THE SECRETION AND PROPERTIES OF MILK 1241 



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. 



The fitness of caseinogen for building up the tissues of the body is evident 

 when we compare as in the Table on page 1242 the products of its hydrolysis 

 with those of all the proteins in other food-stuffs. It will be seen that 

 practically every amino-acid and allied substance employed in the building 

 up of the various proteins is represented in caseinogen. The only exception 

 is glycine, which can be easily formed from other amino-acids. 



In another point we find an adaptation of the milk to the growth of the 

 young animal, and that is in its lecithin content. Lecithin is probably 

 employed to the largest extent in the building up of the central nervous 

 system, where it forms the most important constituent of the medullary 

 sheaths of the nerve fibres . There is a corresponding proportionality between 

 the lecithin content of milk and the relative brain weight of the young 



