42 PHYSIOLOGY. 



on its surface, which is chiefly composed of caseinogen inmeshing some 

 fat-particles. This film is formed by the drying of proteid at the 

 surface of the milk. The chief proteid of milk is a phospho-proteid 

 called caseinogen, which can be precipitated by adding to the diluted 

 milk a weak acid or by saturating it with a neutral salt. The chief 

 peculiarity of caseinogen is its coagulating power when treated with 

 a ferment, rennin, in the presence of lime salts. The coagulation of 

 milk depends upon the change of a soluble proteid, caseinogen, into 

 an insoluble body, casein, by means of the enzyme, rennin, and the 

 presence of lime salts is necessary. It is probable that the rennin 

 first splits the caseinogen into two bodies, the more important being 

 soluble casein, which then combines with the calcium salts to form a 

 casemate of calcium, while the other passes into solution in the whey 

 as whey proteid, or lacfoserum proteose. 



The casein thus generated inmeshes the fat-granules and forms 

 milk-curd. This curd, like the blood-clot, shrinks after a few hours 

 and an opalescent fluid, or serum, called whey, is expressed. 



This whey contains, besides the whey-proteid, or lactoserum pro- 

 teose, traces of other proteids and also lactose and milk salts. The 

 casein of cows' milk forms large masses on coagulation, while 

 women's milk forms very fine flakes. 



The lactose, or sugar of milk, does not readily ferment with 

 yeast, but it is capable of undergoing a special fermentation, by which 

 it is changed by the lactic acid bacillus into lactic acid, and this 

 lactic acid is further split up into butyric acid. These two acids, 

 lactic and butyric, precipitate the caseinogen and produce the curd in 

 sour milk; but this curd is quite a different body from that pro- 

 duced by rennin, for it can be dissolved by a weak alkali, and then the 

 rennin will clot it. Potassium oxalate, which precipitates the cal- 

 cium in the milk, prevents the clotting of the milk. The other pro- 

 teids in milk, besides caseinogen, are lactalbumin and lactoglobulin. 



Kumiss is mare's milk fermented. It contains 10 per cent, of 

 solids, 3 per cent, of alcohol, 2 per cent, of fat, 2 per cent, of sugar, 

 1 per cent, of lactic acid, 1 to 2 per cent, of casein, and 1 volume 

 per cent, of carbonic acid. 



Kephir is cows' milk fermented by kephir grains. 



Matzoon is prepared by adding to milk a ferment consisting of 

 some form of yeast and the lactic acid bacilli. It, however, contains 

 very much less alcohol and carbonic acid than kumiss. Plasmon 

 is prepared by precipitating casein from fresh milk. Then it is dis- 

 solved in sodium bicarbonate in the presence of free carbon dioxide, 



