CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 361 



tyrosin. If these be removed the peptone may be precipitated 

 from the concentrated filtrate by the addition of a large excess 

 of alcohol and separated by filtration. The second filtrate 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 di- 

 gested, 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. We said that in gastric 

 digestion more than one kind of peptone was probably formed, 

 and the same may be said of pancreatic digestion. We may now 

 add that in both gastric and pancreatic digestion two kinds of 

 peptone are probably formed, one of which resists the action of 

 trypsin, and undergoes no further change, but the other of which, 

 whether arising from gastric or pancreatic digestion undergoes 

 further change by the action of trypsin and it is this which 

 is the source of the leucin and other bodies of which we are 

 speaking. 



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 H 13 N0 2 or C 5 H ]0 .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 n N0 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 



