ACTION OF THE BILE IN DIGESTION 221 



place in starchy and fatty matters when they are exposed to the action 

 of the pancreatic juice out of the body, and they pass through the 

 various stages of transformation respectively into lactic acid and the 

 fatty acids. Putrefactive action, however, does not readily take place 

 in proteids that have been precipitated after having been cooked, or in 

 raw gluten or casein. The presence of fat also interferes with putre- 

 faction. 



Trypsin, in an alkaline medium, changes proteids into their respec- 

 tive peptones, in much the same way and involving nearly the same 

 intermediate processes as in the digestion of these substances by the 

 gastric juice, except that the first change is into alkali-albumin instead of 

 acid-albumin. In the upper part of the intestine the vigor of this pro- 

 teolytic action is much increased by admixture with the intestinal secre- 

 tions; and in the lower part, where the contents are more strongly 

 alkaline, the activity of the process is increased by sodium carbonate. 

 If the action of the pancreatic juice on proteids is prolonged in vitro, 

 the changes continue and substances are formed which yield leucin, 

 tyrosin and other analogous products. The final putrefactive changes 

 resulting in indol, skatol, phenol, etc., some of which have a distinctly 

 fecal odor, probably are due to the action of micro-organisms. 



Action of the Pancreatic Juice on Fats. The pancreatic juice is the 

 only digestive secretiorf that is capable of forming a complete and 

 permanent emulsion with fats, and this persists when the emulsion is 

 diluted with water. Steapsin, extracted from the fresh pancreas, has 

 the property of decomposing the fats into their fatty acids and glycerin ; 

 but the fatty acids do not appear in the chyle. The emulsification of the 

 fats by the pancreatic juice is to a great extent a mechanical process, 

 dependent on the general physical characters of the liquid ; but 

 although the fat contained in the lacteal vessels is always neutral, it 

 is thought that steapsin assists in rendering the emulsion fine and 

 permanent. 



ACTION OF THE BILE IN DIGESTION 



The physiological anatomy of the liver and the general properties 

 and composition of the bile will be considered again in connection with 

 the physiology of secretion and excretion ; and here it will be necessary 

 only to study the action of the bile in digestion. 



The question whether the bile is simply excrementitious or is con- 

 cerned in digestion was formerly the subject of much discussion ; but it 

 is now admitted by all physiologists that the action of the bile in diges- 

 tion and absorption, whatever its office may be as an excretion, is essen- 

 tial to life. The experiments of Swann, Nasse, Bidder and Schmidt, 



