130 THE HUMAN MECHANISM 



(sigmoid flexure) between the descending colon and the 

 rectum. The ascending colon is always filled, while the rest 

 of the tube may be empty. It is chiefly in this first part of 

 the tube that the abstraction of water occurs. When, as the 

 result of the discharge of new material from the small intes- 

 tine into the large, the ascending colon becomes distended, 

 some of its contents are pushed into the transverse colon, 

 and this material is rather rapidly passed by peristalsis 

 through the descending colon, in the lower part of which it 

 accumulates, being prevented from entering the rectum by 

 the sigmoid flexure. Finally, with sufficient accumulation of 

 this more solid material at the sigmoid flexure, stronger peri- 

 staltic contractions move the mass on into the rectum, which 

 thereby becomes distended, and this gives the desire to empty 

 the bowels. From this it will be seen why the bowels are 

 more readily emptied after meals. It is also highly advisable 

 to empty the bowels when this desire comes on, since other- 

 wise the distending stimulus loses its effectiveness and the 

 continued abstraction of water hardens the feces. 



35. Microbic life in the intestine. Occurring simultaneously 

 with the chemical changes produced by the digestive juices 

 are others produced by microbes (Part II), which are always 

 found in the intestine in large quantities. The acidity of the 

 gastric juice keeps down the numbers of these germs in the 

 stomach and, under healthy conditions, greatly limits their 

 activity in that organ. We have seen, however, that some 

 portions of the contents of the stomach are not acid in reac- 

 tion during certain periods of digestion, and it not infre- 

 quently happens for this reason that unhealthy living and, 

 especially, improper feeding may result in serious gastric 

 indigestion with excessive bacterial decomposition of the food. 

 The production of gas, leading to flatulence or belching, is 

 one of the most familiar results of such bacterial action. 



In the intestine the less strongly acid (or even neutral or 

 slightly alkaline) reaction is much more favorable to bacterial 



