MAMMARY SECRETION. 615 



Milk is coagulated by all acids. Several, especially acetic acid in 

 excess, again dissolve the precipitate of casein. Casein is precipitated 

 by acids only when a certain degree of acidity is reached, since alkaline 

 phosphates must be first neutralized ; if the alkaline phosphates are 

 removed, even carbon dioxide is then capable of precipitating casein. 

 The spontaneous coagulation of milk is produced by the development of 

 lactic acid, which is formed from milk-sugar in the milk by the action 

 of bacteria introduced from without or by the action of the lactic acid 

 ferment which appears to be present in milk. In this process the neutral 

 alkaline phosphate is converted into acid phosphate. The casein is sepa- 

 rated from its combination with calcium phosphate and is precipitated, 

 the sugar being decomposed into lactic acid and carbon dioxide.' When 

 milk coagulates spontaneously, or, in other words, curdles, it separates 

 into a tough, jelly-like substance or curd, which consists of the insoluble 

 casein and fat, floating in an opalescent acid fluid, or whey. The whey 

 contains the greater part of the salts of the milk, the lactic acid, the 

 undecomposed milk-sugar, and certain amounts of fat and the albumi- 

 noids. That the spontaneous curdling of milk is due to the action of the 

 ferment contained itself in milk (which decomposes the milk-sugar into 

 lactic acid)- is proved by the fact that boiling greatly retards the spon- 

 taneous coagulation, evidently through the destruction of this ferment. 

 This lactic acid ferment may be precipitated from milk by the addition of 

 an excess of alcohol. If the ferment is then dissolved in water and added 

 to solutions of milk-sugar it will produce rapid fermentation. The action 

 of the milk-curdling ferment, as pointed out in the chapters on Digestion, 

 is very different. Here, the casein is rendered insoluble by the action of 

 the rennet ferment, even in alkaline fluids and without at all calling in 

 the aid of lactic acid. The salts in milk, especially the calcic phosphate, 

 are essential to the action of milk-curdling ferment ; for, when milk is 

 subjected to dialysis, rennet is then rendered incapable of producing 

 coagulation. 



Spontaneous coagulation of milk may be prevented by the addition 

 of sodic carbonate, boracic or salicylic acids. So, also, the addition of 

 one drop of the ethereal oil of mustard to twenty cubic centimeters of 

 milk will likewise preserve its fluid condition. 



Colostrum, sows' milk, and the milk of carnivora coagulate when 

 heated. Boiled milk coagulates spontaneously only with difficulty, and 

 is also more difficult to coagulate with rennet, but when acidulated the 

 casein coagulates even more readily than in fresh milk. 



A high temperature facilitates both forms of coagulation. Milk 

 which has become acid, but which is still fluid, will coagulate when 

 heated. ' 



In addition to casein, milk contains other albuminoids, one of which 



