A new concept of the anatomy of the anal sphincter mechanism and the physiology of defecation. III. The longitudinal anal muscle: anatomy and role in anal sphincter mechanism.

A study of the longitudinal anal muscle was performed in 16 cadaveric specimens. The study comprised dissection and microscopic examination. The bundles of the longitudinal were found arranged in three layers: medial, intermediate, and lateral; each has a different origin and is separated from the other by a fascial septum. Four fascial septa related to the longitudinal muscle could be identified. They split and decussate below the lower end of the longitudinal muscle to form the "central tendon." The central tendon lies between the base loop of the external anal sphincter and the longitudinal muscle. It gives rise to multiple small fibrous septa in different directions; those which penetrate the base loop split and decussate to form the corrugator ani cutis. A mechanism of action of the corrugator is presented. The role of the longitudinal muscle in the anal sphincter mechanism and during defecation is discussed. The muscle plays its major role during defecation. The part played by the muscle in anal fixation is considered. It helps to fix the anal canal to the side wall of the pelvis during defecation, thus preventing anal prolapse.