DEFECATION. 269 



upper portion. The sensation which leads to an effort to discharge the faeces 

 is due to the accumulation of matters in the sigmoid flexure, which finally 

 present at the contracted, upper portion of the rectum. This constriction, 

 situated at the most superior portion of the rectum, is sometimes called the 

 sphincter of O'Beirne. 



The above is the mechanism of the descent of faecal matter into the rec- 

 tum in defaecation, as the act is usually performed ; but under certain condi- 

 tions, faeces must accumulate in the dilated portion of the rectum. Ordina- 

 rily, the discharge of fasces takes place only after the efforts have been con- 

 tinued for a certain time, and when the evacuation is " figured," the whole 

 length discharged frequently exceeds so much the length of the rectum, that 

 it is evident that a portion of it must have come from the colon ; but in 

 cases in which the faeces are very fluid, or when the call for an evacuation 

 has not been regarded and has become imperative, the immediate discharge 

 of matters when the sphincter is relaxed shows that the rectum has been 

 more or less distended. 



In the process of defaecation, the first act is the passage, by peristaltic 

 contractions, of the contents of the sigmoid flexure of the colon through the 

 slightly constricted opening of the rectum into its dilated portion below. 

 The faecal matter, however, is not allowed to remain in this situation, but it 

 passes into the lower portion of the rectum, in obedience to the contractions 

 of its muscular coat, assisted by the action of the abdominal muscles and the 

 diaphragm. The circular fibres of the rectum undergo the ordinary peri- 

 staltic contraction ; and the action of the longitudinal fibres is to render the 

 rectum shorter and more nearly straight. The internal and the external 

 sphincters present a certain resistance to the discharge of the faeces, particu- 

 larly the external sphincter, which is a striated muscle of considerable power. 

 There is always, however, a voluntary relaxation of this muscle, or rather a 

 cessation of its semi-voluntary contraction, which immediately precedes the 

 expulsive act. The dilatation of the anus is also facilitated by the action of 

 the levator ani, which arises from the posterior surface of the 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 rec- 

 tum, and by its contractions, favors the relaxation of the sphincter and 

 draws the anus forward. 



The diaphragm and the abdominal muscles merely compress the abdom- 

 inal organs, and consequently those contained in the pelvis, and assist in the 

 expulsion of the contents of the rectum. The diaphragm is the most im- 

 portant of the voluntary muscles concerned in this process ; and during the 

 act of straining, the lungs are moderately filled and respiration is inter- 

 rupted. 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. Although more or less 



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