THE CHEMISTRY OF DIGESTION 467 



neither particularly harmful nor beneficial. The fermentation 

 products are probably absorbed and used by the Body, but they 

 would be used equally well if absorbed as sugar without fermenta- 

 tion. In the case of one particular carbohydrate, however, cellu- 

 lose, bacterial fermentation affords the only means by which it 

 can be made available in man for the use of the Body. It seems 

 to be well established that tender cellulose, such as is eaten in 

 lettuce, for example, may be digested by bacteria to a considerable 

 extent; where it is less tender, as in most fruits and vegetables, it 

 remains, as stated earlier, practically undigested. 



Intestinal fermentation is not essential to health as is shown 

 by the possibility of living normally in arctic regions, where, it is 

 said, intestinal bacteria are sometimes wholly wanting. When 

 the fermentation becomes excessive intestinal disturbances may 

 readily result. The production of fermentation acids in too great 

 concentration leads to irritation of the intestinal wall and causes 

 diarrhea. 



In the large intestine the bacterial action is chiefly putrefaction 

 of proteins, rather than fermentation of carbohydrates. The 

 difference is not due to the presence of different species of bac- 

 teria, but to the different nature of the available food. Where 

 carbohydrate is present in excess, as in the small intestine, fer- 

 mentation is the normal action. By the time the intestinal con- 

 tents reach the large intestines the digestible carbohydrates are, 

 as we shall learn (p. 500), all absorbed out into the blood. There 

 remains, however, a portion of the protein, including all indigestible 

 meat fragments. In this environment, largely protein, the normal 

 bacterial action is of the nature of putrefaction. The character- 

 istic features of the contents of the large intestine are the results 

 of this putrefaction. In connection with it various toxic substances 

 are formed which may be^-absorbed from the intestine into the 

 blood. The symptoms of heaviness and general ill-feeling that 

 frequently accompany sluggishness of the large intestine are to 

 be referred to the presence of these toxins in the blood. The con- 

 dition is known as autointoxication. The obvious method of 

 avoiding this condition is by using care that material shall not 

 stagnate in the colon. 



The Prevention of Self -Digestion. A question of much in- 

 terest to physiologists has been why the stomach and intestinal 



