CHAP. iv. THE CASEIN OF MILK. 283 



both arrested, and appear as a more or less coherent mass of 

 gelatinous matter. 



If to the liquid which passes through the porcelain filter a little 

 acid be added, a white granular substance is separated which is still 

 casein. Filtered through filter-paper and the clear liquid heated, another 

 precipitate (albumin) makes its appearance. This albumin is regarded, 

 however, by Duclaux, as simply a ph} r sical modification of casein. 

 Milk contains, in fact, of albuminoid matter properly so called, only 

 casein. But this casein exists really in three states, in a state of 

 perfect solution, capable of passing through a porcelain filter ; in a 

 state of mucous coagulation, uniformly diffused throughout the liquid ; 

 in a state of suspension from which when milk is left at rest it falls to 

 the bottom of the liquid. For the sake of simplicity, and also because 

 it is difficult to separately determine the quantities of the two last- 

 named forms, Duclaux gives to these two collectively the name of 

 solid casein, and he calls that which passes through the porcelain filter 

 dissolved casein. 



In normal samples of milk the proportion of solid casein is liable to 

 variation, but the percentage of dissolved casein is always approxi- 

 mately the same. The weather, the temperature, the addition of 

 water, the action of acids or alkalies employed in feeble proportions, 

 of salts, &c., affect the latter little or not at all. The same is true of 

 the action of rennet. It evades consequently all the operations to 

 which milk is submitted in order to obtain therefrom its nutrient 

 ingredients. In the manufacture of cheese, only the gelatinous casein 

 and the casein in suspension are utilised. The action of rennet is to 

 cause gelatinous casein to pass into the state of suspended casein, 

 that is, of curd easily separable from the serum or whey. 



Experiments made by Thierfelder l lead him to believe that milk- 

 sugar is produced from blood serum by the agency of a ferment which 

 he calls saccharogen, though he has not succeeded in isolating this 

 substance. Casein he regards as most probably formed from serum- 

 albumin by an analogous ferment likewise present in the mammary 

 gl uid. 



Hence, we learn that the mammary gland, by the direct metabolic 

 activity of its secreting cells, appears capable of forming, out of its proto- 

 plasm, typical representatives of the three great classes of food-stuffs, 

 (1) proteids, albuminoids, or nitrogenous organic compounds represented 

 by the casein and albumin, (2) carbohydrates represented by the milk- 

 sugar, (3) fats represented by the oil globules. In other words, the secre- 

 tion of milk may be regarded as " a process of moulting of the epithelial 

 cells, which undergo decomposition, and discharge the resulting pro- 

 ducts into the excretory ducts." But, in order to discharge this com- 

 plex function, the protoplasm must be nourished ; wonderfully capable 

 as it is, it yet would be powerless to do the work which living matter 

 alone can do were it not furnished with the material with which to 

 operate. And this material is abundantly, almost lavishly, supplied to 



1 " Biedcrmann's Ceutralblatt fur Agricultur-Chemie," 1884. 



