Dependence upon Oirculation 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 
