BIOLOGICAL CLASSIFICATION OF FOODS 47 



In a mixed diet, there is usually, but not always, a sufficiency 

 of the compounds of the different elements mentioned. We 

 quote Sherman upon this point: " There must also be main- 

 tained in the body a proper balance between sodium and calcium 

 (the metal of lime). For example, the rhythmical contraction 

 and relaxation of heart muscle, which constitutes the normal 

 beating of the heart, is dependent upon this muscle being bathed 

 by a fluid containing the proper concentration and quantitative 

 proportions of sodium and calcium. Calcium is not always suf- 

 ficiently abundant even when the food is freely chosen; hence the 

 richness of a food in calcium is a factor affecting its value." 1 

 McCollum and Simmonds found as a result of their experiments, 

 " that the deficiency in mineral elements in wheat and other seeds 

 is limited to three elements, calcium, sodium and chlorine." 2 



The ash of milk is present in liberal quantity, is of high qual- 

 ity and well balanced, and is rich in its lime content as a source 

 of calcium. There is more lime in a pint of milk than in a pint of 

 limewater. 



Two Unidentified but Essential Food Substances One of These 

 in Milk-fat but not in Ordinary Fats 



There are, in association with some of the foodstuffs, sub- 

 stances which have not as yet been identified chemically, pos- 

 sibly on account of the minute quantities in which they are 

 present; and yet observation, in feeding experiments, has shown 

 that they are indispensable. If these are absent from their 

 foods, animals will neither grow nor retain vigor. 



As early as 1906, Hopkins of Cambridge (England) showed 

 conclusively that on an apparently complete food made up of 

 purified proteins, ordinary fats, carbohydrates and salts young 

 rats would not grow, but that when a very small amount of milk 

 was used enough to make up about 4 per cent of the dry matter 

 of the food growth became entirely satisfactory. This led him 

 to conclude that there were present in milk unidentified food 

 substances which he termed " accessory " articles of the diet. 



1 Food Products, pp. 19 and 20. 



2 The Newer Knowledge of Nutrition, p. 23. 



