292 THE CHEMISTRY OF DAIRYING 



very small (cf. p. 177). Any increase in the yield during the 

 first three months or so is generally accompanied by a slight 

 reduction in the percentages both of fat and of non-fatty solids. 

 When the yield diminishes as the stage of lactation advances, 

 the percentages of both ingredients, but especially that of the 

 fat, gradually increase again. As a rule, the milk is richer 

 towards the end of the lactation than at the commencement. 



When the udder is full of milk it becomes distended, and 

 pressure is set up. This pressure retards the rate of secretion, 

 and also affects the quality of the milk. The rate of secretion 

 is fastest soon after the cow has been milked, and gradually 

 becomes slower as the udder fills. Consequently, the oftener 

 a cow is milked the larger is the total yield obtained. It is 

 said that the yield may be increased 20 per cent, by milking 

 three times a day instead of twice. 



The pressure set up by distension of the udder causes 

 increased resistance to the passage of the fat through the 

 gland, and milk secreted under pressure is poorer in quality. 

 Frequent milking tends to reduce the pressure in the udder, 

 and, therefore, produces richer milk as well as more of it. 



When cows are milked at equal intervals of, say, twelve 

 hours, no appreciable difference in the composition of the 

 morning and evening milk is observed. When the intervals 

 are unequal, the milk obtained after the longer interval is 

 larger in quantity, but inferior in quality, as compared with 

 that obtained after the shorter interval. This is probably due 

 to the same cause the increased pressure due to the accumu- 

 lation of milk in the gland. In town dairies the cows are 

 generally milked about 5 a.m. and 2 p.m., making two intervals 

 of nine and fifteen hours' duration respectively. Consequently, 

 the evening supply is nearly always the richer, but the yield is 

 smaller. 



The milk obtained in the course of any one milking is not 

 of uniform quality throughout. Each successive portion con- 

 tains a higher percentage of fat than the preceding one. The 

 " fore milk," i.e. the portion first drawn from each teat, contains 

 less fat than the bulk, and the " strippings," or last-drawn por- 

 tions, contain more. The magnitude of the differences due to 



