THE FOOD AND DIGESTION 335 



It is characterised by being precipitated on neutralisation 

 and by being insoluble in distilled water. On filtering off 

 these, the filtrate yields a precipitate on saturating with 

 sulphate of ammonia, indicating the formation of a deutero- 

 proteose, and, if the filtrate from this be tested, the presence 

 of a proteid may be demonstrated. Peptone has been 

 produced. (Chemical Physiology, p. 18.) 



These changes may be represented in the following 

 table : 



Coagulated Proteid. 



Soluble Native Proteid. 

 Acid Proteate. 



| I 



Proto-proteose. Hetero-proteose. 



Deutero-proteose. Deutero-proteose. 



Peptone. Peptone. 



The process is one of breaking down a complex molecule 

 into simpler molecules, probably with hydration. 



The object of this was formerly supposed to be to allow of 

 the diffusion of the proteid in the form of peptone through 

 the wall of the intestine. It is now known that absorption 

 is not due to diffusion, and it is more probable that the 

 change to the simplest proteid molecule is a necessary step 

 in the building up of the proteid into the special protoplasm 

 of the body of the particular animal. 



On certain proteids and their derivatives the gastric juice 

 has a special action. On collagen the HC1 acts slightly in 

 converting it to gelatin. On gelatin the gastric juice acts, 

 converting it to a gelatin peptone. 



On nucleo-proteids it acts by digesting the proteid part 

 and leaving the muclein undissolved. 



Haemoglobin is broken down into hsematin and globin, and 

 the latter is changed into peptone. 



The casein of milk is first coagulated and then changed to 

 peptone. The coagulation is brought about by the presence 

 of the second enzyme of the gastric juice rennin. 



