XV. MILK AND MILK PRODUCTS 331 



The great fluctuations in the fat content of the milk of a cow, 

 kept under as constant conditions as it is possible to secure, were 

 called attention to by the writer in 1901 1 and the results, obtained 

 from the examination of the morning and evening milk from each of 

 17 cows, have figured largely in many prosecutions for alleged milk 

 adulteration. 



This investigation showed, in a most emphatic manner, that 

 morning's milk was much poorer in fat than the evening milk from 

 the same cow, when the intervals between the milkings were unequal, 

 but that the percentage of fat in either the morning or evening milk 

 was liable to enormous variation from day to day, even when the con- 

 ditions were, so far as they could be controlled, unchanged. 



When the mixed milk of many cows is analysed, these fluctua- 

 tions are often, to a great extent, masked, since they rarely occur in 

 the same direction, simultaneously in many animals. 



The results have been entirely corroborated, both by further in- 

 vestigations by the writer and by others. The fluctuations are greatest 

 in cows yielding much milk ; with advancing lactation they generally 

 tend to become less marked. 



These irregularities are shown by the percentage of fat only, the 

 other constituents exhibiting little variation. The yield of milk varies, 

 somewhat, but there is little correlation between yields at, say, suc- 

 cessive morning milkings and percentages of fat in the morning milks. 



To show the character of the variations, the results given by the 

 analyses of the morning and evening milks of two cows out of the 

 nineteen studied, in 1901, for forty consecutive days, are represented 

 graphically in Figs. 12 and 13, the cows whose milks are represented 

 being in the 134th day (Fig. 12) and the 70th day (Fig. 13) of lactation 

 at the time the experiment began. The upper lines in the diagrams re- 

 present the percentage amounts of solids-not-fat in the morning (dotted 

 line) and evening (continuous line) milks each day ; the lower con- 

 tinuous line gives the percentage of fat in the evening, the lower dotted 

 line that in the morning milk, while the vertical columns represent 

 by their height (1 per cent corresponding to 10 Ib. milk) the weight of 

 milk yielded at each morning (shaded column) and evening (black 

 column) milking. 



As the diagrams show, variations of 1 per cent of fat in the 

 milk from successive morning or evening milkings are not infrequent, 

 so that when the milk of individual cows is concerned, even the " ap- 

 peal to the cow," so often regarded as reliable, in actions for alleged 

 tampering with milk, may furnish misleading evidence. In the mixed 

 milk of many cows, the fluctuations are much less marked, but even 

 in this case, great differences in fat content are shown between morn- 

 ing and evening milk, if the intervals be very unequal. 



As to the cause of these great variations in the proportions of fat 

 in the milk of a cow from day to day, little is really known. Since the 

 casein, albumin, milk sugar and ash do not share in the irregularities, 

 it seems obvious that variations in the activity of the fat-producing 

 organs, i.e., the mammary gland itself, must be the cause. But as to 



1 Trans. High, and Agrie. Soc. Scotland, 1901, 218. 



