PHYSIOLOGY OF MILK SECRETION. 205 



cent, in quantity. An attempt has been made to explain this by attributing 

 it to the fall in temperature during the night, requiring some of the fat to 

 keep up the body heat and to the lessened activity of the animal. Fleisch- 

 mann and Vieth experimented upon a German herd of 119 cows, and found 

 that the fat in the evening milk not only varied within wider limits than 

 in the morning's milk, but also observed that from March until July, the 

 period of greatest activity of the gland, that the morning milk was "richer 

 in fats than the evening milk. The difference in the quantity of milk 

 drawn morning and evening is due in part to the greater length of time 

 allowed to elapse between the evening and morning milking. This may 

 possibly also account for some of the differences in the percentage of fat. 

 In general the milk richest in fat is that drawn after the shortest period, 

 and this has been shown to be true in cases where cows have been milked 

 three or four times a day. After the third or fourth week of lactation the 

 percentage of fat in the milk remains nearly constant until the seventh 

 or eighth month, or until the quantity of milk begins to rapidly diminish. 



The daily variations in the milk are sometimes considerable. Such 

 variations may be ascribed to changes in the weather, temperature, food, 

 surroundings, indisposition, etc. 



The monthly variations in the quantity and quality of the milk are less 

 marked, than the daily variations. Cows coming fresh in the spring rapidly 

 better the quality of their milk, beginning about five months after calving, 

 but cows coming in in the fall maintain a fairly even quality throughout their 

 entire period of lactation. The quantity of milk is augmented when cattle 

 are first turned out to pasture and during drouth, but this must be ascribed 

 to food conditions, and not to seasonal variations. The richest milk is 

 produced after the seventh month. 



The yearly changes in quantity are slight. The increase in the second 

 and third producing years is marked, but after that it is rarely more than 

 3 per cent. The changes are so dependent upon feeding that no conclusions 

 can be drawn upon this point. 



The percentage of the fat in milk is affected by the age of the cow. The 

 young cow produces a milk poorer in fat than during vigorous middle age. 

 The fat may fall to a very low per cent in old age. 



There is considerable difference in the percentage of fat in the milk 

 taken at the different parts of the milking. Schmidt made complete analysers 

 of first drawn milk and last drawn milk, and found that this difference 

 was almost wholly due to the fat, there being eight times more fat in the end 

 milk than in the fore milk. We have made many tests of the fat and found 

 the per cent, to be about five times as great in the end as in the fore milk. 

 The explanation offered for this is that the fat at first lodges or adheres 

 to the lactiferous ducts, and that in reality a separation of cream begins 

 in the udder, and this fat would as far as circumstances permitted, seek 

 to float on the denser fluid in the cisterns and teats. The udder of a cow 

 killed immediately after milking showed on examination, that the ducts 



