282 THE COMPLETE GRAZIER. BOOK n. 



view in the microscopical examination of normal milk are seen to vary 

 considerably in size, the largest ones being several times as big as an 

 individual secreting cell from the epithelial lining of the alveolus. 

 These large globules are produced by the fusion of the smaller ones 

 after their expulsion from the alveoli, and during their passage along 

 the lactiferous ducts. 



The milk-sugar, or lactose, of the milk is, like the fat, also a product 

 of the metabolic activit}^ of the protoplasm of the secreting cells of the 

 mammary gland. Of the exact mode whereby the protoplasm of the 

 epithelium cell elaborates milk-sugar from the constituents of the 

 blood, little or nothing is known, but that the formation of milk-sugar 

 is effected by these cells is proved by the fact that this particular form 

 of sugar occurs nowhere else in the animal body, though grape-sugar, 

 or dextrose, is a normal constituent of blood, chyle, and lymph. 

 Another proof is afforded by the circumstance, that although milk- 

 sugar is a typical carbohydrate, its occurrence in the milk is not 

 dependent on the presence of carbohyd rates in the food, for it is 

 maintained in abundance in the milk of carnivorous animals when 

 these are fed exclusively on meat, a nitrogenous food as free as 

 possible from any kind of sugar or other carbohydrate. Of all the 

 constituents of milk, the milk-sugar is least influenced by external 

 conditions. 



With regard to the casein of the milk, is this also a product of the 

 metabolic activity of the protoplasm of the secreting cell, that is, is it 

 manufactured in and by this protoplasm, or is it simply separated from 

 the blood ? Here, again, evidence points to the former as the correct 

 interpretation of the origin of the casein, for when the action of the 

 secreting cells is imperfect, as at the beginning and at the end of 

 lactation, the albumin which normally is less than one-seventh of the 

 casein is actually in excess of it, and albumin that particular variety 

 known as serum-albumin is a normal constituent both of blood and 

 of milk. But when the secreting cells are in full activity, the casein 

 comes prominently forward as the leading nitrogenous constituent of 

 milk. 



Certain physical peculiarities are associated with the presence of 

 casein in milk. As has been demonstrated by Duclaux, casein is found 

 in milk in two forms. It is partly in suspension, and on account of 

 its different specific gravity this sinks to the bottom of a vessel of 

 fresh milk left at rest. It is also, on the other hand, partly in a kind 

 of gelatinised condition, in which state it remains diffused through the 

 milk. These two forms of casein pass insensibly the one into the 

 other, and although it is possible by rigorous methods to distinguish 

 between them, yet there is no fundamental difference. That which is 

 described as being in a gelatinous semi-liquid condition is, indeed, 

 as much in suspension as is the other form, the proof of which is 

 that if milk is filtered, not through a paper filter the pores of 

 which are large enough to permit the passage even of granules of 

 butter-fat, but across unglazed porcelain, the two forms of casein are 



