COMPOSITION OF THE ANTERIOR WALL OF THE EXTERNAL ANAL SPHINCTER IN FEMALE: SUPPORTING SYSTEM BY TWO MUSCLE SLINGS

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Background. The external anal sphincter (EAS) is generally recognized as the oval tube of the striated muscle, and it is well known that EAS is partly adjoined by the superficial transverse perineal muscle (STP) anteriorly and the puborectalis (PR) posteriorly. However, it is still unclear the arrangements of the muscle bundles of EAS. Aim. The aim of this study is to clarify the precise morphological structure of EAS especially the anterior region. Material and Methods. We used 19 pelvic halves from 11 female cadavers (average 81.3 years old). The muscles of the pelvic floor were dissected from the inferior aspect. Then, the perineal muscles and organs were removed from the coxal bone en bloc to dissect from the inner surfaces. Results and Discussion. The bulbospongiosus (BS) attached to the anterolateral surfaces of lower part of EAS. The anterior wall of EAS was composed of three muscle bundles, and they were arranged in parallel in order from the top: 1) anterior bundle of PR as the upper bundle; 2) STP and 3) the proper circular muscle. The STP ran on the superomedial surface of BS to run between PR and the proper circular muscle. The anterior bundle of PR and STP were connected with the same muscle bundle on the contralateral sides. The posterior bundles of PR formed a muscle sling on the posterior part of the anal canal. Conclusions. The anterior region of the anal canal is supported by the two muscle slings by the anterior bundles of PR and STP.