562 Tilt MECHANICAL PROCESSES OF DIGESTION [CH. XXXVII. 



certainly mix up the csecal contents very thoroughly. They have, 

 however, only been seen in the exposed intestine of animals, and 

 therefore may be artificially produced. A study of X-ray shadows 

 does not reveal their existence in man. If retro-peristalsis does 

 occur, regurgitation is effectually prevented into the small intestine 

 partly by the ileo- csecal valve, and mainly by a strong band of 

 circular muscular fibres called the ileo-caecal sphincter; this is 

 normally kept in a state of tonic contraction by impulses carried by 

 the splanchnic nerve ; it is relaxed when this nerve is cut, and then 

 the contents of the two intestines mix freely. (T. E. Elliott.) 



Defcecation. The rectum is a short tube about 4 or 5 inches long 

 in man, which is normally empty until immediately before defaeca- 

 tion. In a person of regular habits, a glass of cold water on rising, 

 the stimulus of a cold bath, the taking of breakfast, and the after- 

 breakfast pipe or cigarette combine to produce peristalsis of the 

 colon, so that a small quantity of faeces enters the rectum, and then 

 arises the desire to defaecate. At the end of the rectum is the anal 

 canal, closed by a strong internal sphincter (a thickening of the 

 involuntary circular fibres of the muscular coat), and by the external 

 sphincter, which is a voluntary muscle made of transversely striated 

 fibres. 



The "call to defecation" having been thus produced, the act 

 itself is started by the increase in intra-abdominal pressure brought 

 about by the voluntary contraction of the abdominal wall, the 

 diaphragm and the levator ani. The diaphragm is kept down by deep 

 inspirations, followed by closure of the glottis; this depresses the 

 colon, so that the shadow of its transverse portion arid the flexures 

 may be lowered as much as 2 inches. The transverse colon may 

 not rise to its normal position until even an hour has elapsed 

 from the act of straining during defaecation. Accompanying the 

 action of these voluntary muscles, the whole colon from the caecum 

 onwards enters into powerful peristalsis; the contents of the 

 transverse colon are thus forced into the descending colon, from 

 which they are evacuated together with the faeces already present 

 between the splenic flexure and the anus. The entrance of more 

 faeces into the rectum until they reach the anal canal irritates afferent 

 nerves in the wall of the rectum ; the nerve impulses so generated 

 pass to a centre or centres in the lumbo-sacral region of the spinal 

 cord, where efferent impulses are set in action upon which depend 

 the reflex acts required to complete the process ; these are : 



1. Strong peristalsis of the whole colon. 



2. Continued contraction of the abdominal muscles. 



3. Eelaxation of both the anal sphincters and of the levator ani. 

 The last traces of faeces are expelled by voluntary contractions of 



the levator ani. 



