394 A MANUAL OF PHYSIOLOGY 



unfavourable at the end of intestinal digestion, and favourable at 

 the beginning of gastric digestion. 



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

 medium ; and although the cause of the acidity and the char- 

 acter 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, 

 or a great part of it, 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 per- 

 mitted 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 putre- 

 faction is constantly 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. Indeed, from the time when 

 the first micro-organism enters the digestive 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 starch and dextrin which have escaped 

 the action of the saliva are changed into maltose by the amylopsin. 



