LARGE INTESTINE 121 



the most important are indol and skatol. They represent pro- 

 teid putrefaction; they are responsible for the fecal odor; hence 

 the characteristic difference in the odor of the contents of the 

 ileum and colon. The reaction of fecal matter varies. The 

 amount for the average person is about four and a half ounces 

 per day. 



Gases. Hydrogen, nitrogen and carbon dioxide are found 

 normally in the small intestines. They serve to keep the tube 

 patulous, and avoid obstruction, and also to prevent concussion. 

 In the large intestine bacterial activity increases the number of 

 gases present. Here, in addition to those found in the small 

 intestine, there are carbureted and sulphuretted hydrogen, with 

 others at times. 



Movements of the Large Intestine. The muscular contrac- 

 tions of the colon forcing the feces onward are of the same general 

 character as those of the small intestine, though less violent. 

 The contents thus passed analward by peristalsis accumulate 

 gradually in the sigmoid flexure until defecation occurs. 



Defecation. The act of defecation is both voluntary and in- 

 voluntary voluntary in the relaxation of the external sphincter 

 and involuntary in the peristalsis which brings the fecal matter 

 to present at that muscle. It is probable that the rectal pouch 

 does not usually contain feces, but that the desire to defecate is 

 brought about by the entrance of the mass into it from the sig- 

 moid. Then, if the desire is obeyed, peristalsis of the non- 

 striated muscular coat continues, the internal sphincter of plain 

 muscle relaxes, as does also the external of striped muscle, and 

 evacuation takes place. 



Usually, by an effort of the will, evacuation can be voluntarily 

 prevented by maintaining the tonic contraction of the external 

 sphincter. If the desire to defecate be disregarded, the fecal 

 accumulation probably returns to the sigmoid, leaving the rec- 

 tum comparatively empty. The act of evacuation is commonly 

 aided further by voluntary contraction of the diaphragm and 



