372 LECTURE XVI. 



isodynamic only as regards their combustion value. The law of isody- 

 namics would undoubtedly hold in the case of sucklings in this direction 

 were it not for the fact that its body substance is increased so largely 

 that it is necessary to arrange its diet according to particular lines. We 

 must, to be sure, admit that by the transformation of carbohydrates to 

 fat, and perhaps also by the reverse process, one of these nutrients may 

 replace another, even in the construction of the cells. 



The assumption that rickets is caused by a lack of lime in the nourish- 

 ment is contrary to numerous observations. Rickets appears sometimes 

 with a food that is rich in lime, even when the child is being fed upon 

 mother's milk, though much less frequently than with other food. It 

 might be thought that there still may be a deficiency in lime, and perhaps 

 on account of the fact that, owing to a disturbed process of absorption, an 

 insufficient supply of lime becomes available to the tissues. This brings 

 us to the question as to the state of the lime when it is absorbed and 

 assimilated. 



We meet here with one of the most remarkable circumstances in 

 the entire subject of physiological nutrition. Whereas our knowledge 

 concerning the occurrence of organic nutrients is considerable, we know 

 very little concerning the way in which the inorganic substances are com- 

 bined in the food. We do not know whether they are present as inorganic 

 salts, in the animal and vegetable tissue, or whether complicated 

 organic compounds are at hand which contain these inorganic elements 

 in a state of more or less firm combination. It is conceivable that lime, for 

 example, can take part in the construction of tissue only when it is present 

 in a definite state of combination. Such an assumption was especially 

 justifiable at the time when it was not recognized that it was possible for 

 the human organism to accomplish syntheses. Now that we have seen, 

 however, that the animal cells are capable of accomplishing most com- 

 plicated syntheses, it becomes more and more probable that they are also 

 able to make use of inorganic salts in the formation of their tissue. 



Although we know very little concerning the way in which lime is con- 

 tained in the ordinary foods, still, on the other hand, it is to be expected 

 that an explanation of the way lime is present in milk will throw the most 

 light upon this question. There are a number of possibilities to consider. 

 It may be that the lime is in some way combined with the protein in milk, 

 or that it may be dissolved in it in the form of an inorganic salt. At all 

 events, the fact that the lime is not present in any firm state of combination 

 is proved by the following experiment performed by Bunge. 1 On diluting 

 cow's milk with water and precipitating the casein by careful addition of 

 acetic acid in the cold, only a trace of lime is found in the albuminous 



Z. Biol. 45, 532 (1901). 



