1388 



PHYSIOLOGY 



yeast test is the best means of distinguishing lactose from dextrose 

 in the urine. It gives the ordinary tests for reducing sugar. The salts 

 of milk include insoluble salts, soluble calcium salts, sodium and 

 potassium, phosphates and chlorides. 



Mere enumeration of the constituents of milk presents but little 

 interest unless we realise how closely the composition of this fluid is 

 adapted to the needs of the growing animal. In the first place, we 

 find a proportionality between the total solids of the milk and the 

 rate at which the young animal grows. It must be remembered that 

 the milk taken by the animal serves only in part for the production 

 of energy in its body, a great proportion of it being required for the 

 building up of new tissue. In no respect is this correspondence seen 

 better than in the comparative analyses of the ash of milk and 

 of the young animal of the same species which were made by 

 Bunge. The following Table shows the composition of the ash of a 

 rabbit fourteen days old, of the milk which it was receiving from its 

 mother, of the ash of rabbit's blood and blood-serum. Nothing could 

 be more striking than the marvellous way in which the cells of the 

 mammary gland have picked out from the salts of the circulating 

 plasma exactly those salts which are needed for the growing animal 

 and in the same proportion : 



This close correspondence is only necessary where growth is very 

 rapid, so that the greater part of the constituents of the milk have to 

 be utilised in the building up of the animal tissues. As Bunge has shown, 

 the slower the growth of the animal the greater the divergence between 

 the composition of the milk and that of the new-born animal. We 

 may compare, for instance, the rabbit, which doubles its weight in six 

 days, with the dog, which doubles its weight in ninety-six days, and 

 the human infant, which takes one hundred and eighty days to double 

 its weight at birth. 



The last column of the following Table represents the composition of 

 the ash of cow's milk, and shows how very inefficiently this milk can be 

 regarded as replacing human milk, the natural food of the infant. 



