THE LARGE iNfeklNE 101 



yond this point may be freed of its contents. If this hap- 

 pens once in twenty-four hours the age of the food residues 

 discharged will evidently vary from thirty-three hours, for 

 the matter longest retained, down to nine hours, in the case 

 of that which barely came within the section evacuated. 

 There must be extremely wide departures from these fig- 

 ures in individual subjects. 



The descending colon is reported to be found empty, as a 

 rule, when observed with the #-ray. This is taken to mean 

 that it is an irritable segment which is stimulated to ad- 

 vance all that enters it to the sigmoid flexure without de- 

 lay. The sigmoid flexure, on the contrary, permits a rela- 

 tively large accumulation before it is excited to contract. 

 This region of the colon is interesting from a biologic stand- 

 point, because it seems to be an adaptation to the erect posi- 

 tion of the body. Quadrupeds hold the large intestine, 

 roughly speaking, in a horizontal plane, and with them 

 the effect of gravity on the contents is immaterial. These 

 animals have no sigmoid flexure, the descending colon in- 

 clining toward the midline, and joining the rectum without 

 the S-shaped connection. When the erect position is as- 

 sumed there will naturally be a tendency on the part of the 

 fecal material to settle toward the anus. The develop- 

 ment of the sigmoid in the apes and in man provides a 

 place of lodgment for the burden and spares the rectum 

 from a constant distention. 



The sharp bend between the sigmoid and the rectum, 

 amounting almost to a kink, such as one may see in a rub- 

 ber tube that has been doubled, is not easily passed by the 

 feces. It is. only when the quantity has become consider- 

 able that a vigorous peristalsis overcomes the resistance 

 and fills the rectum. Here for the first time the pressure 

 arouses distinct sensations. If defecation is postponed the 

 tone of the rectum may be lowered and these sensations 

 cease to be felt. If the occasion favors, the anal sphincters 

 are inhibited and the rectum is emptied by its own peris- 

 talsis, reinforced by external pressure developed through 

 contractions of the abdominal muscles and the diaphragm. 



