452 TRYPTIC DIGESTION. [BOOK n. 



If a pancreatic digestion mixture be freed from the alkali-albumin 

 by neutralisation and nitration, the nitrate yields, when concen- 

 trated by evaporation, a crop of crystals of tyrosin. If these be 

 removed the peptone may be precipitated from the concentrated 

 nitrate by the addition of a large excess of alcohol and separated 

 by filtration. The second nitrate upon being concentrated by 

 evaporation yields abundant crystals of leucin and traces of tyrosin. 

 Thus by the action of the pancreatic juice a considerable amount 

 of the proteid, which is being digested, is so broken up as to give 

 rise to products which are no longer proteid in nature. From this 

 breaking up of the proteid there arise leucin, tyrosin, and probably 

 several other bodies, such as fatty acids and volatile substances. 



As is well known, leucin and tyrosin are the bodies which 

 make their appearance when proteids or gelatin are acted on by 

 dilute acids, alkalis, or various oxidising agents. Leucin is a body, 

 which in an impure state crystallizes in minute round lumps with 

 an obscure radiate striation, but when pure, forms thin glittering 

 flat crystals. It has the formula C 6 H 13 NO 2 or C ? H 10 . NH 2 (CO. OH) 

 and is amido-caproic acid. Now caproic acid is one of the " fatty 

 acid " series, so that leucin may be regarded as a compound of 

 ammonia with a fatty acid. Tyrosin, C 9 H U NO 3 , on the other 

 hand, belongs to the " aromatic " series ; it is a phenyl compound, 

 and hence allied to benzoic acid and hippuric acid. So that in 

 pancreatic digestion the large complex proteid molecule is split 

 up into fatty acid and aromatic molecules, some other bodies 

 of less importance making their appearance at the same time. 

 We infer that the proteid molecules are in some way built up 

 out of " fatty acid " and " aromatic " molecules together with 

 other components, and we shall later on see additional reasons for 

 this view. 



Among the supplementary products of pancreatic digestion 

 may be mentioned the body indol (C 8 H 7 N), to which apparently 

 the strong and peculiarly faecal odour which sometimes makes its 

 appearance during pancreatic digestion is due. Indol, however, 

 unlike the leucin and tyrosin, is not a product of pure pancreatic 

 digestion, but of an accompanying decomposition due to the action 

 of organised ferments. A pancreatic digestive mixture soon be- 

 comes swarming with bacteria, in spite of ordinary precautions, 

 when natural juice or an infusion of the gland is used. When 

 isolated ferment is used, and atmospheric germs are excluded, or 

 when pancreatic digestion is carried on in the presence of salicylic 

 acid, or thymol, or some other agent which prevents the develop- 

 ment of bacteria and like organisms but permits the action of the 

 trypsin, no odour is perceived, and no indol is produced. 



After long-continued digestion, especially when accompanied by 

 putrefactive decomposition, the amount of proteids which are carried 

 beyond the peptone stage and broken up, may be very great. 



In gastric digestion such a profound destruction of proteid 



