PROBLEM 2. The Digestive System Makes Food Usable 201 



10. Why are the muscles of the digestive tube beyond the throat called 

 involuntary muscles? Distinguish between voluntary and involuntary 

 muscles in appearance. What is the other name for voluntary mus- 

 cles; for involuntary muscles? Describe peristalsis. 



11. What starts the secretion of the gastric glands? Of what is gastric 

 juice composed? What is the importance of each substance? 



12. Name and describe the tissues which are found in the wall of the 

 stomach. Which important tissues are not found? 



13. Describe the muscles in the walls of the stomach and tell how they 

 function. How does food get into the small intestine? 



14. Sum up the changes that have taken place in food by the time it 

 reaches the small intestine. Name three juices it meets there. Explain 

 the changes brought about by pancreatic juice in three kinds of food. 

 Explain how intestinal juice completes the work of the pancreatic 

 juice. 



15. Is emulsification a physical or a chemical change? Explain. How does 

 emulsification help? In what two ways is the bile of help in digestion? 



16. Why is there little absorption of food in the stomach? 



17. The small intestine is long and narrow; it has folds; it has villi. Explain 

 how each of these structures is useful in digestion or absorption. 



18. Normally, which parts of our food reach the large intestine? 



19. What is glycogen? Explain its relation to the liver. In what sense 



are amino acids building stones? How does the liver put amino acids 

 to good use? 



20. Where is secretin made? How does it function? Why is secretin called 

 a hormone? Explain three means by which digestive glands can be 

 stirred to action. How is the secreting of the digestive glands some- 

 times stopped? 



Exercises 



1. Can starch enter an artificial cell? Prepare an artificial cell by filling 

 a gelatin capsule with some white of t^^ and sugar solution. Make a thin 

 starch paste by heating a small amount of starch in a large amount of 

 water. Cool. Place the capsule in the paste. After two hours remove the 

 capsule and test the contents for starch. What test will you use? What 

 do you observe? What conclusions can you draw about the entrance of 

 starch into the cell? 



2. Does saliva change starch into sugar? Prepare a solution of boiled 

 starch. Pour some into each of two test tubes. Add saliva to one test tube 

 and let it stand in a warm place for half an hour or more. Now test part 

 of the solution in each of the test tubes for sugar. (Use Benedict's or Feh- 

 Ung's solution.) Test another part for starch. Before you draw a conclu- 

 sion make sure that you have eliminated every other possible conclusion. 

 What else must you do? 



