8o Life and Health 



exactly to the amount of energy expended. This continual 

 loss of substance must be made good by the introduction 

 into the body of new material. This "new material" is 

 the food we eat and is introduced into our bodies by means 

 of the alimentary canal. 



The process known as digestion, by which the food thus 

 introduced is acted upon so that it can pass through the 

 delicate walls of the alimentary canal into the blood and 

 lymphatics, will be treated in Chapters VI and VII. 



Meantime, let us learn something of the nature and 

 composition of food, which is so essential to the welfare of 

 the bodily tissues. 



117. Classification of Foods. Foods may be conveniently 

 divided into four great classes, to which the name foodstuffs, 

 or alimentary principles, has been given. 



I. Proteids, or Nitrogenous Foods. 

 II. Starches and Sugars, or Carbohydrates. 



III. Fats and Oils. 



IV. Inorganic or Mineral Foods, Water, Salt. 



118. Proteids, or Nitrogenous Foods. The proteids, fre- 

 quently spoken of as nitrogenous foods, are rich in one 

 or more of the following organic substances : albumin, 

 casein, fibrin, gelatin, myosin, gluten, and legumin. 



The type of this class of foods is egg albumin, well known 

 as the white of an egg. The serum of the blood is very 

 rich in albumin, as is lean meat. The curd of milk con- 

 sists mainly of casein. Fibrin exists largely in blood and 

 flesh foods. One of the chief constituents of muscular 

 fiber is myosin. Gluten exists largely in the cereals, wheat, 

 barley, oats, and rye. The proteid principle of peas and 

 beans is legumin, a substance resembling casein. 



