SURVEY OF DIGESTION AS A WHOLE 415 



by the alkali of the pancreatic juice, enterokinase and trypsin by 

 the hydrochloric acid of the gastric juice, trypsinogen by the pepsin 

 and hydrochloric acid. Trypsin has no destructive effect on entero- 

 kinase or trypsinogen (Mellanby). 



Trypsin, like pepsin, performs its work in part in an acid medium ; 

 and although the cause of the acidity and the character of the 

 medium are far from being the same as in the gastric juice, it is 

 obviously an advantage that the chief proteolytic ferment should 

 be able to act upon the proteins in all parts of the intestine and 

 at every stage of intestinal digestion whether the reaction is acid 

 or alkaline. The proteins of the chyme are all carried by the 

 trypsin to the stage of peptone, and the peptone, even in 

 perfectly normal digestion, is further split up into amino- and 

 diamino-acids by the trypsin and by the erepsin of the succus 

 entericus. 



In the lower portions of the small intestine bacteria of various 

 kinds are present and active; and it is not unlikely that even 

 throughout its whole length a certain range of action is permitted 

 to them, checked by the acidity of the chyme, though scarcely by 

 the feeble antiseptic properties of the bile. 



The lower end of the small intestine is not cut off by any bacteria- 

 proof barrier from the large intestine, in which putrefaction is con- 

 stantly going on. It has been actually shown that small particles, 

 such as lycopodium spores, suspended in water, soon reach the 

 stomach when injected into the rectum. So that micro-organisms, 

 aided by the antiperistalsis of the colon, may be able to work their 

 way above the ileo-colic sphincter and valve, even against the 

 downward peristaltic movement of the small intestine. But even if 

 this were not the case, a few bacteria or their spores, passing through 

 the stomach with the food, would be enough to set up extensive 

 changes as soon as they reached a part of the alimentary canal 

 where the conditions were favourable to their development. In- 

 deed, from the time when the first micro-organism enters the diges- 

 tive tube soon after birth, it is never free from bacteria; and their 

 multiplication in one part of it rather than another depends not so 

 much on the number originally present to start the process, as on 

 the conditions which encourage or restrain their increase. 



A certain amount of already emulsified fats is broken up into 

 their fatty acids and glycerin in the stomach, unemulsified fats 

 entirely by the fat- splitting ferment of the pancreatic juice. The 

 acids will form soaps with alkalies wherever they meet them in the 

 intestinal contents, or even in the mucous membrane. A portion 

 of those soluble soaps may be immediately absorbed; the rest will 

 aid in the emulsification of the fats not yet chemically decomposed, 

 and thus greatly hasten the fat-splitting action of the pancreatic 

 juice. The phosphatides are in all probability acted upon in the 



