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The external anal sphincter (or sphincter ani externus ) is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the skin surrounding the margin of the anus. ==Anatomy== It measures about 8 to 10 cm in length, from its anterior to its posterior extremity, and is about 2.5 cm opposite the anus, when defecation occurs the sphincter muscle retracts. It consists of two strata, superficial and deep. * The ''superficial'', constituting the main portion of the muscle, arises from a narrow tendinous band, the anococcygeal raphe, which stretches from the tip of the coccyx to the posterior margin of the anus; it forms two flattened planes of muscular tissue, which encircle the anus and meet in front to be inserted into the central tendinous point of the perineum, joining with the Transversus perinæi superficialis, the Levator ani, and the Bulbocavernosus. * The ''deeper'' portion forms a complete sphincter to the anal canal. Its fibers surround the canal, closely applied to the internal anal sphincter, and in front blend with the other muscles at the central point of the perineum. In a considerable proportion of cases the fibers decussate in front of the anus, and are continuous with the Transversi perinæi superficiales. Posteriorly, they are not attached to the coccyx, but are continuous with those of the opposite side behind the anal canal. The upper edge of the muscle is ill-defined, since fibers are given off from it to join the Levator ani. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「External anal sphincter」の詳細全文を読む スポンサード リンク
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