CYCLE OF CARHOX 85 



The animals that can digest cellulose most completely are 

 those in which the food is retained in the body for a long 

 period of time. Jn the case of the cow and the sheep this 

 extends to six or seven days. In the laboratory the decom- 

 position of cellulose is slow, hut the conditions are not so 

 favorable as in the animal, where the temperature and 

 moisture conditions are at the optimum: the constant re- 

 moval of the by-products is again of the utmost importance 

 in determining the rate at which any biological process will 

 be maintained. 



Whether this is the only service to the animal that is 

 rendered by the immense numbers of microorganisms grow- 

 ing in the intestinal tract of animals may be doubted. 

 Many experiments have been made in the hope of deter- 

 mining whether the life of the higher animals would be pos- 

 sible without the presence of bacteria in the alimentary 

 tract. It is very difficult to maintain an animal in a per- 

 IVctly sterile condition and still keep it otherwise normal. 

 The general conclusion to be drawn from such experiments 

 as have been most successful is that, while the bacteria may 

 not be necessary for the life of the higher animals, they are 

 of great importance in aiding the animal to utilize its food, 

 and that probably such action is not confined to the cel- 

 luloses. The relation existing between the animal and the 

 bacteria of the intestinal tract is one in which the animal 

 is deriving some benefit. There is a more or less character- 

 istic flora for each kind of animal. If this flora is replaced 

 by an abnormal one, the helpful relation may be changed 

 to one in which the animal is injured. It is believed that 

 the condition known as autointoxication in man is due to 

 the replacement of the normal acid-producing flora by one 

 that acts primarily on proteins, with the production of 

 poisonous substances that are absorbed from the alimentary 

 tract and exert a cumulative effect on the animal. Metchni- 



