THE NOURISHMENT OP THE BODY 251 



would have to be eaten. As a matter of fact, we commonly 

 use a combination of foods in our diet, if allowed a choice, 

 which includes representatives of the three classes of foods. 

 Combinations of this kind are bread and butter, rice and milk, 

 pork and beans, potatoes and meat, etc. The ratio of proteins 

 to carbohydrates in the ordinary food should be about 1 to 

 6. Fats, which contain more carbon in proportion to the oxy- 

 gen than do carbohydrates, are useful sources of heat, and in 

 cold countries, and in the colder months of our year, have an 

 increased representation in our food. Certain articles of food 

 are of much greater value in sustaining life than others. 

 Among those of greatest value are corn, beans, peas, potatoes, 

 and the various grains. Nuts have a high food value because 

 of the oil and proteins they contain. Cucumbers, lettuce, 

 spinach, and radishes contain very little food material but are 

 valuable for the mineral salts which they contain and for giving 

 bulk to the food. Most fruits are of little value as nourish- 

 ment, though the acids, salts, and flavors found in them make 

 them most desirable additions to our food supply. The value 

 of a food is usually measured by the number of kilocalories it 

 contains, a kilocalorie being defined as a thousand ordinary 

 calories. In books on food, this unit is spoken of simply as 

 calorie, but the kilocalorie is always meant. Those who work 

 hard with their muscles require enough food to give about 

 5000 kilocalories daily. Students and those whose occupa- 

 tions do not require vigorous exercise need much less. High 

 school students need about 2500 kilocalories daily. 



216. The Digestive System. In the one-celled animals, 

 each cell, of course, secures and digests its own food, but in 

 the more complex animals which often contain many billions 

 of cells, the majority of which cannot come unto direct con- 

 tact with the food, means must be provided for securing and 

 transporting this food to them. Digestion is therefore carried 

 on by certain cells for the entire body and the material 



