The AiA art piece shows the most distal part of the colon derived from rendered from a barium enema which uses X-ray technique.  The rectum (Latin straight) in the lateral projection is far from straight.  It conforms to the shape of the sacrum making a gentle “C” shaped curve.  At the anorectal junction it makes a backward turn, and at the point of turning it is called the anorectal flexure.

The puborectalis muscle forms a sling around the distal rectum and functions in the evacuation process. The puborectalis muscle acts as a sling around the anorectal junction.  It is normally contracted so that it pulls the anorectal junction toward the pubis resulting in a kink in the junction which helps to maintain continence by acting as a valve with both involuntary and voluntary action.  In this image the muscle is relaxed and the anorectal angle is thus reduced allowing the act of defecation.

Related categories: colon, shape, valve, X-ray, muscle, function physiology