186 CHILDHOOD OF ANIMALS 



life. In the young and growing body a much larger proportion 

 of protein is necessary, as the tissues themselves are growing and 

 have to be built up. Next comes the fat, which rises to the surface 

 in the form of cream, or can be separated as butter. In different 

 milk it varies in quantity from about one per cent, to ten per cent, 

 and is present in different chemical forms. Fats are used to a very 

 small extent in the actual building of the framework of the body. 

 They are burnt in the tissues to supply heat and energy. The fats 

 are suspended as little globules in the liquid of the milk and give 

 it a white appearance. All milk contains sugar in proportions 

 ranging from three to seven per cent. Milk sugar is chemically 

 different from cane sugar or grape sugar, and also differs in different 

 animals. The sugar of mares' milk, for instance, can be fermented; 

 producing alcohol, and this property is employed to make weak 

 fermented beverages such as the well-known kephir and kumiss of 

 the Caucasus and the Russian steppes ; the lactose or milk sugar 

 of cows' milk does not ferment in this way. The sugars, like the 

 fats, play little part in the actual composition of tissues, but serve 

 as fuel. Lastly, all milk contains a small amount of dissolved 

 mineral matter, the ash which is left when it is dried and burnt, 

 and this is practically the same as the ash when flesh is similarly 

 treated. 



Although in a general way the milk of all animals is similar, as 

 it has to build up and nourish tissues which are similar, there are 

 striking practical differences. Unfortunately we do not know very 

 much about the exact constitution and properties of milk except 

 in a small number of animals, but we do know enough to re- 

 cognise four distinct types adapted to four types of structure. 

 I can explain this best by giving a Table which sets out the main 

 facts, and then taking the four groups in turn. In the first column 

 I have given the kinds of animals to which the types belong. In 

 the second and third columns I have taken the whole cubical 

 capacity of the stomach and intestines at 100 and set down the 

 relative proportions of the stomach capacity and the intestinal 

 capacity. Next follows the nature of the curdling, the kind of 

 change which occurs when the milk is mixed with the digestive 

 fluids of the stomach, and in the last four columns the percentages 

 of the four chief substances of which milk is composed. The Table 

 must not be taken as an accurately worked-out scientific statement 

 of the case, because there is not sufficient knowledge for this, but 

 only as true in a general way. 



