492 DIGESTION IN THE INTESTINES [CH. XXXII. 



magnesium. The alkalinity of the juice is due to phosphates and car- 

 bonates, especially of sodium. 



1. Action of Trypsin. Trypsin acts like pepsin, but with certain 

 differences, which are as follows : 



(a) It acts in an alkaline, pepsin in an acid medium. 



(&) It acts more rapidly than pepsin ; deutero-proteoses can be 

 detected as intermediate products in the formation of peptone ; the 

 primary proteoses have not been detected. 



(c) An albuminate of the nature of alkali-albumin is formed in 

 place of the acid-albumin of gastric digestion. 



(d) It acts more powerfully on certain albuminoids (such as elastin) 

 which are difficult of digestion in gastric juice. It does not, however, 

 digest collagen. 



(e) Acting on solid proteids like fibrin, it eats them away from the 

 surface to the interior ; there is no preliminary swelling as in gastric 

 digestion. 



(/) Trypsin acts further than pepsin, on prolonged action decom- 

 posing the peptone which has left the stomach into simpler products, 

 of which the most important are leucine, tyrosine (see p. 499), poly- 

 peptides (see p. 500), arginine (see p. 404), aspartic acid [arnino-succinic 

 acid C 2 H 3 (NH 2 )(COOH) 2 ], glutamic acid [amino-pyrotartaric acid, 

 C 3 H 5 (NH 2 XCOOH) 2 ], ammonia, and a substance called tryptophan 

 [indole-amino-propionic acid], which gives a red colour with chlorine 

 or bromine water, and also the Adamkiewicz reaction (p. 398). 



The action of proteolytic enzymes is, by a process of hydrolysis, to split the 

 heavy proteid molecule into smaller and smaller molecules ; first, we get proteoses, 

 then peptones and polypeptides, and, finally, simple products like leucine and 

 tyrosine. A variable fraction of the proteid molecule is broken off with comparative 

 case, but the whole breakdown is more easily performed by the powerful tryptic 

 enzyme than by the comparatively feeble agent pepsin. Pepsin, however, is not 

 entirely inactive in this direction, for although leucine and tyrosine are not usually 

 found in a peptic digest, unless the action has been very prolonged, yet there are 

 analogous substances of low molecular weight (aspartic acid, hexone bases, etc.), 

 which were incorrectly grouped together by the earlier workers as a peptone 

 (antipeptone). The essential difference between pepsin and trypsin is one of 

 velocity of action. 



2. Action of Amylopsin. The conversion of starch into maltose 

 is the most powerful and rapid of all the actions of the pancreatic 

 juice. It is much more powerful than saliva, and will act even on 

 unboiled starch. The absence of this ferment in the pancreatic juice of 

 infants is an indication that milk, and not starch, is their natural diet. 



3. Action on Fats. The action of pancreatic juice on fats is a 

 double one : it forms an emulsion, and it decomposes the fats into 

 fatty acids and glycerin by means of its fat-splitting ferment steapsin. 

 The fatty acids unite with the alkaline bases to form soaps (saponifi- 

 catiori). The chemistry of this is described on p. 395. The fat- 

 splitting power of pancreatic juice cannot be studied with a glycerin 



