168 PHYSIOLOGY OF ALIMENTATION. 



strange that larger numbers of micro-organisms should also 

 live in some portions of the gut than in others. The exist- 

 ence of a greater variety of bacteria in certain portions of 

 the alimentary tract would also increase the chances for a 

 symbiotic development here. At any rate in the absence 

 of systematic observations on the effect of external con- 

 ditions upon bacteria we must be exceedingly careful in 

 accepting any explanation of their distribution throughout 

 the alimentary tract which is based upon purely vitalistic 

 conceptions. 



6. Much has been and more will be written concerning 

 the influence of the ordinary bacterial inhabitants upon the 

 general health of the host. The majority of investigators 

 seem agreed that under ordinary circumstances the bac- 

 teria of the alimentary tract (even the pathogenic which 

 are found here) do not penetrate the intestinal mucosa. 

 Under various as yet entirely unknown conditions, however, 

 these micro-organisms may pass readily through the wall 

 of the gut. This is very generally the case shortly after 

 death, and during life is perhaps the cause of certain local 

 or general infections which start from the intestinal tract. 

 Why the intestine should under certain circumstances lose 

 its power of holding back these micro-organisms is not yet 

 explained. It is not improbable, however, that the taking 

 up of bacteria by the cells of the intestinal wall is deter- 

 mined at least in part by the same circumstances which 

 determine the taking up of bacteria by the leucocytes, 

 namely, alterations in surface tension. It would not be 

 strange to find that all those conditions which determine 

 the entrance of bacteria into the intestinal mucosa are such 

 as alter the surface tension of the cells composing it. 



Even if under ordinary circumstances bacteria do not pass 

 through the walls of the alimentary tract the same cannot 

 be said of their products. Many of these are readily diffusible 

 and so are absorbed. These products of bacterial activity 

 are at times harmless, at other times intensely poisonous. 



