-
Evacuation is initiated by straining which
raises intraabdominal pressure. It begins with several centimeters of
descent of the pelvic floor. This is followed by relaxation of the puborectalis muscle and opening of the anal canal.
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As the process of
evacuation continues, the anorectum changes from a funnel shape to an
almost straight tube, with practically complete loss of the anorectal
angle.
-
During evacuation, defecography normally demonstrates an increase
in the anorectal angle, a descent of the anorectal junction, and a
widening of the anal canal.
-
Normal subjects are able to eliminate more
than two thirds of the contrast material within 30 sec. Complete
emptying of the rectum is seen in only one half of patients.
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