398 DIGESTION OF MILK. [BOOK n. 



which moreover is diffusible; this may be called gelatin-peptone , 

 and there appear to be more forms than one of this substance. 

 Intermediate products, especially substances analogous to albu- 

 moses, and termed glutoses, also make their appearance. Chondrin 

 and elastin, the special constituent of elastic tissues, also undergo 

 under the influence of gastric juice changes which are at least in 

 some respects similar to those undergone by gelatin ; so also does 

 mucin. Nuclein, on the other hand, appears to be unaffected by 

 gastric juice. Hence when the bodies called nucleo-albumins ( 29), 

 which appear to be compounds of some proteid or other with nuclein, 

 are treated with gastric juice, they are split up, the proteid con- 

 stituent is digested and dissolved, while the nuclein is unaffected 

 and so may be separated out. Keratin, the special constituent of 

 horny tissues, seems also to be unaffected by gastric juice. 



207. 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 a peculiar 

 form, or peculiar forms of albumin, fats, milk-sugar and various 

 salines, the peculiar proteid casein. In natural milk casein is 

 present in solution, and ' curdling ' consists essentially in the soluble 

 casein being converted (or more probably as we shall see presently, 

 split up) into an insoluble modification of casein, which as it is 

 being precipitated carries down with it a great deal of the fat and 

 so forms the ' curd/ 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 neutralised gastric 

 juice, or neutral rennet, is equally efficacious. 



The curdling action of rennet is closely dependent on tempera- 

 ture, 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. Moreover, 

 as a matter of fact, a curdling ferment may be extracted by 

 glycerin and by the other methods used for preparing ferments. 

 The ferment however is not pepsin but some other body ; and the 

 two may be separated from each other. If magnesium carbonate 

 in powder be cautiously added to gastric juice or to an infusion of 

 calves' stomach a copious precipitate is formed. If the addition 

 of magnesium carbonate be stopped as soon as any further pre- 



