THE TYPES 199 



of refreshening, although she is usually rebred so as to 

 come in annually. 



220. Feeding capacity. After being furnished with a 

 reason for becoming functionally active it is necessary 

 that the udder be supplied with the materials from 

 which to produce milk. This function devolves upon 

 the digestive system as a source of the nutrients and 

 the circulatory system as a means of conveyance to the 

 udder. 



221. Constitution. In view of the record performances 

 of some cows it is apparent that high production involves 

 the expenditure of an enormous amount of energy and 

 nerve force to sustain it. A race horse is hardly required to 

 have more stamina and constitution than a cow on test, 

 while resistance to disease is of even greater importance 

 in her case on account of the artificial conditions under 

 which she is kept producing. 



222. Udder. The udder has been referred to as the 

 essential machine of the milk factory, the cow. It 

 is not a mere reservoir in which the milk accumulates 

 and from which it is simply withdrawn, but the udder 

 is a gland with secretion as its function. While the glandu- 

 lar arrangement does provide a receptacle for the milk 

 as it is produced, secretion goes on most actively during 

 the milking process, and is even controlled, within limits, 

 by the cow at will. The active factors in milk production 

 are the gland tissue cells which intervene between the 

 terminal capillary ramifications of the blood vessels and 

 the ultimate divisions of the interior of the udder (Fig. 99) . 

 The water, salt and some free albumen pass directly from 

 the blood into the interior of the gland, but the greater part 

 of the albumen undergoes transformation in its passage 

 through the cell and reappears as casein. Fat and lactose 



