx. The pharynx acts as an intersection between the esophagus and the trachea . The esophagus leads to the stomach, whereas the trachea leads to the lungs . The epiglottis, a cartilaginous flap, covers the trachea during swallowing to prevent food and fluid from entering the lungs. Food then enters the esophagus and involuntary muscles contract to push food into the stomach.
Invertebrates have comparable feeding and digestive mechanisms to those of vertebrates. Invertebrates do not possess true teeth. However, they often have beaks or toothlike structures for biting and holding. Insects have three pairs of appendages on the head that serve multiple functions. The first pair, the mandibles, are primarily for crushing. The second pair, the maxillae, serve as grasping jaws, and the third, the labia, as probing and tasting tongues. Food that has been broken down into smaller pieces enters the mouth, often with the help of extended maxillae. As with vertebrates, salivary glands produce enzymes to help break down food. The reduced food then enters the crop, an organ similar to the vertebrate stomach, for further digestion.
In insects, the form and structure of mouthparts varies dramatically depending upon the type of feeding. Locusts eat leaves and have grinding and cutting mandibles. Mosquitoes and butterflies have sucking or siphoning mouthparts. The common housefly has spongelike mouthparts with which they lap up food they have liquefied with salivary secretions. Other invertebrates have interesting structures used for procuring and reducing food. For example, Nereis, a carnivorous polychaete (marine worm), have a muscular pharynx with chitinous jaws that they turn inside out quickly to seize prey. The pharynx then retracts and the prey is swallowed. Crustaceans often reduce the size of food using shredding devices like the tearing, beaklike jaws in cephalopod mollusks (i.e., squid). Snails have a radula, a rasping structure in the mouth, for scraping algae off rocks.
see also Digestive System.
Alisha K. Holloway
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Hickman, Cleveland P., Jr., Larry S. Roberts, and Allan Larson. Integrated Principles of Zoology, 9th ed. Dubuque, IA: Wm. C. Brown Publishers, 1995.
Hildebrand, M. Analysis of Vertebrate Structure, 3rd ed. New York: John Wiley & Sons, Inc., 1988.