A new concept of the anatomy of the anal sphincter mechanism and the physiology of defecation

The role of the external and internal anal sphincters in the mechanism of anal continence is presented. The external sphincter induces continence by 1) preventing internal sphincter relaxation, what I have called the "voluntary inhibition action," and 2) mechanical compression of the rectal neck and anal canal proper. The mechanism of both actions is described. The internal sphincter plays a significant role not only in involuntary, but also in voluntary, continence. The importance of this role in the correction of anal incontinence is clarified. "Stress defecation," a condition which follows internal sphincter damage, is discussed. A "single loop continence" theory is presented, based on the fact that each of the three loops of the external sphincter has its own innervation, attachment, and direction of muscle bundles; each loop thus acts as a separate sphincter. The clinical application of this theory is presented.