64 Cattle Problems. 



But numbers of cows do not retain control of their mam- 

 mary artery blood, or retain the contractile power which 

 is natural in these arteries, and this is the reason why many 

 cows continue their yield, during a considerable period, 

 while pregnant, some of them to near the full term of 

 pregnancy, and others, as farrow cows, during two, and in 

 some cases, three seasons in succession. 



When cows hold up their milk by contracting the chan- 

 nels of their teats, this is an evidence of strong instinctive 

 desire to nourish their young, and that the contractile 

 power here exerted is strong. Naturally, contractility is re- 

 quired in the arteries; but in many cows contractile power 

 in the arteries is much reduced by relaxation from engorg- 

 ing, degrees of increase in blood. In such cases, the udder 

 skin is more or less thinned by increase of milk, and the bag 

 becomes flimsy and uncontracting, which, though a favor- 

 ite indication of yield, with some, is a certain evidence of 

 weakened contractility in the udder skin, and correspond- 

 ingly weakened contractile power in the udder supply ar- 

 teries, or reduced power in the cows to dry up their yield. 

 So instinct naturally controls duration of yield, which is 

 reduced at about the same rate at which embryo growth 

 increases. 



The supply of blood to the milk glands is naturally re- 

 duced in advance of the actual demand by the embryo, 

 because this is a necessity of breeding. But the length 

 of time in which cows dry up their yield in advance dif- 

 fers with the strength of instinct, or contractile power un- 

 der nerve control, the cows having the strongest con- 

 tractility in their arteries and milk glands, drying as a rule, 

 earlier than others of equal yield, that have weaker con- 

 tractile power in the udder supply arteries, and also in the 

 udder, in some cows. 



It is probable that about half the best cows in the coun- 

 try — and particularly in Cheese Factory Districts — yield- 



