MILK. 287 



\Yhen the water of milk is found as high as 91.5 per cent 

 of the whole the sum of the fat, protein, sugar and salts can 

 be only 8.5 per cent in place of 12.5 per cent, which should be 

 expected in the mixed market milk. 



Market Milk. Experience has shown that the mixed milk 

 from a herd of well-kept cows should have a composition not 

 far from that given in the table above under " mean." Laws 

 have been passed in most of the large cities of the United 

 States and Europe requiring that milk sold as pure must be 

 of a quality not inferior to this mean value. Indeed, in some 

 places a market milk of still higher standard is required. 



PHYSICAL COMPOSITION OF MILK. 



The exact nature of the mixture of the component parts of 

 milk has long been a debated question. When taken from 

 the udder the fats, proteins, sugar and salts are mixed homo- 

 geneously, and no immediate tendency is observed toward a 

 separation of the light fat from the other and heavier solids. 

 In time, however, such a separation takes place and the fat 

 rises in the form of cream. Milk cannot be looked upon as 

 a transudation from the blood because it contains substances 

 not found in that fluid; the casein of milk and the lactose are 

 different from the proteins and sugar normally existent in 

 the blood, and the fat of milk is more complex probably than 

 the blood fat. It is necessary to admit, then, that some of 

 the milk components are produced in or from the substance of 

 the mammary glands themselves. It is held by some authori- 

 ties that the nucleo-proteids of the gland cells are similar to 

 or identical with the casein, which therefore has its origin in 

 the gradual breaking down of those cells. In regard to the 

 milk fat it is known that certain fats can pass but little changed 

 from the food through the blood and appear finally in the 

 milk, imparting peculiar properties. But, on the other hand, 

 milk fat is produced when the food of the animal contains no 

 fat whatever, and certainly no fats resembling the characteris- 

 tic volatile fats of the milk. In the carnivora, confined to an 



