::. The Act of Defecation .::

At the beginning of defecation, the subject voluntarily raises intra-abdominal pressure by muscular contraction of the quadratus lumborum, rectus abdominis, external and internal obliques, transversus abdominis and diaphragm. The puborectalis muscle surrounding the anorectal junction then relaxes so as to allow the straightening of the anorectal angle. Remember that the puborectal sling usually produces an angle of about 90 between the rectal ampulla and the anal canal, so that it is closed off. However, as it straightens, the angle increases to about 130 to 140 so the canal straightens and the feces, or in this case barium, can be evacuated. The external anal sphincter then constricts and lengthens the anal canal. It accentuates the action of the puborectalis by also pulling the anorectal junction forward anteriorly to further increase the anorectal angle. The fatty tissue of the ischiorectal fossa allows for changes in position and size of the anal canal and anus during defecation. During evacuation, the anorectal junction moves down and backwards and the pelvic floor usually descends slightly.

The internal anal sphincter, which usually closes off the upper and middle portion of the canal, relaxes to allow passage. The circular muscles of the rectum then stimulate a wave of contraction to push feces towards the anus. As feces emerges from the anus, the longitudinal muscles of the rectum and levator ani bring the anal canal back up, the feces is expelled, and the anus and rectum rise back up and forward to return to their normal position. The anal canal is closed tightly once again.

In the video, when the patient performs the squeeze maneuver, the anal canal can be seen tightly closed between the anorectal junction and the external anal orifice. Then after contraction, towards the end of defecation, the rectum is emptied. Notice how the pelvic floor has slightly descended and the anal canal is wide open. Also, notice that the anorectal angle is greater than at rest or squeezing.

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