290 HAND-BOOK OF PHYSIOLOGY. 



monly unperceived by the mind; but they are perceptible when they are 

 accelerated under the influence of any irritant. 



The movements of the intestines are sometimes retrograde; and there 

 is no hindrance to the backward movement of the contents of the small 

 intestine. But almost ^complete security is aiforded against the passage 

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

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

 caecum (at the borders of which orifice are the folds of mucous membrane 

 which form the valve) is encircled with muscular fibres, the contraction 

 of which prevents the undue dilatation of the orifice. 



Proceeding from above downward, 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 faeces, which are gradually 

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

 nation 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 externally, and as 

 we have seen above, holds the orifice close by a constant slight tonic con- 

 traction. 



Experimental irritation of the brain or cord produces no evident or con- 

 stant effect on the movements of the intestines during li^e; yet in conse- 

 quence of certain conditions of the mind the movements are accelerated or 

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

 ened in their power, and costiveness, with a tympanitic condition, ensues. 

 Immediately after death, irritation of both the sympathetic and pneumo- 

 gastric 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 

 ganglionic plexus before referred to. 



Influence of the Nervous System on Intestinal Digestion. 

 As in the case of the oesophagus and stomach, the peVistaltic movements of 

 the intestines are directly due to reflex action through the ganglia and nerve 

 fibres distributed so abundantly in their walls (p. 255); 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 coeliac and 

 other abdominal plexuses. 



The splanchnic nerves are in relation to the intestinal movements, 

 inhibitory these movements being retarded or stopped when the splanch- 

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

 nics are also much concerned in intestinal digestion. 



