The anal sphincter is composed of two layers of muscles that overlap each other. Its outer layer is the external anal sphincter, which is a skeletal muscle and is random. The inner layer is the internal anal sphincter, which is the non-random smooth muscle part. The internal anal sphincter is a continuation of the rectal ring muscle layer, which is smooth muscle, pearly white, its upper boundary is flat in the plane of the rectal ring muscle of the anal canal, and the lower reaches the intersphincter sulcus, wrapping around the upper 2/3 of the anal canal. The internal anal sphincter muscle bundle is oval, continuously arranged in a stacked tile pattern, with the upper fibers sloping inward, the middle gradually horizontal, and some fibers slightly upward in the lower part. The lower end of the internal anal sphincter is the most hypertrophic, forming a clear ring-like free edge, surrounded by elastic fibers of the united longitudinal muscles. The external anal sphincter, on the other hand, is the outermost muscle of the anal canal, a transverse muscle, innervated by the somatic nerve, and a random muscle. It can be divided into three layers, namely the subcutaneous, superficial and deep layers. The subcutaneous layer surrounds the lower end of the anal canal. The superficial layer is an elliptical bundle of muscles that surrounds both sides of the anal canal, ending at the tip of the coccyx at one end and connecting to the superficial transverse perineal muscle in the perineum anterior to the anus. The deep layer is a circular bundle of muscles that surrounds the anal canal and ends at the sciatic tuberosity on each side of the anus.