212 HUMAN PHYSIOLOGY 



without this segment of the canal and so does a man- 

 Enthusiasts have advocated its general abolition but 

 the surgical ordeal is too severe to be courted. Persons 

 who have lost the colon lose a good deal of water but no 

 substantial nutriment in the discharges. 



In the lower animals at least, the ascending colon is 

 most of the time swept by waves which run counter to 

 what we are inclined to call the normal direction. They 

 are said to be antiperistaltic. The backward thrust 

 tends to keep the food (now called so only by courtesy) 

 packed into the first part of the colon. Its return to the 

 small intestine is hindered by a combination of sphincter 

 and mechanical valve at the junction of the two divi- 

 sions. At long intervals a strong contraction of the 

 cecum and ascending colon drives a mass of the contents 

 onward as far as the left-hand half of the transverse 

 colon. Here, near the spleen, there is often a period 

 of retention. 



When this section of the tube in its turn is aroused 

 to react, a vigorous peristalsis forwards the contents to 

 another place of quiescence, the sigmoid flexure. This 

 portion of the colon is about horizontal when the human 

 body is upright. There is nothing to correspond with 

 it in the quadrupeds and it is looked upon as an adapta- 

 tion to the erect position. If the human colon were 

 curved like that of a cat any material that passed the 

 spleen might be jarred down into the rectum and dis- 

 tend it disagreeably. The sigmoid receives and bears 

 up the load. 



The sigmoid in most subjects thrusts its fecal contents 

 into the rectum about once in twenty-four hours; in 

 many cases this happens with clock-like regularity. 

 The distended rectum at once begins to develop peris- 

 taltic waves which the sphincters of the anus at first 

 resist. This strife is attended with discomfort and 

 furnishes the call to -defecate. If there is a postpone- 

 ment the rectum may relax and give no sense of fulness. 

 If, however, the time is favorable the sphincters may 

 be relaxed and the rectum emptied by its own contrac- 



