CHAPTER III 



DIGESTION 



AN examination of all the various substances that ordinarily 

 serve mankind as food reveals the fact that they consist of 

 one or more simpler components known as foodstuffs. These 

 may be defined as materials absolutely necessary to the main- 

 tenance of the normal composition of the body; to the growth 

 of new tissue and to the repair of tissue waste; or as materials 

 yielding energy to the body. Futhermore, a food must exert 

 no injurious effect upon the organism. Viewed from this 

 point of view, foodstuffs fall into five or six groups water, 

 inorganic salts, proteins, albuminoids, carbohydrates, and 

 fats. Oxygen is also absolutely necessary to the maintenance 

 of life, but it is not ordinarily regarded as a foodstuff. In 

 addition to the abovs there are other substances ingested with 

 food whose value is an indirect one in that they make eating 

 more agreeable. These accessory articles of diet are .flavors, 

 condiments, and stimulants. 



Foodstuffs. 1. Water and inorganic salts are constantly 

 being lost from the body, so that they must be replaced in the 

 food. They serve in maintaining proper osmotic relations 

 between tissue elements, but furthermore, enter most inti- 

 mately into vital reactions. In the case of salts some of these 

 reactions are undoubtedly ionic effects. 



2. Proteins and Albuminoids. Proteins are absolutely essential 

 to the body, since they are the sole available source of nitrogen. 

 They serve for the repair of old tissue, for the formation of 

 new tissue and as a source of energy. They contain the ele- 

 ments carbon, hydrogen, nitrogen, oxygen, sulphur, and some- 

 times phosphorus and iron. Hundreds of atoms are contained 

 in a molecule; the formula for egg albumin, for instance, has 

 been given as C 204 H 322 N 52 O 66 S 2 . When proteins are broken down 



