404 HANDBOOK OF PHYSIOLOGY. 



communication between the ileum and caecum (at the borders of which 

 orifice are the folds of mucous membrane which form the valve) is en- 

 circled 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 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 ex- 

 ternally, and as we have seen above, holds the orifice close by a con- 

 stant 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. Stimulation of pneumo-gastric nerves, if not too strong, induces 

 genuine peristaltic movements of the intestines. Violent irritation 

 stops the movements. 



Influence of the Nervous System on Intestinal Digestion. 



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

 ments of the intestines may be directly set up in the muscular fibres by 

 the presence of chyme acting as the stimulus. Few or no movements 

 occur when the intestines are empty. The intestines are connected 

 with the central nervous system both by the vagi and by the splanchnic 

 nerves, as well as by other branches of the sympathetic which come to 

 them from the cceliac and other abdominal plexuses. 



The relations of these nerves respectively to the movements of the 

 intestine and the secretions are probably the same as in the case of the 

 stomach already treated of. 



Duration of Intestinal Digestion. The time occupied by the journey 

 of a given portion of food from the stomach to the anus varies consid- 

 erably even in health, and on this account probably it is that such dif- 

 ferent opinions have been expressed in regard to the subject. About 

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

 the small intestine, and twenty-four to thirty-six hours by the passage 

 through the large bowel. 



The contents of the large intestine, as they proceed toward the rec- 

 tum, become more and more solid, and losing their more liquid and 

 nutrient parts, gradually acquire the odor and consistence characteristic 



