ALIMENTARY TRACT AS AN EXCRETORY SYSTEM. 321 



The acidity was due to organic acids. These acids no doubt 

 inhibit the development of those bacteria which are capable 

 of producing putrefaction of the protein substances present 

 in the small intestine. For this reason no protein putrefac- 

 tion occurs here under normal circumstances, or only rarely. 



If what has been said above concerning the reaction of 

 the intestinal contents is true, then how are we to reconcile 

 it with the observations that have been made on various fer- 

 ments and their behavior toward acids and alkalies? Alkali- 

 pro teinase, for example, is generally stated to act best in an 

 alkaline medium. In the body of various animals, including 

 man, this ferment must however work for the most part in 

 an acid, at the best in a neutral medium. Evidently, there- 

 fore, alkali-pro teinase does not work under the most favorable 

 conditions, so far as reaction of surrounding medium is con- 

 cerned, in the body; or else the statements of the various 

 students of the effect of acids and alkalies on alkali-pro teinase 

 must be revised. So far as the former of these possibilities is 

 concerned, we know that alkali-proteinase will act in neutral 

 or acid media, even very powerfully in neutral media. So far 

 as the latter possibility is concerned, it is entirely probable 

 that the use of indicators better adapted to the investigation 

 of the problem will show that the alkalinity in which al- 

 kali-proteinase acts best is lower than we formerly supposed, 

 and that some of the solutions in which alkali-proteinase is 

 known to act very well and which are ordinarily stated to 

 be alkaline are really neutral or even acid in reaction. Under 

 normal circumstances, therefore, alkali-proteinase may still 

 be considered as acting in the intestine under only slightly 

 unfavorable circumstances. 



What has been said of alkali-proteinase holds also for the 

 other enzymes. Unquestionably the acid reaction of the 

 intestinal contents may be of service to certain of the fer- 

 ments, such, for example, as the acid-proteinase of the 

 stomach and duodenum. Finally, even if the acid reaction 

 of the contents of the small intestine reduces the activity 



