246 GASTRIC JUICE. [BOOK n. 



simple splitting up of larger proteid molecules. Peptone closely 

 resembling, if not identical with, that obtained by gastric di- 

 gestion, may be obtained by the action of strong acids, by the 

 prolonged action of dilute acids especially at a high temperature, 

 or simply by digestion with super-heated water in a Papin's 

 digester. The role of pepsin therefore is only to facilitate a change 

 which may be effected without it. 



All proteids, so far as we know, are converted by pepsin into 

 peptone. Of its action on other nitrogenous substances not truly 

 proteid in nature, we need only say that mucin, nuclein, and the 

 chemical basis of horny tissues are wholly unaffected by it, but 

 that the gelatiniferous tissues are dissolved and changed into a 

 substance so far analogous with peptone, that the characteristic 

 property of gelatinisation is entirely lost. Chondrin and the 

 elastic tissues are also dissolved. 



Action of gastric juice on milk. It has long been known that 

 an infusion of calves' stomach, called rennet, has a remarkable 

 effect in rapidly curdling milk, and this property is made use of in 

 the manufacture of cheese. Gastric juice has a similar effect ; 

 milk when subjected to the action of gastric juice is first curdled 

 and then digested. If a few drops of gastric juice be added to a 

 little milk in a test tube, and the mixture exposed to a tempera- 

 ture of 40, the milk will curdle into a complete clot in a very 

 short time. If the action be continued the curd or clot will be 

 ultimately dissolved and digested. Milk contains, besides albumin, 

 fats, milk, sugar and various salines, a peculiar proteid called 

 casein 1 , a body allied to the so-called alkali-albumin. In natural 

 milk casein is present in solution, arid ' curdling ' consists essen- 

 tially in the casein becoming insoluble and being precipitated in a 

 solid form, a great deal of the fat being generally carried down 

 with it. Now casein is readily precipitated from milk upon the 

 addition of a small quantity of acid, and it might be supposed that 

 the curdling effect of gastric juice was due to its acid reaction. 

 But this is not the case, for neutralized gastric juice, or neutral 

 rennet, is equally efficacious. Moreover the substance thrown down 

 by an acid is not quite exactly the same as that which appears in 

 curdling. 



The effect is closely dependent on temperature, being like the 

 peptic action of gastric juice favoured by a rise of temperature up 

 to about 40. Moreover the curdling action is destroyed by 

 previous boiling of the juice or rennet. These facts suggest that a 

 ferment is at the bottom of the matter ; and indeed, all the 

 features of the action support this view. The ferment however is 

 not pepsin but some other body; and the two may be separated 

 by cautiously adding magnesium carbonate to gastric juice or to 

 an infusion of calves' stomach. The clear fluid, left above the pre- 



1 See Appendix. 



