92 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



to the acid, since neutral extracts of the gastric mucous mem- 

 brane will themselves curdle milk. After coagulation the 

 action of pepsin begins and the casein is converted into pep- 

 tones in the usual manner. The value of the curdling pro- 

 cess is not apparent. 



Action of Gastric Juice on Foods. (A) On Proteids. A 

 familiar test for the proper performance of gastric digestion 

 is the observation of the effect of the juice in a given case 

 upon the white of an egg (proteid). In normal gastric 

 juice, or in a properly prepared artificial solution, the egg is 

 seen to swell up and dissolve. This soluble proteid is now 

 called peptone, and it differs from the proteid of the egg in 

 certain important respects, to be noted later. .But, although 

 peptone is the final product of pepsin-hydrochloric action, 

 there are certain substances produced intermediate between 

 the initial proteid and the final peptone, just as in the case 

 of the formation of maltose by ptyalin. Some of these sub- 

 stances have been called acid-albumin, parapeptone, propep- 

 tone, etc. But whatever they may be, the nomenclature of 

 Kuhne is being largely followed at present. He supposes 

 that the first product is an acid albumin which he calls syn- 

 tonin ; that syntonin under the influence of pepsin undergoes 

 hydrolysis, taking up water and splitting into primary pro- 

 teoses; that each of these primary proteoses takes up water 

 and splits into secondary proteoses; that these last undergo 

 a similar change with the production of peptones; so that 

 the successive substances are proteid, syntonin (acid-albu- 

 min), primary proteoses, secondary proteoses, peptones. 



Peptones can be shown to be different from syntonin and 

 the proteoses by chemical reaction. The chief object of pro- 

 teolytic digestion is to get the proteids into a diffusible 

 condition. Peptones differ from proteids in at least three im- 

 portant respects: (i) They can pass through animal mem- 

 branes, that is, can be absorbed; (2) they are no longer co- 

 agulable by heat or many acids; (3) they are capable of as- 

 similation by the cells after they have been absorbed. 



