TYPES AND MARKET CLASSES OF LIVE STOCK 161 



Breeds Solids Fat 



per ct. per ct. 



Jersey 14.70 5.35 



Guernsey 14.71 5.16 



Shorthorn 13.38 4.05 



Ayrshire 12.61 3.66 



Holstein-Friesian 11.85 3.42 



The quantity of milk given by the different breeds is almost 

 inversely proportional to the fat content, so that the total quan- 

 tity of solids and fat is nearly the same for all dairy breeds. The 

 highest average percentage of fat in a regular milking that has 

 been reported is 10.7, and the cow did not give milk of this rich- 

 ness regularly. In a few cases, tests showing 9 per cent, have 

 been made and it is only rarely that a cow averages 7 per cent. 



Secretion of milk. The udder is a true organ of secretion. 

 Milk is not merely strained from the blood, but contains sub- 

 stances not found in the blood, these being formed in the gland 

 itself. Surrounding the alveoli are capillaries, and through 

 the walls of the capillaries the fluids of the blood pass freely 

 into the cavity of the alveoli by osmosis. At the same time, 

 the epithelial cells lining the alveoli are the seat of secretory 

 activities which produce some of the most important constituents 

 of the milk. Thus milk is formed partly from the osmosis of 

 blood serum and white blood corpuscles directly into the cavity 

 of the alveolus, and partly by a chemical elaboration by the 

 epithelial cells. The water from the blood serum passes out of 

 the alveolus, carrying with it some of the mineral constituents 

 of the blood in solution, and a part of the albumin of the blood 

 serum. During its passage from the capillaries to the cavity 

 of the alveolus, by far the larger part of the albumin of the blood 

 is changed by the epithelial cells to the casein of milk. These 

 cells also secrete globules of fat, having an average diameter of 

 about 0.0001 of an inch. Small amounts of fat may also be 

 carried over directly by the blood and appear in the milk without 

 change. Only minute quantities of sugar are found in the blood, 

 hence milk-sugar is no doubt also elaborated by the secretory 

 cells of the udder. 



Milk secretion is not perfectly understood in all its details. 

 In some of its phases, milk secretion is undoubtedly a continuous 

 process, but the agitation of the udder at milking time seems 

 to be very essential in completing the process of milk manufac- 

 ture. Between milkings, the udder is engaged in certain im- 

 portant preliminary steps which make possible the rapid 



