The riddle of the sphincters. The morphophysiology of the anorectal mechanism reviewed.

Although easily taken for granted when functioning properly, malfunction of the anorectal mechanism presents profound inconvenience and embarrassment. The anal sphincters, internal and external, have been the subjects of extensive study yet much appears to remain unsettled. Each sphincter has been proposed, at one time or another, to be the sine qua non of fecal continence, and both clinical and nonclinical studies have often yielded contradictory results. For example, it has been established that the internal sphincter is actually relaxed during the moments of greatest need! On the other hand, the external sphincter can be fully contracted only for a matter of seconds before becoming fatigued. Anatomic studies of the external sphincter date from 1715, yet it has defied consistent and consensual description. A rather persistent theme has been the claimed existence of anatomic subdivisions (e.g., deep, superficial, and subcutaneous). Some form of this traditional description is found in all modern anatomic texts; however, when dissecting, the student is invariably disappointed in his or her efforts to discern the subdivisions. Further, the roles of other important, nonsphincteric mechanisms are often overlooked. This presentation will briefly review the controversial history of the anorectal mechanism and correlate the results of both clinical and anatomic/physiologic studies. By so doing, a functional concept of the structure of the anorectal mechanism emerges.