DEFECATION. 409 



body and ramus of the pubis, the inner surface of the spine 

 of the ischium, and a line of fascia between these two points, 

 passes downward and is inserted into the median raphe of the 

 perineum and the sides of the rectum, the fibres uniting with 

 those of the sphincter. While this muscle forms a support 

 for the pelvic organs during the act of straining, it steadies 

 the end of the rectum, and by its contractions, favors the re- 

 laxation of the sphincter, and draws the anus forward. 



The action of the diaphragm and the abdominal muscles is 

 very simple. They merely compress the abdominal organs 

 and consequently those contained in the pelvis, and assist in 

 the expulsion of the contents of the rectum. The diaphragm 

 is the most important of the voluntary muscles concerned in 

 this process ; and during the act of straining, the lungs are 

 moderately filled and respiration is interrupted. The vigor 

 of these efforts depends greatly upon the consistence of the 

 faecal mass, very violent contractions being frequently required 

 for the expulsion of hardened faeces after long constipation. 

 Though more or less straining generally takes place, the con- 

 tractions of the muscular coats of the rectum are frequently 

 competent of themselves to expel the fasces, especially when 

 they are soft. This can be shown by arresting all voluntary 

 muscular action during an easy act of defecation, when the 

 fasces may be passed by contractions of the rectum alone. 



By a combination of the movements above described, the 

 floor of the perineum is pressed outward, the anus is di- 

 lated, the sharp bend in the lower part of the rectum is 

 brought more into line with the rest of the canal, and a por- 

 tion of the contents of the rectum is expelled. Yery soon, 

 however, the passage of faeces is interrupted by a contraction 

 of the levator ani and the sphincter, by which the anus is 

 suddenly and rather forcibly retracted. This muscular 

 action may be effected voluntarily ; but after the sphincter 

 has been dilated for a time, the evacuation is interrupted in 

 this way, notwithstanding all efforts to oppose it. After a 

 time, another portion of faeces is discharged, until the matters 



