232 THE HUMAN MECHANISM 



The same consideration applies in a larger way to the food 

 as a whole. Some foods, like meat, are chiefly protein; 

 others, like the cereals, have an excess of starch, while 

 others, like butter or olive oil, are chiefly fat. A diet com- 

 posed of several kinds of food, that is, a mixed diet, is more 

 likely to avoid an excess of any one nutrient than when 

 any one food unduly preponderates. 



19. Other indispensable constituents of the food. I. Inor- 

 ganic salts. In addition to the proteins, fats, and carbo- 

 hydrates, which together make up almost the whole (96 to 98 

 per cent or even more) of the food we eat, two other groups 

 of substances are required in much smaller quantities, but 

 they are none the less absolutely indispensable. The first 

 of these is the group of inorganic salts. In the body are 

 found chlorides, carbonates, and phosphates of sodium, potas- 

 sium, calcium, and magnesium. These occur both in the 

 living cells and in the blood and lymph, and they are con- 

 stantly being removed from the blood in the urine and per- 

 spiration. This loss must be made good by the food. Most 

 foods contain salts, and our usual food contains most of the 

 inorganic salt necessary for making good the loss. The table 

 salt used in cooking and to develop the flavor of food at 

 tablets for the greater part in excess of the actual needs of 

 the body, the excess being promptly excreted by the kidneys. 

 The addition of some salt, however, seems to be necessary. 

 The craving of herbivorous animals for salt in which their 

 food is deficient is well known, and in parts of India salt 

 famines have occurred during which the price of salt was 

 higher than that of gold. 



20. Other indispensable constituents of the food. II. " Vita- 

 mines. " Finally, it is known that certain other compounds, 

 found in small quantities in many foods, are necessary 

 for adequate nourishment. The exact chemical nature of 

 these substances is still a matter of investigation, but it is 

 known that they are neither protein, fat, carbohydrate, nor 



