A new concept of the anatomy of the anal sphincter mechanism and the physiology of defecation. The external anal sphincter: a triple-loop system.

Since anal sphincters are used as the continent sphincters in some urologic operations, a study of their anatomic structure and function seems necessary. The anatomy of the external anal sphincter has been studied in 18 cadavers by dissection and serial histologic sections. The muscle has been found to consist of a series of U-shaped loops which are distinguishible as three main "loops": top, intermediate, and base. The puborectalis and the deep portion of the external sphincter have been found to be one muscle which is given the name "top loop." No concentric circular muscle bundles could be detected at any level of the external sphincter except in the base loop. A new concept of the mechanism of action of the external sphincter in anal continence and during defecation is presented. An air-tight occlusion of the anal canal could be achieved by the "triple-loop system" of the external sphincter which compresses opposed alternating anal segments. An incomplete anal occlusion by a single loop contraction is completed and potentiated by the succeeding loop action. The last fecal portion is dispelled from the anal canal by a process of "vermicular contractions" which is the result of the loop arrangement of the muscle bundles. Single-loop continence has been discussed. It is suggested that unless all three of the loops are destroyed, any single loop can act as a sphincter which maintains continence to solid stools but not to fluid ones or flatus.