vi INTESTINE AS AN OEGAN OF EXCEETION 369 



external defaecation. It is preceded by peristaltic movements of 

 the muscles of the large bowel, which differ from those of the 

 small in being slower and less vigorous, the muscles of the large 

 intestine being comparatively scanty in comparison with the 

 diameter of the canal, which far exceeds that of the small intestine. 

 The three longitudinal bands must shorten the canal in contract- 

 ing, and compress the sacculi, thus facilitating the propulsion of 

 the faecal masses from one saccule to another. While one of 

 these empties itself by the combined contraction of the circular 

 and the longitudinal fibres, the next dilates by the relaxation of 

 its muscles, and fills with the faecal masses expelled from the 

 preceding. 



The mechanism of the movements of the large intestine corre- 

 sponds on the whole with that of the small intestine, as shown by 

 Bayliss and Starling (1900-1), who extended their earlier work 

 (see p. 240 et seq.^) on the small to the large bowel. But while 

 antiperistaltic movements are, as we have seen, the exception 

 in the small intestine, Cannon's experiments (1901) with the 

 Rontgen rays show that they occur normally in the large bowel. 



The peristalsis of the large bowel is not a continuation of that 



of the small ; while the latter ceases at the ileocaecal valve, the 



former commences at the fundus of the caecum and extends from 



the caecum to the colon, till it reaches the end of the sigmoid 



flexure, where the faeces are supported by the bladder and sacrum. 



O'Beirne of Dublin (1833) was the first to state on the strength 

 of numerous observations that under normal conditions, i.e. when 

 they attain a certain degree of consistency, the faeces remain in 

 the sigmoid colon, and do not descend into the rectum during the 

 interval between one evacuation and the next. It is a fact that 

 on digital exploration of the rectum, even at the moment that 

 precedes evacuation, i.e. when the subject feels the need to 

 defaecate, the rectal ampulla is empty, and the exploring finger is 

 seldom smirched. Even after an operation for anal fistula in which 

 the sphincters are divided, the patient can usually retain the 

 faeces during the interval between two evacuations. These ob- 

 servations show that the faeces normally stay in the sigmoid colon, 

 supported by the bladder and sacrum, and only descend into the 

 rectum when the excessive accumulation of the faeces, and the 

 pressure these exert upon the walls of the colon, arouse the need 

 of defaecating, and reflexly exaggerate the peristaltic movements 

 of the large bowel. 



After the faecal mass has descended into the rectum, the 

 mechanical excitation of its sensory nerves still further increases 

 the desire to defaecate. 



Defaecation can be inhibited for a certain time by voluntary 

 contraction of the external sphincter and the levator ani. By 

 this mechanism it is sometimes possible to postpone the act for 



VOL. II 2 B 



