434 THE GLANDULAR ORGANS. 



providing that a sufficient amount of free hydrochloric acid is present ; 

 but, as we have seen, the gastric juice is further capable of effecting 

 the coagulation of caseinogen even though hydrochloric acid is absent. 

 This is brought about through the specific activity of the milk- 

 curdling ferment ; but it is to be noted that the coagulation of milk 

 is in this case not directly comparable to the action of an add. 

 Our knowledge of the mechanism of rennin coagulation of milk has 

 been materially advanced through the researches of Loevenhart. 

 It has thus been ascertained that rennin first changes caseinogen to 

 paracasein and renders the calcium salts available for the precipita- 

 tion of the latter ; subsequently the paracasein is precipitated by the 

 calcium salts as casein. In this connection it is to be noted that the 

 " rendering of calcium salts available " does not necessarily mean 

 that they are soluble. Sodium citrate, for instance, prevents the 

 coagulation of milk by rennin ; the calcium present combines with 

 the citric acid and is thus rendered unavailable (exactly as in the 

 case of the coagulation of the blood). Nevertheless, calcium citrate 

 is soluble to a sufficient degree, particularly in the cold. 



In all the work previously done on rennin coagulation one phase 

 of the subject has in a measure been neglected. It has been claimed 

 that the caseinogen exists in the milk in solution as a calcium salt, 

 viz., as a so-called neutral calcium salt (neutral to litmus, but acid 

 to phenolphthalein), and that in this form it becomes subject to the 

 action of rennin. Paracasein then results, but to obtain coagulation 

 it is still necessary to heat or to add a calcium salt. To produce 

 rennin coagulation directly under such conditions phosphoric acid 

 (or possibly some other acid) is necessary in certain concentration. 

 The part which this plays, however, has not yet been determined. 



The pepsin of the gastric juice plays no part whatever in the 

 coagulation of the milk. But after this has taken place the actual 

 digestion of the precipitated caseinogen begins. As I have pointed 

 out, this is then decomposed, with the formation of a paranuclein and 

 albumin, which latter is digested in the usual manner. A hetero- 

 albumose, however, is not formed during the process. 



From the above considerations it is clear that all those factors 

 which tend to increase the amount of soluble lime salts in the milk 

 will increase the tendency of the caseinogen to coagulate upon the 

 subsequent addition of chymosin, while this is diminished if the 

 soluble salts are transformed into the insoluble form or if their 

 amount is diminished. It is for this reason also that boiled milk 

 does not coagulate so rapidly as fresh milk, as the free carbonic acid, 

 which holds a certain amount of calcium in solution, is thereby 

 removed. The common addition of lime-water to milk similarly 

 increases the tendency to coagulation, but does not render it more 

 digestible, as is generally supposed. 



The coagulation of the milk which occurs spontaneously on 

 standing is analogous to that which results upon the addition of a 

 mineral acid, and is referable to the formation of lactic acid from 

 lactose as a result of bacterial action. 



