THE SECRETION AND PROPERTIES OF MILK 1295 



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

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



CHEMICAL CONSTITUTION OF DIFFERENT PROTEINS 



animal. Thus in the calf the brain is only ^-fcr of the whole animal. In 

 cow's milk lecithin is present in the proportion of 14 per cent, of the total 

 protein. In the puppy the brain is ^V of the whole body and the proportion 

 of lecithin to protein in the milk is 2-11 per cent. In the infant the brain 

 forms -f of the body weight, while the lecithin is 3-05 per cent, of the protein 

 of human milk. 



We thus see that under normal conditions the young animal is supplied 

 through its natural food with all the foodstuffs in the proportions which it 

 requires for its normal nourishment and growth. It is impossible therefore 

 satisfactorily to replace the natural milk of an animal by that of another 

 species. In civilised communities it is becoming more and more the custom 

 to endeavour to feed the child with cow's milk, more or less modified, in the 

 vain endeavour to reproduce the properties of human milk. Among all 

 classes this involves the administering of a milk differing in its qualities 

 and in the relative proportions of its proteins, fats, carbohydrates, and 

 salts, from human milk. So-called ' humanised ' milk is only a rough imita- 

 tion of the natural mother's milk. Among the poorer classes this artificial 

 feeding means the replacement of a natural sterile food, throwing very little 



