412 THE GLANDULAR ORGANS. 



magnesium sulphate to saturation. Metallic salts, such as copper 

 sulphate, also precipitate a neutral solution completely. 



In the milk the casein exists as a neutral lime salt, but does not 

 occur in a state of actual solution, as has been pointed out. On 

 filtering milk through a Chamberlain filter under pressure it 

 remains behind, together with the fat and calcium phosphate, as 

 a jelly-like material. On treating milk with a dilute acid the 

 casein is precipitated, as in the case of the aqueous solution of its 

 salts. To a certain extent this may occur in the stomach, 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 lime-casein even though hydrochloric acid is absent. 

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

 curdling ferment (chymosin) ; but it is to be noted that the coagula- 

 tion of milk is in this case not directly comparable to the action of 

 an acid ; for while the latter merely brings about the separation of 

 the casein by the removal of its basic component, Hammarsten has 

 shown that the chymosin previously causes a partial decomposition 

 of the lime-casein by hydrolysis. As a result, a small amount of an 

 albumose-like substance is split off, which is found in the whey, 

 while the greater portion of the lime-casein is transformed into so- 

 called lime-paracasei'n. The paracasei'n is likewise a nucleo-albumin 

 with acid properties, and forms salts with the alkalies and lime, 

 which, like those of casein, are readily soluble in water. These 

 salts are then further apt to combine with soluble calcium salts 

 to form double salts, which are insoluble in nearly neutral solu- 

 tions. As the milk is nearly neutral in reaction, and as soluble 

 calcium salts are at the same time present, coagulation consequently 

 occurs. The resulting clot constitutes what is commonly known as 

 cheese. 



In the absence of soluble calcium salts coagulation does not 

 occur after addition of the chymosin. Lime-paracasein, however, is 

 manifestly formed, as upon subsequent treatment with a soluble 

 calcium salt the fluid coagulates in the usual manner. That the 

 ferment takes no part in the process of coagulation itself, but merely 

 prepares the lime-casein, as it were, for this end, can readily be 

 demonstrated by boiling the solution of chymosin and lime-casein 

 after having been kept at a temperature of about 35 C. for a few 

 minutes. The ferment is thus destroyed, but coagulation occurs 

 nevertheless if a soluble calcium salt is now added. 



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 lime-paracasei'n begins. As I have 

 pointed out, this is then decomposed, with the formation of a para- 

 nuclein and albumin, which latter is digested in the normal 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 



