CHANGES IN INTESTINES. THE FECES. 1 93 



tive ferments with them; and some of these at times work 

 through the stomach into the intestine, where they start reac- 

 tions of their own. Just what changes take place in the small 

 intestine depends on the character of the food. Following the 

 alkaline zone where the pancreatic secretion, the bile and intes- 

 tinal juice rapidly effect the changes already described, there 

 is a zone of acid or neutral reaction where certain fermentation 

 processes of bacterial origin take place. If the food is rich 

 in carbohydrates this fermentation may be considerable, result- 

 ing in the formation of appreciable quantities of lactic, butyric 

 and acetic acids. The liberation of these acids at this stage 

 is a matter of very considerable importance, since it prevents 

 the breaking up of not yet absorbed protein by bacterial putre- 

 faction. If the acids were not present bacteria would reach 

 the small intestine in enormous numbers from the large intes- 

 tine and greatly modify the conditions there. While, along 

 with the acid-forming bacteria, a few others are always pres- 

 ent in the small intestine, the real putrefactive changes do not 

 begin to a marked extent until later, when what remains of 

 the food passes down into the large intestine. Ordinarily the 

 small intestine is devoid of disagreeable odor, showing the 

 absence of putrefactive changes. 



It is evident therefore that the chemical nature of the food 

 is a factor of great importance in determining the character 

 of the complex reactions which follow the real digestive proc- 

 esses in the upper part of the intestine. Here we have nor- 

 mally the work of enzymes, and this is always followed by 

 bacterial destruction of what is left. But we must distinguish 

 between fermentation changes and putrefactive changes, the 

 former being characteristic of carbohydrate food and the lat- 

 ter of protein food. As one or the other of these pre- 

 dominates, the chemical processes taking place must vary. 

 Throughout the length of the small intestine, and in the be- 

 ginning of the large intestine active absorption takes place, 

 but between the enzymes and the bacteria a struggle for the 

 possession of the field is in progress all the time. Theoret- 

 14 



