204 UNCONSCIOUS NERVOUS OPERATIONS 



ing animals, and they are therefore spoken of as " perfect 

 foods." 



288. Characteristics of a Healthful Diet. (1) A health- 

 ful diet must contain all five chemical classes of food 

 elements in due proportion. (2) It must be adapted to 

 the climate, to the age of the individual, and to his mode 

 of life, that is, to the kind and amount of work which 

 he does. (3) Not only must the different kinds and 

 amounts of necessary food appear in the diet, but they 

 must appear in digestible form. 



289. Classification of Foods. With reference to the use 

 made of them, foods are divided into three classes : (1) 

 tissue builders ; (2) force generators ; (3) force regu- 

 lators. The proteids constitute the first class, though 

 we are not certain but that they may be sometimes oxi- 

 dized, yielding force without having entered into the 

 composition of living cells. In the second class are placed 

 the oxidizable substances, the fats and carbohydrates. 

 They may also assist in forming tissue to some extent. 

 The third class includes the inorganic compounds, 

 water, salts, etc., and certain food accessories. 



290. Proteids are the most important substances which en- 

 ter into the animal organism, being absolutely essential to 

 the phenomena of life. Plants are the great manufacturers 

 of proteids ; no process has yet been discovered for making 

 them artificially. Proteids are complex chemical com- 

 pounds and are characteristically represented by the 

 casein of milk and the albumin of egg. All are com- 

 posed of carbon, hydrogen, nitrogen, and oxygen in 

 various proportions, with a trace of sulphur. Proteids 

 are the only group of foods which contain nitrogen, and 

 whose chief if not sole purpose is the building of tissue. 

 Animals can live without carbohydrates and fats, but not 



