THE USE OF FOOD 377 



it may be absorbed into the blood. Then the heart must 

 pump the blood and the blood vessels carry the food to the 

 cells. In the cells, as the food is used, waste products 

 are formed which must be removed and the blood must 

 carry these away. A third system of organs brings the 

 oxygen from outside the body so that it may get into 

 the blood and then be carried to the cells. These are the 

 organs of respiration, or the breathing organs. 



These processes are all different, but each is tied up 

 with the other. Food must be digested, then transported 

 to the cells. Then oxygen must be used to burn the food 

 before energy can be released and work done. In addi- 

 tion, some of the food may be used to build up the cells, 

 and finally materials not usable must be removed a 

 complicated process, but if we take one thing at a time, 

 we may be able to understand what happens. 



Demonstration 3. What Do We Mean by Digestion? 



Place a piece of soda cracker -in water in a test tube. Add a 

 few drops of iodine. What happens? If the mixture turns blue- 

 black, it shows the presence of starch. 



Take another piece of the same cracker treated in the same 

 way, but place in the test tube an equal amount of a blue sub- 

 stance called Fehling's solution. Heat to almost boiling. If the 

 mixture in the test tube turns brick red, this shows the presence 

 of grape sugar. What happens? 



Now chew a piece of soda cracker until it tastes sweet. Con- 

 tinue to chew it until it is thoroughly mixed with saliva. Then 

 place the chewed cracker with plenty of saliva in a test tube. Set 

 the tube in warm water for 20 minutes. Then test the contents 

 with Fehling's solution, treating as before. What happens? 



This is an example of a controlled experiment by which 

 we have proved that something in the saliva must have 

 caused the cracker to change from starch to grape sugar. 

 Starch will not dissolve in water, but sugar will. If food 

 is to get into the cells, it must be in solution. These 

 experiments show the necessity for digestion, 



