374 SECRETION. 



The proportion of water in milk is subject to a certain amount of variation, but this 

 is not so considerable as might be expected from the great variations in the entire quan- 

 tity of the secretion. In treating of the quantity of milk in the twenty-four hours, we 

 have seen that the influence of drinks, even when nothing but pure water has been taken, 

 is very marked ; and, although the activity of the secretion is much increased by fluid 

 ingesta, the quality of the milk is not usually affected, and the proportion of water to the 

 solid matters remains about the same. 



Nitrogenized Constituents of Milk. Very little remains to be said concerning the 

 nitrogenized constituents of human milk, after what has been stated under the head of 

 alimentation. The different principles of this class undoubtedly have the same nutritive 

 function and they appear to be identical in all varieties of milk, the only difference being 

 in their relative proportion. It is a matter of common experience, indeed, that the milk 

 of many of the lower animals will take the place of human milk, when prepared so as to 

 make the proportions of its different constituents approximate the composition of the nat- 

 ural food of the child. A comparison of the composition of human milk and cow's milk 

 shows that the former is poorer in nitrogenized matters and richer in butter and sugar ; 

 and consequently, the upper strata of cow's milk, appropriately sweetened and diluted 

 with water, very nearly represent the ordinary breast-milk. 



Caseine is by far the most important of the nitrogenized principles of milk, and it sup- 

 plies nearly all of this kind of nutritive matter demanded by the child. Lacto-proteine, 

 a principle described by Millon and Commaille, is not so well defined, and albumen 

 exists in the milk in very small quantity. That albumen always exists in milk, can readily 

 be shown by the following process described by Bernard : If milk, treated with an 

 excess of sulphate of magnesia so as to form a thin paste, be thrown upon a filter, the case- 

 ine and fatty matters will be retained, and the clear liquid that passes through shows a 

 marked opacity upon the application of heat or the addition of nitric acid. 



The coagulation of milk depends upon the reduction of caseine from a liquid to a 

 semisolid condition. When milk is allowed to coagulate spontaneously, or sour, the 

 change is effected by the action of the lactic acid which results from a transformation of a 

 portion of the sugar of milk. Caseine, in fact, is coagulated by any of the acids, even 

 the feeble acids of organic origin. It differs from albumen in this regard and in the fact 

 that it is not coagulated by heat. It has been suggested that, in fresh milk, the caseine 

 exists in combination with carbonate of soda, and that coagulation always takes place 

 from the action of acids upon this salt, by which the caseine is set free. It is true that 

 coagulated caseine may be readily dissolved in a solution of carbonate of soda, but it has 

 been shown that coagulation may be induced by the agency of certain neutral principles, 

 while the milk retains its alkaline reaction. If fresh milk be slightly raised in tempera- 

 ture and be treated with an infusion of the gastric mucous membrane of the calf, coagu- 

 lation will take place in from five to ten minutes, the clear liquid still retaining its alka- 

 line reaction. Simon has observed that the mucous membrane of the stomach of an 

 infant a few days old, that had recently died, coagulated woman's milk more readily than 

 the mucous membrane of the stomach of the calf. 



Non- Nitrogenized Constituents of Milk. Non-nitrogenized matters exist in abun- 

 dance in the milk. The liquid caseine and the water hold the fats, as we have seen, in 

 the condition of a fine and permanent emulsion. This fat has been separated from the 

 milk and analyzed by chemists and is known under the name of butter. In human 

 milk, the butter is much softer than in the milk of many of the inferior animals, particu- 

 larly the cow ; but it is composed of essentially the same constituents, although in differ- 

 ent proportions. In different animals, there are developed, even after the discharge of 

 the milk, certain odorous principles, which are more or less characteristic of the animal 

 from which the butter is taken. 



The greatest part of the butter consists of margarine. It contains, in addition, oleine, 

 with a small quantity of peculiar fats, which have not been very well determined, called 



