THE DIGESTIVE SYSTEM. 303 



with glycerol. By adding some of the glycerol extract to 0'2 per 

 cent, hydrochloric acid an artificial gastric juice is obtained. 



(1) If a few flakes of fibrin be placed in a test tube containing such 

 an artificial juice and the tube be kept at a temperature of 37 C., it 

 will be observed that the fibrin gradually swells up and then dissolves. 

 If the solution is neutralised, a precipitate of acid metaprotein is formed. 

 If this is removed by filtration and the filtrate is boiled, a coagulum of 

 soluble globulin may be formed. When this is removed by filtration, 

 the solution gives a pink colour on the addition of dilute copper sulphate 

 and caustic soda, owing to the presence of proteoses and peptones. 



Further analysis of this filtrate shows that several varieties of 

 proteose and at least two kinds of peptone are present. The proteoses 

 are classified as primary and secondary, the primary group being pre- 

 cipitated by the addition of an equal volume- of saturated solution of 

 ammonium sulphate \ when this precipitate has been removed by filtra- 

 tion, the secondary varieties are precipitated by full saturation with the 

 same salt. When the latter precipitate is removed, the solution contains 

 peptones only, although, if the digestion be prolonged for several days, 

 some amino-acids may bejDresent. 



The changes produced in the fibrin are due to the activity of an 

 enzyme, pepsin, which in the presence of dilute hydrochloric acid brings 

 about hydrolysis of the protein molecule and breaks it up into smaller 

 and more soluble molecules. The first change produced is the solution 

 of the fibrin with the production of a substance known as soluble 

 globulin. This is then changed into acid metaprotein, and, by successive 

 hydrolytic stages, the various proteoses, and finally peptones, are formed. 

 Digestion in the stomach never passes beyond the peptone stage, and in 

 fact the conversion into peptone is not complete when the contents of 

 the stomach are passed on into the small intestine. 



The stages of pejptic digestion of protein may be represented in 

 tabular form thus 



Protein 



i 



Soluble globulin 



I 

 Acid metaprotein 



I 

 Primary proteoses 



Secondary proteoses 



J 



Peptones 



Some proteins which occur in food do not undergo these changes. 

 Thus elastin is unaffected by peptic digestion in the time available in 



