i EXCHANGE OF MATERIAL 41 



With the ordinary nutritive substances, a considerable quantity 

 of mineral compounds are introduced into the organism. They 

 are obviously necessary during the anaplaslic period for the 

 formation of the bones and tissues. Since the child at the breast 

 grows and lives solely on his mother's milk, it is clear that this 

 food must contain everything which is necessary for the develop- 

 ment of the child. 



The ashes of milk are, according to Bunge, almost identical in 

 quality and quantity with these which can be extracted from the 

 organism of the sucking child. The former contain rather more 

 potassium and less sodium than the latter. This is due to the 

 development of the muscles, which contain more potassium, and 

 to the gradual ossification of the cartilages, which contain more 

 sodium. Chlorine is also found in -larger quantities in the ashes of 

 the milk than in those of the infant, possibly because chlorides are 

 used in the formation of the acid of the gastric juice, and are not all 

 absorbed when they pass into the intestines, and of those absorbed 

 a part is discharged by the kidneys. 



When the child is weaned, does it find in other articles of food, 

 as it did in milk, the salts which are necessary for its growth ? 

 The composition of the ashes of the principal articles of food 

 shows that they contain in an even greater proportion all the 

 mineral elements found in milk, with the exception of calcium, of 

 which there is a small quantity as compared with that found in 

 milk. Yolk of egg is the only food which contains a larger 

 quantity of calcium than milk ; it is therefore the best substitute 

 for milk which can be given to children. 



Cow's milk contains more salts than human milk ; this is 

 accounted for on teleological grounds by the fact that the calf grows 

 more rapidly than the child. From this point of view the adult 

 organism, once the anaplastic period is over, should not need to 

 take in mineral substances which, being saturated compounds 

 and possessing no potential energy, are not consumed during the 

 metabolism of the tissues. We must, however, bear in mind that 

 mineral substances are chemically combined with organic sub- 

 stances, and when these latter are decomposed, the salts remain 

 free, and since they are not or only in part employed in the 

 formation of new organic complex molecules, they diffuse in the 

 fluids and are discharged in the excretions. They are indeed 

 eliminated in large quantities even when the body is in an 

 absolutely fasting condition. This explains why it is necessary 

 for adults to take constantly fresh mineral substances with their 

 food. 



Forster in 1873 fed two dogs on the residue of meat from 

 which Liebig's extract had been made ; these residues, washed in 

 distilled water and dried, contain only 0'8 per cent of salts. To these 

 proteins containing very little salt he added fat, sugar, and starch, 



