CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 427 



former case the fibrin does not swell up, but remains as opaque as 

 before, and appears to suffer corrosion rather than solution. But 

 there is a still more important distinction between pancreatic and 

 peptic digestion of proteids. Peptic digestion is essentially an 

 acid digestion ; we have seen that the action only takes place in 

 the presence of an acid, and is arrested by neutralisation. Pan- 

 creatic digestion, on the other hand, may be regarded as an alkaline 

 digestion ; the action is most energetic when some alkali is present ; 

 and the activity of an alkaline juice is hindered or delayed by 

 neutralisation and arrested by acidification at least with mineral 

 acids. The glycerine extract of pancreas is under all circumstances 

 as inert in the presence of free mineral acid as that of the stomach 

 in the presence of alkalis. If the digestive mixture be supplied 

 with sodium carbonate to the extent of 1 p.c., digestion proceeds 

 rapidly, just as does a peptic mixture when acidulated with hydro- 

 chloric acid to the extent of *2 p.c. Sodium carbonate of 1 p.c. 

 seems in fact to play in tryptic digestion a part altogether 

 comparable to that of hydrochloric acid of '2 p.c. in gastric di- 

 gestion. And just as pepsin is rapidly destroyed by being heated 

 to about 40 with a 1 p.c. solution of sodium carbonate, so trypsin 

 is rapidly destroyed by being similarly heated with dilute hydro- 

 chloric acid of '2 p.c. Alkaline bile, which arrests peptic digestion, 

 seems, if anything, favourable to tryptic digestion. 



Corresponding to this difference in the helpmate of the ferment, 

 there is in the two cases a difference in the nature of the products. 

 In both cases peptone is produced, and such differences as can be 

 detected between pancreatic and gastric peptones are relatively 

 small ; but in pancreatic digestion the bye-product is not, as in 

 gastric digestion, a kind of acid-albumin, but, as might be ex- 

 pected, a body having more analogy with alkali-albumin. More- 

 over, before the alkali-albumin is actually formed, the fibrin 

 becomes altered and takes on characters intermediate between 

 those of alkali-albumin and of ordinary albumin ; and when fresh 

 raw, i.e. unboiled, fibrin is acted upon by pancreatic juice, one or 

 more globulins appear as initial products. 



Further, there are evidences that differences, of even a more 

 profound nature than the above, exist between pancreatic and 

 gastric digestion. One of these is the appearance, in the pan- 

 creatic digestion of proteids, of two remarkable nitrogenous crys- 

 talline bodies, leucin and tyrosin. When fibrin (or other proteid) 

 is submitted to the action of pancreatic juice, the amount of 

 peptone which can be recovered from the mixture falls far short 

 of the original amount of proteids, much more so than in the case 

 of gastric juice ; and the longer the digestive action, the greater is 

 this apparent loss. If a pancreatic digestion mixture be freed from 

 the alkali-albumin by neutralisation and filtration, the filtrate 

 yields, when concentrated by evaporation, a crop of crystals of 

 tyrosin. If these be removed the peptone may be precipitated 



