GASES FOUND IN THE ALIMENTARY CANAL 



237 



garded, the desire may pass away, after a little time the act becoming 

 impossible. It is probable that the feces are then passed out of the 

 rectum by antiperistaltic action. The sensation that leads to an effort 

 to discharge the feces is due to the accumulation of matters in the 

 sigmoid flexure, which finally present at the contracted upper portion 

 of the rectum. 



The above is the mechanism of the descent of fecal matter into the 

 rectum in defecation, as the act is usually performed ; but under certain 

 conditions, feces must accumulate in the dilated portion of the rectum. 

 Ordinarily, the discharge of feces takes place only after the efforts have 

 been continued for a certain time, and when the evacuation is " figured," 

 the whole length discharged frequently exceeds so much the length 

 of the rectum, that it is evident that a portion of it must have come 

 from the colon ; but in cases in which the feces are very liquid, or when 

 the usual call for an alvine evacuation has not been regarded and has 

 become imperative, the discharge of matters at the moment when the 

 sphincter is relaxed shows that the rectum has been more or less dis- 

 tended. 



The diaphragm and the abdominal muscles compress the abdominal 

 organs, and consequently those contained in the pelvis, and assist in the 

 expulsion of the contents of the rectum. The diaphragm is the most 

 important of the voluntary muscles concerned in this process ; and dur- 

 ing the act of straining, the lungs are moderately filled and respiration 

 is for the time interrupted. The vigor of these efforts depends to a 

 considerable extent on the consistence of the fecal mass, violent con- 

 tractions frequently being required for the expulsion of hardened feces 

 that have accumulated during long constipation. Although more or less 

 straining usually takes place, the contractions of the muscular coats of 

 the rectum frequently are competent of themselves to expel the feces, 

 especially when they are soft. 



GASES FOUND IN THE ALIMENTARY CANAL 



The gases in the stomach appear to have no definite office. They 

 usually exist in small quantity and are sometimes absent. The oxygen 

 and nitrogen are derived from bubbles of air incorporated with the ali- 

 mentary bolus during mastication and insalivation. The other gases 

 probably are evolved from the food during digestion ; at least, there 

 is no satisfactory evidence that they are produced in any other way. 

 Magendie and Chevreul collected and analyzed a small quantity of gas 

 from the stomach of an executed criminal a short time after death and 

 ascertained that it had the following composition : 



