Is the Anatomy of External Anal Sphincter Fully Understood?

The external anal sphincter is an elliptical cylinder of striated muscle that envelops the entire length of the inner tube of smooth muscle, but it ends slightly more distal than the internal anal sphincter. Knowledge of the exact anatomy of the anal sphincter complex is essential for management of pathology in this area. Many anatomical and radiological studies of the external anal sphincter have been conducted till date and yet there is no consistent and consensual description of EAS. Various researchers have described the anatomy of external anal sphincter; some authors suggested it to be a single continuous sheet; others described it to be a bilaminar structure and still further it was suggested to be a triple loop system. However, most of the modern-day anatomy and surgery textbooks describe the three parts of the external anal sphincter as subcutaneous, superficial and deep. Milligan & Morgan [1] reported that the external anal sphincter consists of subcutaneous, superficial and profundus components. Goligher et al. [2] on the other hand, found no suggestion of division of the external anal sphincter into separate parts: the muscle was considered to be a single continuous sheet.