40 INORGANIC FOODSTUFFS 



of vegetable origin which are most remote in their composition from the 

 proteins of animal tissue. In such a diet a large proportion of the nitro- 

 gen is wasted because, as we shall see in a subsequent chapter, the 

 amino-acids into which the protein splits up on digestion are present 

 in the wrong proportion and have to be resorted and selected in very 

 different proportions in order to build up proteins of the animal type. 

 It has been found that an animal subsisting on a diet of this kind suffers 

 not only a large wastage of nitrogen, necessitating the consumption 

 of a large quantity of food to maintain nitrogenous equilibrium, but 

 also a large wastage of mineral constituents, so that it cannot be 

 maintained in health or nutritive equilibrium without the addition 

 to the diet of a considerable excess of mineral substances over the 

 amount which would be required by an animal subsisting on a more 

 varied diet. 



CALCIUM. 



During the early months of the growth of a suckling infant or animal, 

 lime is very rapidly being absorbed and utilized by the tissues for the 

 formation of bones. This calcium is totally derived from milk. Now 

 the lime in milk is present therein in two forms, namely, in the form 

 of calcium phosphate and in the form of a bulky, indiffusible compound 

 with one of the proteins of milk, casein. The calcium phosphate is, of 

 course ionized, but the calcium caseinate, on the contrary, does not 

 yield calcium ions in solution. 



When we add acids to milk, or when owing to the action of bacteria 

 upon the milk-sugar which it contains lactic acid is produced in the 

 milk, it assumes the curdled appearance which we are accustomed 

 to associate with "sour milk." This appearance is due to the separa- 

 tion of free Casein, uncombined with calcium, which has been abstracted 

 from the calcium caseinate by the acid. Free casein is insoluble 

 in water or very dilute acids and hence is precipitated in curds or 

 flocculi, while the calcium is now present in the "sour" milk in the form 

 of the calcuim salt of the acid which has been added. 



Precisely the same thing happens when milk which has been ingested 

 by the suckling comes into contact with the hydrochloric acid which 

 is contained in the gastric juice. Free casein ; more or less modified by 

 partial digestion, is precipitated and calcium is set free as calcium 

 chloride. 



There has been a good deal of discussion in the past as to whether the 

 two forms of lime in milk are equally readily utilized by the suckling. 

 In view of the above-mentioned facts there would appear to be no very 

 good reason for distinguishing between them, since in the stomach, 

 where absorption begins, both forms of calcium are reduced to a 

 common level by the conversion of the calcium caseinate into the 

 ionized and diffusible chloride. 



Notwithstanding this fact it has been frequently argued that the 



