162 TYPES AND MARKET CLASSES OF LIVE STOCK 



culmination of the act of secretion when the stimulus afforded 

 by agitation is given. (The theory that milk secretion 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 con- 

 taining 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. Agitation of the udder is necessary to com- 

 plete the process of secretion. Observation of a calf, lamb, or 

 kitten when nursing shows that considerable agitation is a promi- 

 nent feature of nature's method of milking. Under this stimulus 

 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. 



Experiments made at the Wisconsin Experiment Station 

 in manipulating the udders of cows, after the regular milkiirgs, 

 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 twenty 

 cents per pound, the value of the extra product would amount 

 to $6.00 per head annually, or $6,000,000 for all the cows in 

 the state. 



Colostrum. Because of more or less incompleteness of the 

 various processes of secretion at the start of the lactation period, 

 the first milk differs in composition from that afterwards se- 

 creted, and is called colostrum. It is thick and viscous, con- 

 tains less water than normal milk, five times as much albumin, 

 twice as much ash, slightly more fat, and about one-half the 

 usual percentage of sugar. It has a laxative effect on the calf, 

 useful in properly starting the work of digestion. In about one 

 week following birth, the yield of milk usually increases and 

 becomes normal in composition. 



Stimuli to secretion. The cow has no control over the 

 secretion of milk, it being involuntary. During the latter part 

 of pregnancy, the embryo calf makes very rapid growth, and 



