NON-NITROGENOUS CONSTITUENTS OF MILK 299 



properly sweetened 'and diluted with water, very nearly represent the 

 ordinary breast-milk. 



Casein is by far the most important of the nitrogenous constitu- 

 ents of milk, and it supplies nearly all of this kind of nutritive matter 

 demanded by the child. Lactoprotein is not so well denned, and lact- 

 albumin, a proteid resembling serum-albumin, with lactoglobulin, exists 

 in milk in small quantities. These proteids, however, are of little 

 physiological interest. 



The coagulation of milk depends on the reduction of casein from 

 a liquid to a semisolid condition. When milk is allowed to coagulate 

 spontaneously, the change is effected by the action of the lactic acid 

 which results from a change in a part of the sugar of milk. Casein, in 

 fact, is coagulated by nearly all acids, even the feeble acids of organic 

 origin. It differs from albumin in this regard and in the fact that it is 

 not coagulated by heat If fresh milk is slightly raised in temperature 

 and 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. The action on casein of the rennin 

 of the gastric juice has already been described in connection with 

 gastric digestion. 



Non- Nitrogenous Constituents of Milk. Non-nitrogenous matters 

 exist in abundance in the milk. The liquid casein and the water hold 

 the fats in the condition of a fine and permanent emulsion. This fat 

 may easily be separated from 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 that are more or less characteristic of 

 the animal from which the butter is taken. 



The greatest part of the butter consists of palmitin. It contains in 

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

 been very well determined called butyrin, caprin, capro'fn, caprylin, 

 with some other analogous substances. Palmitin and olein 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 con- 

 stituents 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 



