CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 451 



same way as pepsin is isolated, and to which the name trypsin has 

 been given. 



The appearance of fibrin undergoing pancreatic digestion is 

 however different from that undergoing peptic digestion. In the 

 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 glycerin 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 in their broad features, as 

 in the reactions given in 203, pancreatic peptone and gastric 

 peptone are alike. Albumoses also similar to gastric albumoses 

 may be found in a pancreatic digesting mixture. The bye-products 

 however are different ; some amount of alkali-albumin makes its 

 appearance, and at an early stage the fibrin becomes altered and 

 takes on characters intermediate between those of alkali-albumin 

 and of ordinary albumin; 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 a more pro- 

 found nature than the above, exist between pancreatic and gastric 

 digestion. One of these is the appearance, in the pancreatic diges- 

 tion of proteids, of two remarkable nitrogenous crystalline 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. 



