32 INFECTIONS OF THE DIGESTIVE TRACT 



rule, but they do not alter the general applicability of the 

 statement that putrefactive microorganisms are com- 

 monly few in the upper two thirds of the small intestine. 

 The chief factors in bringing about this state of relative 

 freedom from anaerobic organisms are apparently the 

 defensive action of the stomach juices and the rapid 

 passage of the chyme through the upper intestine, which 

 offers little chance for the multiplication of anaerobes, 

 or indeed of bacteria of any kind. To what extent the 

 moderate amount of oxygen present inhibits bacterial 

 growth is uncertain. 



In the human ileum, the character of the bacterial 

 decompositions in healthy persons appears to differ 

 rather widely according to a variety of conditions, some 

 of which are still obscure. Perhaps on account of the 

 approach to the ileocsecal valve, which offers some 

 degree of mechanical difficulty to the passage of the con- 

 tents of the small intestine, there is an accumulation of 

 bacteria in the ileum. This mechanical obstacle, in the 

 human intestine and in that of many lower animals 

 (e.g. dog), consists partly in the narrowing of the lumen 

 of the gut, but partly also in the presence of inspissated 

 intestinal contents on the further side of the valve, in 

 the colon itself. In man there is within a foot or two 

 of the colon a marked increase in the number of bacteria 

 in the ileum, and this increase relates not merely to the 

 total number of bacteria present, but also to the varieties. 

 The mere accumulation of material affords opportunity 

 for anaerobic conditions of life, and we find in fact that 

 the numbers of strict anaerobes in the lower part of the 



