SECRETION. 311 



neutral or alkaline. By the clotting, caseinogen is converted into a 

 coagulated proteid, casein, and a proteid residue called whey-proteid. 

 Casein carries down with it the fat, and the two materials form cheese. 

 As in the case of blood, coagulation cannot occur except in the presence 

 of calcium salts. When caseinogen is acted on by rennin, it is split by 

 hydrolytic cleavage into two parts, paracasein and whey-proteid. Para- 

 casein combines with the calcium salts to form the insoluble compound 

 casein; the whey-proteid remains behind in solution in the whey. By 

 reference to the coagulation of the blood, the similarity of the two proc- 

 esses will be seen. Caseinogen is also precipitated from milk in the 

 presence of an excess of acid. When milk curdles after "souring," it is 

 due to the formation of lactic acid from the milk-sugar by micro- 

 organisms. 



Lact-albumin differs in some of its reactions from serum-albumin (p. 

 115); it coagulates when milk is boiled, but this scum is also partly due 

 to the drying up of the caseinogen on the surface of the milk. 



Lacloglobulin, another proteid of milk, is similar to the paraglobulin 

 of the blood. 



(3.) Fats. The fats of milk are those usually found in animal tis- 

 sues, viz., oleiu, stearin, and palmatin (p. 122). There are also others, 

 especially that of butyric acid in combination with glycerin. Lecithin 

 and cholesterin and a lipochrome may also be present. The fat, split 

 up into minute particles, which are lighter than the remainder of the 

 constituents, rises to the surface when the milk stands, forming cream; 

 arid cream, when its fatty molecules have run together, forms butter. 



(4.) Lactose. This sugar, the reactions of which are mentioned at 

 p. 124, is apt to undergo lactic-acid fermentation if the milk be exposed 

 to the air, from the action of the organized ferment, the bacterium 

 lactis. When this occurs milk becomes sour and the caseiuogen is 

 thrown down. 



(5.) Salts. The chief salt of milk is calcium phosphate. Without 

 its presence caseinogen cannot form casein. The gases are carbon 

 dioxide and nitrogen. 



SALTS IN WOMAN'S MILK (Rotch). 



Calcium phosphate . . . . . .23.87 



Calcium silicate . ... . 



Calcium sulphate ........ 



Calcium carbonate 



Magnesium carbonate . . . . . 3.77 



Potassium carbonate . . . . . . 23.47 



Potassium sulphate ...... 



Potassium chloride 12.05 



Sodium chloride 21.77 



Iron oxide and alumina ..... 0.87 



100.00 



