LARGE INTESTINE 121 



dol and skatol. They represent proteid 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 av- 

 erage person is about four and a half ounces per day. 



Gases. Hydrogen, nitrogen and cafbon dioxide are found 

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

 patulous, and avoid obstruction, and also to prevent con- 

 cussion. In the large intestine bacterial activity increases the 

 number of gases present. Here, in addition to those found 

 in the small intestine, there are carbitretted and sulphuretted 

 hydrogen, with others at times. 



Movements of the Large Intestine. The muscular con- 

 tractions 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 peristal- 

 sis accumulate gradually in the sigmoid flexure until defeca- 

 tion occurs. 



Defecation. The act of defecation is both voluntary and 

 involuntary 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 the desire to 

 defecate is brought about by the entrance of the mass into it 

 from the sigmoid. 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 will, evacuation can be voluntarily 

 prevented by maintaining the tonic contraction o'f the exter- 

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

 fecal accumulation probably returns to the sigmoid, leaving 

 the rectum comparatively empty. The act of evacuation is 

 commonly aided further by voluntary contraction of the 

 diaphragm and abdominal muscles. The lungs are filled, 

 "the breath is held" (forcing down and holding the dia- 



