Types and Market Classes of Live Stock 185 



composition of milk varies a great deal; there are differences between 

 breeds, differences between individuals, and differences in the milk 

 from the same cow at different times. As lactation advances, the 

 daily milk yield decreases, but the percentage of fat constantly increases 

 as does the percentage of all solids. The specific gravity of average 

 milk is 1.032 and its composition is as follows: 



(Water 87 per cent 



if Ash .7 per cent 



[Solids not I Casein 2.7 per cent 



fat 9.1 per cent 1 Albumin .7 per cent 

 Solids 13 per cent ] [Sugar 5.0 per cent 



[Fat 3.9 per cent 



The results of various American experiments indicate that the 

 average composition of the milk of the various breeds is as follows: 



Breeds Solids Fat 



Per cent Per cent 



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 in- 

 versely proportional to the fat content, so that the total quantity 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 richness 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 substances 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 con- 

 stituents 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 



