THE REQUISITES OF THE DIET 345 



must serve. Milk is made by the cells of the mammary 

 gland. The antecedent materials are presented to 

 these cells by the blood through the medium of the 

 lymph. Yet the selective power of the gland tissue 

 is such that the milk is radically unlike the blood in 

 almost all its characters. 



This is most strikingly true of the salts. In blood 

 the leading saline compound is sodium chlorid. In 

 milk this salt is sparingly present and calcium phosphate 

 stands first. The lime has been derived from blood in 

 which it occurs in only a minute percentage. While 

 the ash of milk is utterly unlike that of blood or blood 

 plasma, it is remarkably like the ash obtained by cremat- 

 ing the young animal entire. The high percentage of 

 lime in milk answers to the need for developing a 

 skeleton. The gland is not compelled to deliver a 

 filtrate such as might be anticipated from a study of 

 the plasma, but is able to prepare a secretion prophetic 

 of a body which does not yet exist. One may well say of 

 milk that it "is the substance of things hoped for, the evi- 

 dence of things not seen." 



One discrepancy between the ash of milk and that 

 of the young animal has been reported: a deficiency in 

 iron. But it has been shown at the same time that 

 new-born animals have more iron for their weight than 

 they will have later. It is thus normal for them to 

 receive a food that is poor in this element for a while. 

 Iron, by the way, is one of the indispensable elements. 

 We cannot have hemoglobin without it and it is doubt- 

 less needed in other connections. It is best utilized when 

 it is taken in the form of complex organic combinations. 

 Among the foods which furnish it in appreciable amounts 

 are meats, yolk of eggs, apples, strawberries, asparagus, 

 and spinach. 



Sodium Chlorid. While food ordinarily contains a 

 great variety of mineral compounds there is one which 

 we deliberately add to our diet. This is common or 

 table salt (sodium chlorid). As much as 10 pounds 

 may be eaten in a year by a man of average tastes. 



