52 THE NORMAL COMPOSITION OF MILK 



All these results show that the chief variation in the com- 

 position of milk at various stages of milking is due to fat, and 

 that the relative proportions of the plasma constituents remain 

 comparatively constant. The proteid is the most variable 

 component of the plasma and this fact is reflected in the in- 

 creasing -rr ratio and the decreasing ash percentage. The 



- - ratio is entirely different to those previously stated and 

 i a t 



shows the entire lack of organic relation between these two 

 constituents. This points to the variation in the fat content 

 being due to mechanical causes and not to changes in meta- 

 bolism. This is also the view of Kirchner, 14 who considered 

 that the fat globules are mechanically retained in the fine 

 ducts of the udder and escape in the strippings. Eckles and 

 Shaw point out, in support of this, that the larger the pro- 

 duction of milk the greater the increase in fat as the milking 

 proceeds; which is explained by the hypothesis that, in the 

 heavier milking cows, the udder is more congested and the 

 openings of the ducts reduced by compression. The relative 

 size of the fat globules at various stages of milking also 

 supports this view. 



Colostrum. The name " colostrum " is applied to the udder 

 secretion before, and immediately after, parturition. A yellow 

 viscous secretion, not unlike that produced by pathological 

 conditions, is often formed, but this is replaced several days 

 before parturition by the colostrum proper. Colostrum is a 

 yellowish, sometimes reddish (due to the presence of blood), 

 slimy liquid with an acid reaction and which shows a tendency 

 to separate. Compared with ripe milk the quantity of proteids 

 in colostrum is very high and is due more to increases in albumin 

 nuclein, and globulin than to an excess of caseinogen. This 

 points to glandular inflammation as a result of physiological 

 irritation. Cholesterol, lecethin, creatinine, tyrosine, and urea 

 are also present. Dextrose is present in addition to lactose, 

 which is slightly diminished in quantity, and the ash is higher 



