130 MANUAL OF PHYSIOLOGY. 



consist of an increased peristaltic contraction of the colon and 

 siginoid 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 abdom- 

 inal pressure required, depend upon the consistence of the faeces. 

 When quite fluid, they constantly tend to come in contact with 

 the sensitive point of the rectum, and a voluntary effort is required 

 to prevent the reflex series of events from taking place, a momen- 

 tary relaxation of the sphincter with voluntary abdominal pres- 

 sure 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 

 removed 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 

 points. One of these, a closely meshed plexus with flattened 

 cords and ganglionic masses at their points of union, lies between 

 the longitudinal and circular layers of muscle (Figs. 56, 57), 

 forming the plexus myenterieus exterior of Auerbach, and most 

 likely has to do with the movements of these layers of muscle. 

 The other lies internal to the circular muscle, in close relation to 

 the muscularis mucosse, and is called the plexus myenterieus inte- 



