Formation of Milk. 39 



as strong as that naturally extended to a cow's own progeny. 



The size of the udder, or its capacity to contain milk, 

 is, like that of the hind quarters, increased by the increase 

 of its contents. The more food the cow digests the more 

 blood she makes, and the larger the supply of blood flow- 

 ing into the mammary arteries, and thence into the milk 

 glands, to supply the ingredients of milk. By gradual or 

 rapid increase of food, and of blood formed from it, the 

 formation of milk and the size of the milk glands sever- 

 ally, and of the udder as a whole, have been gradually 

 increased, as in Dutch or Netherland cows, or rapidly ex- 

 panded, as in cows supplying American cheese factories ; 

 see Plate III., Figs. 2 and 4, and in the width of the 

 back udder, and size of the Yield Mark; see Fig. i, same 

 plate. 



The formation of milk is a hidden vital process, so it 

 cannot be precisely described, nor is this now neces- 

 sary, as we perceive, by its results, as calf food, that milk 

 contains the same kinds of nourishment that builds up the 

 system of the cow, as in fact it must do, for milk and its 

 qualities are derived from the cow's food, and vary as 

 the quality of the cow's food varies. 



It is evident, from the white color of milk, that the 

 red corpuscles, or colored portion of the blood, does not 

 enter the milk glands. The reason of this is that the oxy- 

 gen of the blood passes into the cow's solid tissues, being 

 withdrawn from the artery circulation for the purpose of 

 structural change in the system of the cow herself, leaving 

 but little of the oxygen of the blood in the udder supply 

 arteries. This is the more probable because the calf colors 

 its own blood, and supplies it with oxygen by its own 

 breathing. Hence, there is apparently no occasion for 

 oxygen of the blood in forming milk. The ingredients 

 from which the milk is formed are, however, derived from 

 nutritive elements of the artery blood, which are not taken 



