ACTION OF MICRO-ORGANISMS IN THE INTESTINES 373 



one substance into another. Speaking generally, a special variety of cell 

 is concerned with each ferment action, thus one variety has to do with 

 alcoholic, another with lactic, and another with acetous fermentation. 



A considerable number of species of bacteria exist in the body during life 

 chiefly in connection with the mucous membranes, particularly of the digest- 

 ive tract. Many forms of bacteria have been isolated from the mouth, a few 

 varieties from the stomach, and a very large number from the intestines. It 

 is only in the last-named locality that their multiplication has much effect 

 from a physiological point of view. The normal (hydrochloric acid) acidity 

 of the stomach usually destroys all the micro-organisms taken in with the 

 food, but when the amount of this acid is deficient (and sometimes even 

 when it is normal) some of the spores may escape. On reaching the small 

 intestine these spores begin to develop in its alkaline medium, and may in- 

 crease to such an extent as to stop all intestinal digestion; the point where 

 this occurs varies from day to day. The large intestine always swarms 

 with micro-organisms, though they do not readily pass the ileocecal valve 

 into the small intestine. Some species of bacteria found in the intestine are 

 anaerobic; i.e., they do not develop in the presence of free oxygen. 



The changes induced in the intestine by the activity of micro-organisms 

 are of two kinds, fermentation and putrefaction; the former of these results 

 in the breaking-down of carbohydrate matter, and the latter in the disintegra- 

 tion of protein matter. The process of fermentation is the less complex 

 and probably occurs normally in the small intestine. The lactic acid fermen- 

 tation is the most important, though the butyric acid fermentation is next; 

 under the influence of these bacteria the carbohydrates are butyrates 

 broken down into lactic and butyric acids, and perhaps into acetic 

 acid also. Carbonic acid gas may be formed at the same time and 

 cause flatulence. Cellulose and other insoluble carbohydrates are de- 

 composed, with the formation of marsh gas and hydrogen, which escape 

 by the rectum. 



In putrefaction the process is somewhat similar to that in tryptic digestion, 

 the proteins being broken down into peptones, leucin, tyrosin, and a long row 

 of similar substances. It also results in the production of various gases, such 

 as carbon dioxide, sulphureted hydrogen, ammonia, hydrogen, and methane 

 (marsh gas), and of a high percentage of the volatile fatty acids, valerianic 

 and butyric. Of the aromatic substances the most important are some 

 phenol derivatives and indol and skatol, though their toxicity has been 

 greatly overestimated. Indol and skatol undergo oxidation in the liver after 

 absorption, forming indoxyl and skatoxyl. They are in part carried off in 

 the feces, but when the bowel is obstructed they are absorbed and eventually 

 appear in the urine, indoxyl and skatoxyl forming, respectively, indoxyl- and 

 skatoxyl-sulphuric acids and their salts. Tyrosin is broken down into para- 

 oxy-phenol-propionic acid, paracresol, and phenol; para-oxy-phenol- acetic 



