DIGESTION OF MILK. [BOOK n. 



water in a Papiu's digester, that is to say by means of agents 

 which, iu other cases produce their effects by bringing about 

 hydrolytic changes ; beyond this we cannot at present go. We 

 may add however, as supporting the same view, the statement 

 of some observers that peptone when treated with dehydrating 

 agents or when simply heated to 140 170 C. is in part recon- 

 verted into a body or bodies resembling acid-albumin or globulin. 



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

 into peptone. Concerning the action of gastric juice on other 

 nitrogenous substances more or less allied to proteids but not 

 truly proteid in natiire our knowledge is at present imperfect. 

 Mucin, nuclein, and the chemical basis of horny tissues are wholly 

 unaffected by gastric juice. The gelatiniferous tissues are dissolved 

 by it ; and the bundles and membranes of connective tissue are 

 very speedily so far affected by it, that at a very early stage of 

 digestion, the bundles and elementary fibres of muscle which are 

 bound together by connective tissue fall asunder; moreover both 

 prepared gelatine and the gelatiniferous basis of connective tissue 

 in its natural condition, that is without being previously heated 

 with water, are by it changed into a substance so far analogous 

 with peptone, that the characteristic property of gelatinisation is 

 entirely lost. Chondrin and the elastic tissues undergo a similar 

 change. It is not clear however ho\v far this change is due simply 

 to the acid of gastric juice independently of tin- pepsin. 



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 neutralized 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 



