GASTKIC DIGESTION. 349 



action of the milk-curdling ferment ; for rennet is entirely ineffective 

 when the earthy phosphates are absent. Thus, if casein is precipitated 

 by an acid and dissolved in a small amount of alkali, after careful wash- 

 ing rennet is entirely incapable of producing a coagulum ; so also milk, 

 when subjected to dialysis, by which means the salts are removed, is 

 incapable of coagulating under the influence of rennet. As to whether 

 there is a chemical association of the phosphates in the production of 

 the coagulum by the action of rennet or not, or whether it acts merely 

 mechanically, is not known. 



In addition to the milk-curdling ferment, which, as already stated, is 

 said to be entirely ineffective on milk-sugar, there appears to be still 

 another and third ferment in the gastric juice, different from both pepsin 

 and milk-curdling ferment, and which has for its action the conversion 

 of milk-sugar into lactic acid ; for both pepsin and rennet may be 

 destroyed by the action of a dilute caustic soda solution, and the result- 

 ing fluid will still be able to convert milk-sugar into lactic acid. 



(c) The Acid of Gastric Juice. — The greatest controversy has for a 

 long time existed as to the nature of the free acid of gastric juice. The 

 contradictions on this subject are evidently due to the fact that in the 

 process of analysis the hydrochloric acid usually found might possibly 

 originate from the breaking up of the metallic chlorides which are con- 

 stantly found in this secretion. 



Prout first separated hydrochloric acid from gastric juice by distil- 

 lation, and Lehmann suggested that when metallic chlorides are distilled 

 with lactic acid, hydrochloric acid will always pass into the distillate ; 

 and on account of this objection it was for a long time believed that the 

 acidity of gastric juice was normally due to the presence of free lactic acid. 



Schmidt's analysis of gastric juice, however, overcame this objection 

 raised by Lehmann, as he found in the secretion more hydrochloric acid 

 than could saturate all the bases present. Numerous proofs have since then 

 been brought forward which all tend to demonstrate that hydrochloric 

 acid in a free state is the cause of the acid reaction of this secretion ; thus 

 Richet proved by the degree of solubility in ether, according to the 

 method pointed out by Bertholet, who found that while mineral acids 

 were soluble in ether organic acids were insoluble, that the acid of gastric 

 juice must be a mineral acid, and from what he termed the coefficient of 

 partage with ether that acid was hydrochloric. Still another proof is 

 found in the fact that gastric juice behaves like mineral acids in giving 

 the color of sulphocyanide of iron when added to a solution of sulpho- 

 cyanide of potassium and citrate of iron and quinine (Reoch) ; while, 

 stiil further, the addition of gastric juice to starch-mucilage containing 

 iodide of potassium will develop the blue iodide of starch by liberating 

 the iodine from the potassium. 



