APC 100 The Digestive System Jim Sharp DVM,PhD The digestive system is a multi-organ system involved in the procurement, digestion, absorption, and processing of food, and the elimination of non-absorbed food waste. The basic structure of the digestive system may appear as a straight tube or be rather convoluted and complex. The structure depends on the nature of the organism, its size, body morphology, life habits (filter feeder, nectar collector, herbivore, or carnivore) and the nature of its food (plant and or animal material). The digestive tract is a tube or modified tube-like structure. In terapods (four-legged animals) the proximal portion is the oral cavity followed by the oral pharynx, which leads to the esophagus. Food is propelled down the esophagus into the stomach and then into the intestine. It then exits via the anus or vent. Oral and Pharyngeal Cavities Fish: Fish have a common oropharyngeal cavity, which typically contains teeth. The walls of this cavity contain gills, which are the respiratory organs for fish. They do not have distinct oral and pharyngeal cavities. Mammals: Theoral cavityin mammals typically has an upper and lower arcade of teeth used to prehend and macerate food, and a tongue that has a variety of functions including taste and moving food distally into the oral pharynx. The pharynxin mammals is divided into the nasal and oral pharynx. The nasal pharynx is a respiratory passage from the nose and sinuses to the trachea. It lies above the hard and soft palates. The oral pharynx runs from the oral cavity to the entrance of the esophagus. Food and air cross past one another in the oral pharynx. The esophagus lies dorsal to the trachea in mammals. Birds: Birds also have distinct oral and pharyngeal cavities. They have no teeth; instead a beak is used to prehend food. Maceration of food in birds is typically done in a specialized diverticulum of the esophagus termed the crop. The pharynx of mammals and birds is similar except that birds have a palatine fissure between the oral cavity and the nasal pharynx.
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