*Summary.*—The main structure in the digestive system is the alimentary canal. This provides cavities where important dissolving processes take place, and tubes for joining these cavities, while glands connecting with the canal supply the necessary liquids for changing and dissolving the foods. The general plan of digestion is that of passing the food through the canal, beginning with the mouth, and of acting on it at various places, with the final result of reducing most of it to the liquid state. The digestive fluids supply water which acts as a solvent and carries the active chemical agents, or enzymes, that convert the insoluble foods into substances that are soluble. The muscles in the walls of the canal perform the mechanical work of digestion, while the nervous system controls and regulates the activity of the various organs concerned in this work. Exercises.—1. State the general purpose of digestion. How does digested food differ from that not digested? 2. Name all the divisions of the alimentary canal in the order in which the food passes through them. 3. What other work besides digestion is carried on by the alimentary canal? 4. What is gained by the mastication of the food? Why should mastication precede the other processes of digestion? 5. What is the work of the tongue in digestion? 6. State the purposes served by the gastric juice. 7. Give reasons for regarding the small intestine as the most important division of the food canal. 8. At what places, and by the action of what liquids, are fats, proteids, and starch digested? 9. What enzymes are found in the pancreatic juice? What is the digestive action of each? 10. Describe the work performed by the muscles of the stomach, the mouth, the esophagus, and the small intestine. 11. What advantages are derived from the use of cooked food? 12. State the advantages of drinking pure water. 13. If all the food that one needs to take at a single meal can be thoroughly masticated in fifteen minutes, why is it better to spend a longer time at the table? 14. What is meant by the overlapping of meals? What bad results follow? How avoided? PRACTICAL WORK Examine a dissectible model of the human abdomen (Fig. 75), noting the form, location, and connection of the different organs. Find the connection of the esophagus with the stomach, of the stomach with the small intestine, and of the small intestine with the large intestine. Sketch a general outline of the cavity, and locate in this outline its chief organs. Where it is desirable to learn something of the actual structure of the digestive organs, the dissection of the abdomen of some small animal is necessary. On account of unpleasant features likely to be associated with such a dissection, however, this work is not recommended for immature pupils. [Fig. 75] Fig. 75—Model for demonstrating the abdomen and its contents.