DEFECATION 



397 



EXAMPLE 2 



Normally in man the rectal stimulus gives rise to the consciousness of 

 the desire to defecate and to the initiation of efferent nerve impulses that may 

 increase the contraction of the external sphincter and inhibit the act tempo- 

 rarily. During opening of the bowels however, the voluntary effort leads to 

 relaxation of the external sphincter, and the normal peristalsis of the rectum 

 is suppported by contractions of the abdominal musculature so as greatly 

 to increase the abominal pressure, thus aiding the involuntary reflex which 

 controls the relaxation of 



the internal sphincter and EXAMPLE i 



the contractions of the 

 colon, and rectum. 



The Time of Passage 

 of Food Through the 

 Alimentary Canal. It 

 is important to under- 

 stand the rapidity with 

 which food passes along 

 the different divisions of 

 the alimentary tube. 



The effectiveness of 

 digestion, the dangers 

 from fermentation and 

 putrefaction, and the ills 

 that follow the absorption 

 of toxic by-products are 

 largely dependent upon 

 the time element. 



At the beginning of a 

 meal the activity of the 

 entire alimentary canal is 



low, and one may assume that the various muscular mechanisms are re- 

 latively relaxed. When food is swallowed, immediately small portions 

 begin to pass through the stomach and pyloric sphincter. However, the 

 main portion of the food passes out of the stomach as it undergoes digestion 

 and solution during the succeeding 4 or 5 hours. Hertz, who has given us 

 the most accurate picture of the elements in this process allows 4^ hours 

 for the passage of food along the 22 feet of the small intestine to the iliac 

 sphincter. This he has determined by X-ray photographs after a bismuth 

 meal. This is a comparatively rapid passage averaging about 2\ cm. per 

 minute. The rate is much slower in the large intestine. He allows an 

 average speed of 2 hours each for the ascending, transverse, and descending 

 colon. In the short lengths of the iliac and pelvic colons an equal or even 

 greater time is consumed. Thus he would allow an average of eighteen 



Immediately after 



FIG. 2j6a. The upper pair of figures show the 

 bismuth shadows for normal defecation, the lower 

 after the use of magnesium sulphate. (From Hertz, 

 Cook, and Schlesinger.) 



