Familiarity with the basic steps of evacuation is essential for accurate interpretation of cinede-fecography. As shown in Figure 9.1, the process begins with migration of stool into the rectum. The increased rectal volume leads to stimulation of pressure receptors located on the puborectalis muscle and in the pelvic floor muscles, which in turn stimulate the rectoanal inhibitory reflex (RAIR). The RAIR consists of external anal sphincter contraction and internal anal sphincter relaxation, allowing sampling of anal canal contents. When the anal canal is deemed to have solid contents and a decision is made to evacuate, the glottis closes, pelvic floor muscles contract, and the diaphragm and abdominal wall muscles contract—all of which increase abdominal pressure. The puborectalis muscle relaxes, resulting in straightening of the anorectal angle, and the pelvic floor descends slightly. The external anal sphincter relaxes and anal canal
contents are evacuated. Upon normal complete evacuation, the pelvic floor rises and sphincters contract once more in a "closing reflex."
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