CHEMISTRY OF DIGESTION AND NUTRITION. 249 



diet is used, the acidity being due to organic acids (acetic) and being equal to 

 0.1 per cent, acetic acid. These acids must have come from the bacterial fer- 

 mentation of the carbohydrates, and a number of bacteria capable of producing 

 such fermentation were isolated. The products of bacterial putrefaction of the 

 proteids, on the contrary, are absent, and it has been suggested that the acid 

 reaction produced by the fermentation of the carbohydrates serves the useful 

 purpose, under normal conditions, of preventing the putrefaction of the pro- 

 teids. With reference, therefore, to the point we are discussing namely, the 

 bacterial decomposition of the contents of the intestines we may conclude, 

 upon the evidence furnished by these two cases, that in the human being, when 

 living on a mixed diet, some of the carbohydrates undergo bacterial decompo- 

 sition 'in the small intestine, but that the proteids are protected. We may 

 further suppose that in the case of the proteids the limits of protection are 

 easily overstepped, and that such a condition as a large excess of proteid in the 

 diet or a deficient absorption from the small intestine may easily lead to exten- 

 sive intestinal putrefaction involving the proteids as well as the carbohydrates. 

 In the large intestine, on the contrary, the alkaline reaction of the secretion 

 is more than sufficient to neutralize the organic acids arising from fermentation 

 of the carbohydrates, and the reaction of the contents is therefore alkaline. 

 Here, then, what remains of the proteids undergoes, or may undergo, putrefac- 

 tion, and this process must be looked upon as a normal occurrence in the large 

 intestine. The extent of the bacterial action upon the proteids as well as the 

 carbohydrates may vary widely even within the limits of health, and if excessive 

 may lead to intestinal troubles. Among the products formed in this way, the 

 following are known to occur : Leucin, tyrosin, and other amido-acids ; indol ; 

 skatol; phenols; various members of the fatty-acid series, such as lactic, 

 butyric, and caproic acids ; sulphuretted hydrogen ; methane ; hydrogen ; 

 methyl mercaptan, etc. Some of these products will be described more fully 

 in treating of the composition of the feces. To what extent these products 

 are of value to the body it is difficult, with our imperfect knowledge, to say. 

 It has been pointed out, on the one hand, that some of them (skatol, fatty 

 acids, CO 2 , CH 4 , and H 2 S) promote the movements of the intestine, and may 

 be of value from this standpoint ; on the other hand, some of them are 

 absorbed into the blood, to be eliminated again in different form in the urine 

 (indol and phenols), and it may be that they are of importance in the metab- 

 olism of the body ; but concerning this our knowledge is deficient. On the 

 whole, we must believe that the food in its passage through the alimentary 

 canal is acted upon mainly by the digestive enzymes, the so-called " unorgan- 

 ized " ferments, but that the action of the bacteria, or organized ferments, is 

 responsible for a part of the changes which the food undergoes before its final 

 elimination in the form of feces. These two kinds of action vary greatly 

 within normal limits, and to a certain extent they seem to be in inverse 

 relationship to each other. When the digestive enzymes and secretions are 

 deficient or ineffective the field of action for the bacteria is increased, and this 

 .seems to be the case in some pathological conditions, the result being intes- 



