FOODS AND THE THEORY OF DIGESTION 133 



common salt and sugar. These require no digestive action 

 other than dissolving in water. 



3. Foods that are insoluble in water. These have first 

 to be changed into soluble substances, after which they 

 are dissolved. 



Summary. Materials called foods are introduced into 

 the body for rebuilding the tissues, supplying energy, and 

 aiding in its general work. Only a few classes of sub- 

 stances, viz., proteids, carbohydrates, fats, water, and some 

 mineral compounds have all the qualities of foods and are 

 suitable for introduction into the body. Substances known 

 as drugs, which may be used as medicines in disease, 

 should be avoided in health. Before foods can be passed 

 into the body proper, they must be converted into the 

 liquid form, or dissolved. In this process, known as diges- 

 tion, water is the solvent ; and certain chemical agents, 

 called enzymes, convert the insoluble nutrients into sub- 

 stances that are soluble in water. 



Exercises. i . How does oxidation at the cells make necessary the 

 introduction of new materials into the body ? ^ 



2. What different purposes are served by the foods ? 



3. What is a nutrient ? Name the important classes. 



4. What are food materials ? From what sources are they obtained ? 



5. Name the different kinds of proteids; the different kinds of car- 

 bohydrates. Why are proteids called nitrogenous foods and fats and 

 carbohydrates non-nitrogenous foods ? 



6. Show why life cannot be carried on without proteids ; without 

 water. 



7. What per cents of proteid, fat, and carbohydrate are found in 

 wheat flour, oatmeal, rice, butter, potatoes, round beef, eggs, and pea- 

 nuts ? 



8. State the objection to a meal consisting of beef, eggs, beans, 

 bread, and butter; to one consisting of potatoes, rice, bread, and butter. 

 Which is the more objectionable of these meals and why ? 



9. State the general plan of digestion. 



