DIGESTIVE SYSTEM
The general organization of the digestive tract is the same in all vertebrate, although different elements are emphasized in different groups. In all vertebrates, acid digestion or proteins takes place in the stomach, after which food passes to the upper part of the small intestine, called the duodenum, where a battery of digestive enzymes continues the digestive process. The products of digestion then pass across the wall of the small intestine into the blood stearm. Specialization’s among the digestive systems of different kinds of vertebrates reflect differences in the way these animals live. The initial components of the gastrointestinal tract are the mouth and pharynx. When food passes beyond the teeth to the back of mouth in a mammal, the flowing three things happen: The palate elevates, pushing against the back wall of the pharynx. This seals off the nasal cavity and prevents any food from entering it. Pressure against the pharynx stimulates receptors within its walls to send nerve signals to the swallowing center in the brain stem. The swallowing center sends out nerve signals that keep food from getting into the respiratory tract, which branches off below the pharynx. These signals both inhibit respiration and seal the windpipe by raising the larynx and thus closing the passage between the larynx and the pharynx, called the glottis. A flap of tissue, called the epiglottis, folds back over the opening, providing a seal. After passing the tracheal opening, the food enters a tube called the esophagus, which connects the pharynx to the stomach. No further digestion takes place in the esophagus. Its role is that of an escalator, moving food down toward the stomach. In adult humans the esophagus is about 25 centimeters long, and its lower end opens into the stomach proper. The upper portion of the esophagus is enveloped in skeletal muscle, and the lower two-thirds is enveloped in smooth muscle. Successive waves of contraction of these muscles, which are stimulated by the swallowing center, move food down through the esophagus to the stomach. These rhythmic sequences of waves of muscular contraction in the walls of a tube are called peristalsis. Because the movement of food through the esophagus is primarily caused by these peristaltic contraction, humans can swallow even if they are upside down. The exit of food from the esophagus to the stomach is controlled by a muscular contraction, or sphincter, which opens in response to the pressure exerted by the food. When this sphincter is contracted, it prevents the food in the stomach from moving back up the esophagus. .