336 SECRETION. 



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

 liquid to a semi-solid condition. When milk is allowed to coagulate spon- 

 taneously, 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. If fresh milk be slightly raised in temperature and be treated with 

 an infusion of the gastric mucous membrane of the calf, coagulation will 

 take place in five or ten minutes, the clear liquid still retaining its alkaline 

 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-nitrogeiiized matters exist 

 in abundance in the milk. The liquid caseine and the water hold the fats 

 in the condition of a fine and permanent emulsion. This fat may easily be 

 separated from the milk, 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, particularly the cow; but it is composed of essentially the same 

 constituents, although in different proportions. In different animals, there 

 are developed, even after the discharge of the milk, certain odorous matters, 

 which are more or less characteristic of the animal from which the butter is 

 taken. 



The greatest part of the butter consists of palmitine. Butter contains 

 in addition, oleine, and a small proportion of peculiar fats, which have not 

 been very well determined, called butyrine, caprine, caproi'ne, capriline, with 

 some other analogous substances. Palmitine and oleine are found in the fat 

 throughout the body; but the last-named substances are peculiar to the 

 milk. These are especially liable to acidification, and the acids resulting 

 from their decomposition give the peculiar odor and flavor to rancid butter. 



Sugar of milk, or lactose, is the most abundant of the solid constituents 

 of the mammary secretion. It is this that gives to the milk its peculiar 

 sweetish taste, although this variety of sugar is much less sweet than cane- 

 sugar. The chief peculiarities of milk-sugar are that it readily undergoes 

 change into lactic acid in the presence of nitrogenized ferments, and that 

 it takes on alcoholic fermentation slowly and with difficulty. In the fer- 

 mentation of milk, the lactose is changed first into galactose, and then into 

 alcohol and carbon dioxide. In some parts of the world, alcoholic beverages 

 made from milk are in common use. 



Inorganic Constituents of Milk. It is probable that many inorganic salts 

 exist in the milk, which are not given in the table ; and the separation of 

 these from their combinations with organic matters is one of the most diffi- 

 cult problems in physiological chemistry. This must be the case, for during 

 the first months of extrauterine existence, the child derives all the inorganic 

 as well as the organic matters necessary to nutrition and development, from 

 the breast of the mother. The reaction of the milk depends upon the pres- 

 ence of the alkaline carbonates, and these are important in preserving the 



