186 Types and Market Classes of Live Stock 



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 manufacture. Between milkings, 

 the udder is engaged in certain important preliminary steps which 

 make possible the rapid culmination of the act of secretion when the 

 stimulus afforded by agitation is given. (The theory that milk secre- 

 tion is largely the result of cell growth, division, and degeneration, is 

 no longer held by leading physiologists.) 



If an udder is cut open just before milking time, it is found that 

 the milk-cisterns and ducts are distended with milk containing a very 

 low per cent of butter-fat. The quantity of this milk is only a small 

 part of the total yield which the cow would give at a milking. Agita- 

 tion of the udder is necessary to complete the process of secretion. 

 Observation of a calf, lamb, or kitten when nursing shows that con- 

 siderable agitation is a prominent feature of nature's method of milking. 

 Under this stim.ulus the water of the blood rushes through the walls 

 of the alveoli and carries the butter-fat, which has been manufactured 

 by the epithelial cells, down into the milk-ducts and cisterns, and 

 finally through the teats. Other products of secretion, namely casein 

 and lactose (milk-sugar), are brought down also. The last milk drawn 

 from the udder is the richest in fat; in one experiment, four successive 

 samples taken during the course of milking tested 0.76, 2.60, 5.35, and 

 9.80 per cent of fat respectively. ^ A similar test conducted in 1921 

 by 15 members of the McGregor (Iowa) Cow Testing Association is 

 reported ^ to have given an average of 1.79 per cent of butter-fat in 

 the fore milk and 9.6 per cent in the strippings. These were averages 

 for the cows on the 15 farms. On one farm the strippings tested 14.5 

 per cent and on another farm 12.8 per cent. 



Experiments made at the Wisconsin Experiment Station in 

 manipulating the udders of cows, after the regular milkings, resulted 

 in bringing down considerable milk very rich in butter-fat. It was 

 estimated that if the udders of the million cows in Wisconsin at that 

 time were manipulated after the customary operation of milking, and 

 if butter-fat is worth only 20 cents per pound, the value of the extra 

 product would amount to $6 per head annually, or $6,000,000 for all 

 the cows in the state. 



iJohn W. Decker: Elements of Dairying, p. 38. 

 ^Chicago Daily Drovers Journal, Aug. 1, 1921. 



