Dependence upon Circulation of Blood 11 



cussion of the agencies by means of which this 

 most important result has been brought about 

 would open up the whole question of the selection, 

 breeding and training of cows, as well as every- 

 thing pertaining to the science of foods and feed- 

 ing, which is not here possible. There are, how- 

 ever, some physiological conditions affecting the 

 secretion of milk that may be mentioned. 



Milk is secreted from the blood. The amount 

 of milk secreted will, therefore, depend upon the 

 amount of blood passing through the udder, and 

 this, in turn, will depend upon the number and 

 size of the blood vessels, not only in the udder it- 

 self, but leading to it and away from it, the vigor 

 of the circulation, the supply of food to the ani- 

 mal, and her capacity to eat, digest . and assimilate 

 it and turn it into blood. From or shortly after 

 parturition, there is a constant tendency of the 

 blood vessels in the udder to shrink in size, and 

 consequently a constantly diminishing flow of milk. 

 When the period of lactation advances at the same 

 time that the pastures are growing more scanty 

 and less succulent, this diminution is fairly regular 

 and constant, especially after from three to five 

 months of the period of lactation have passed. This 

 tendency to shrinkage in the size of the blood 

 vessels of the udder may be held in check in great 

 measure by an abundant supply of nutritious food, 

 particularly if it is of a succulent character, and 

 it is not at all uncommon to find cows secreting 

 milk in regular amount, or "holding out," for eight 



