INTESTINAL NERVE-MECHANISM. 121 



tary expiratory effort seldom requires to be continued for more 

 than three or four seconds before some fecal matter reaches the 

 part of the rectum just above the sphincter. When this has oc- 

 curred, no further abdominal pressure is necessary (except when 

 the masses of faeces are large and hard), for the local stimulus 

 starts a series of reflex acts which carry on the operation. These 

 consist of an increased peristaltic contraction of the colon and 

 sigmoid flexure, the waves of which pass along the rectum. These 

 waves are accompanied by synchronous rhythmical relaxation of 

 the sphincter, which replaces its normal condition of tonic con- 

 traction. 



The effect of the voluntary effort, and the amount of the ab- 

 dominal pressure required, depend upon the consistence of the 

 faeces. When quite fluid, they constantly tend to come in con- 

 tact with the sensitive point of the rectum, and a voluntary effort 

 is required to prevent the reflex series of events from taking 

 place, a momentary relaxation of the sphincter with voluntary 

 abdominal pressure is sufficient to eject the contents of the bowel. 

 On the other hand, when the faeces are firm, time is required in 

 order that the slowly acting smooth muscle may pass the mass 

 onward. In common constipation, the difficulty is to get the 

 solid mass down to the sensitive exciting point, in which case a 

 few drachms of warm fluid, used as an enema, may awaken the 

 necessary reflex movements. 



Nervous Mechanism of the Intestinal Motion. 



Many points in the nervous control exerted over the intestinal 

 muscles are obscure. We know that intestinal movements which 

 are peristaltic in their nature occur in a portion of intestine re- 

 moved from the body, and thus separated from all central nerv- 

 ous control. We know also that there are abundant nerve ele- 

 ments in the walls of the intestines which have all the characters 

 of ganglion cells, and therefore probably act as nerve centres. 

 (Figs. 56, 57.) 



With regard to these local nervous agencies, anatomists have 

 made out two distinct sets, both of which have the form of a net- 

 work of nerve fibrils studded with cell elements at their nodal 



