PROBLEM 2. The Digestive Syste?n Makes Food Usable 



189 



Fig. 201 Mail's digestive sys- 

 te7n. The organs are not 

 drawn in correct proportion 

 or in exact position. Food 

 masses are indicated in the 

 stoviacb and at the lower 

 end of the small intestijie. 

 How long does it take food 

 to travel the full length of 

 the alimentary canal? What 

 digestive organs are show?i 

 that are not part of the 

 canal? 



Mouth 



Windpipe 

 (frac/ieaj 



Liver — 

 fporf/y lifted) 



Gall bladd 



Large 

 intestine 



Food mass 

 Appendix 



branes. But the more complex sugars, 

 the starches, the proteins, and the fats do 

 not diffuse through cell membranes. Make 

 an artificial cell and see for yourself 

 whether starch, for example, is able to 

 enter it. See Exercise i. Those food com- 

 pounds that do not diffuse through a cell 

 are first digested in the tube and then 

 pass into the blood. 



What is digestion? You have just read 

 that starches, proteins, fats, and some of 

 the more complex sugars are changed in 

 the digestive tube. Without this change 

 they could not get into the blood nor 

 could they later be used by the body 



Salivary glands 



Food pipe 

 foesopfjogus) 



Stomach 

 Food mass 



Pancreas 



Small 

 intestine 



Rectum 



cells for oxidation and assimilation. The 

 chemists would say that the molecules 

 of protein, starch, fat, and some of the 

 sugars are large. These large molecules 

 are broken up into smaller molecules 

 making new substances. The process by 

 which the large molecules are changed 

 into smaller molecules of other substances 

 is called digestion. 



Digestion is a chemical change; that is, 

 it changes the nature of the substances 

 that are digested. If a piece of bread is 

 broken into crumbs the change is a phys- 

 ical change. The crumbs are still bread. 

 No matter how tiny the crumbs may be 



