New concept of the anatomy of the anal sphincter mechanism and the physiology of defecation. II. Anatomy of the levator ani muscle with special reference to puborectalis.

A study of the surgical anatomy of the levator ani muscles, with special stress on the puborectalis, was performed on 22 cadaveric specimens. The study comprised dissection and microscopic examination. The levator ani was found to consist of two portions only: pubococcygeus and iliococcygeus, the puborectalis being a part of the external anal sphincter. Both levatores decussate at the anococcygeal raphe, which represents a "decussation line" and not a site of insertion for the muscle fibers. A "digastric" pattern of the levator is demonstrated, which is responsible for the harmonic nature of the function of the muscle bundles on each side of the pelvis. The levator hiatus was found to be formed of the medial borders of the pubococcygeus, , and not the puborectalis. A "hiatal ligament" was identified, stretched between the edges of the levator hiatus and the intrahiatal viscera. The role of the pubococcygeus in anal fixation is discussed, and a new concept that the puborectalis does not belong to the levator ani but constitutes na integral portion of the external and sphincter. The puborectalis and the deep external anal sphincter were found to be fused together and identical from the morphologic, histologic, and functional points of view as well as with respect to innervation. Both form a single U-shaped loop which is given the name "top loop."