DIGESTIVE SYSTEM Digestion takes part in the digestive system. It is also called the digestive tract because it is a tube of hollow organs, that is empty, ordered one after the other. Nutrients are used both as building materials and as a source of energy. THE MOUTH AND THE ESOPHAGUS Digestion starts in the mouth where we introduce foods. Its opening is surrounded by the lips. Then there are the teeth, some strong organs. Fixed to the maxilla and mandible, they’re arranged in two rows called the upper and the lower arch. In the mouth the salivary glands that pour out saliva, a liquid that begins to break down some substances present in food. Thanks to maxille and mandible the chewing takes part: teeth shred the food softened by saliva and kneaded by the tongue, that is a muscular organ. The bolus is formed, that is pushed down the pharynx towards the esophagus, with the deglutition. The esophagus is a tube long 20-25cm. It has some smooth muscles which contract like a wave, one after the other, to push the bolus towards the stomach