294 HANDBOOK OF PHYSIOLOGY. 



of the contents of the large into the small intestine by the ileo-csecal 

 valve. Besides, the orifice of communication between the ileum and 

 caecum (at the borders of which orifice are the folds of mucous mem- 

 brane which form the valve) is encircled with muscular fibres, the con- 

 traction of which prevents the undue dilatation of the orifice. 



Proceeding from above downwards, the muscular fibres of the large 

 intestine become, on the whole, stronger in direct proportion to the 

 greater strength required for the onward moving of the fasces, which are 

 gradually becoming firmer. The greatest strength is in the rectum, at 

 the termination of which the circular unstriped muscular fibres form a 

 strong band called the internal sphincter ; while an external sphincter 

 muscle with striped fibres is placed rather lower down, and more exter- 

 nally, and as we have seen above, holds the orifice close by a constant 

 slight tonic contraction. 



Experimental irritation of the brain or cord produces no evident or 

 constant effect on the movements of the intestines during life ; yet in 

 consequence of certain mental conditions the movements are accelerated 

 or retarded ; and in paraplegia the intestines appear after a time much 

 weakened in their power, and costiveness, with a tympanitic condition, 

 ensues. Immediately after death, irritation of both the sympathetic and 

 pneumogastric nerves, if not too strong, induces genuine peristaltic 

 movements of the intestines. Violent irritation stops the movements. 

 These stimuli act, no doubt, not directly on the muscular tissue of the 

 intestine, but on the gan-glionic plexus before referred to. 



Influence of the Nervous System on Intestinal Digestion. 



As in the case of the oesophagus and stomach, the peristaltic move- 

 ments of the intestines are directly due to reflex action through the gang- 

 lia and nerve fibres distributed so abundantly in their walls (p. 258) ; 

 the presence of chyme acting as the stimulus, and few or no movements 

 occurring when the intestines are empty. The intestines are, moreover, 

 connected with the higher nerve-centres by the splanchnic nerves, as 

 well as other branches of the sympathetic which come to them from the 

 cceliac and other abdominal plexuses. 



The splanchnic nerves are in relation to the intestinal movements, 

 inhibitory these movements being retarded or stopped when the 

 splanchnics are irritated. As the vaso-motor nerves of the intestines, 

 the splanchnics are also much concerned in intestinal digestion. 



Duration of Intestinal Digestive Period. The time occupied by 

 the journey of a given portion of food from the stomach to the anus, 

 varies considerably even in health, and on this account probably it is- 

 that such different opinions have been expresed in regard to the subject. 

 About twelve hours are occupied by the journey of an ordinary meal 



