174 PHYSIOLOGY OF ALIMENTATION. 



also changes as we pass through the ileocsecal valve, but 

 here again it is difficult to decide whether the intestinal 

 contents determine the character of the flora or vice versa. 

 The fact remains, however, that while in the small intestine 

 we deal chiefly with the products of carbohydrate decom- 

 position we have to do in the large bowel chiefly with the 

 products of protein decomposition. By virtue of the or- 

 dinary proteolytic enzymes contained in the bacteria the 

 undigested proteins suffer a successive cleavage into pro- 

 fceoses, peptones, and finally amino-acids. The decompo- 

 sition does not cease here but continues in two directions. 

 First of all the amino group may be split off so that simple 

 organic acids are left behind. Acetic acid is in consequence 

 formed from glycocoll, propionic acid from alanin and 

 valerianic acid from amino-valerianic acid. Succinic acid, 

 phenylpropionic acid, oxyphenylpropionic acid, and skatol- 

 acetic acid may also be found. Secondly, carbon dioxide 

 may be split off from the amino-acids with the formation of 

 such substances as cadaverin and putrescin. Through fur- 

 ther oxidation we obtain phenol, indol, and skatol. 1 These 

 in combination with sulphuric acid are excreted in the urine. 

 Their quantitative estimation in the urine has in conse- 

 quence been utilized as a test of the amount of putrefac- 

 tion going on in the alimentary tract. 



To the volatile fatty acids and aromatic compounds and to 

 certain of the gases formed in the action of the bacteria upon 

 the alimentary contents is due the odor of the faeces and 

 flatus. 



From what has been said in the preceding paragraphs it 

 might be concluded that the bacteria of the alimentary canal 

 are either always harmful or at the best of no use to the 

 host. This is, however, not the case. It has been shown 

 through the experiments of NUTTALL, and TniERFELDER 2 that 



1 ABDERHALDEN- Physiol. Chemie, Berlin, 1906, p. 184. 



2 NUTTALL andTniERFELDER: Zeitschr. f. Physiol. Chemie, 1895, 

 p. 109; ibid., 1896, XXII, p. 62. 



