The sphincterani externus is a transverse muscle that surrounds the anal canal and has the function of sphinctering the anus. It can be divided into subcutaneous, superficial and deep parts. (a) The subcutaneous part is located under the skin around the anus, and is a circular muscle bundle that surrounds the lower part of the anal canal. It is attached anteriorly to the central tendon of the perineum and posteriorly to the caudal anal ligament. The caudal ligament is a bundle of connective tissue located between the tip of the tailbone and the anus. The role of the subcutaneous sphincter is not large and, after injury, does not cause fecal incontinence. (b) The superficial part is located on the deeper side of the lower part, outside of the deep part, starting from the caudal bone and the caudal anal ligament and ending forward at the central tendon of the perineum. (c) The deep part is located on the deep side of the superficial part, a circular bundle of muscles surrounding the anal sphincter. The shallow and deep parts of the external anal sphincter, the longitudinal muscle of the lower rectum and the circular muscle thicken to form the internal anal sphincter and the puborectal muscle fibers of the anal raphe together to form a muscular ring around the junction of the anal canal and rectum, called the anorectal ring, which has an important role in sphinctering the anus and controlling defecation, and can lead to fecal incontinence if inadvertently damaged during surgery. The external anal sphincter is innervated by the anal nerve.