**2. Anatomy and physiology**

The anal canal is a continuation of the distal rectum. The surgical anal canal is formed by the hindgut in the proximal part and the anoderm in the distal part, and the border between both is dentate lines. The proximal part of the anus is covered by a mucous line. The superior rectal artery that flows through the anal canal will branch into two, to the left and right. The right branch of the superior rectal artery branches into anterior and posterior branches. This artery will form an arteriovenous plexus located in the right anterior, right posterior, and left lateral regions, which will be covered by mucosa that produces an anal cushion. For practical purposes, from a perineal view, we call it 11.00 o'clock the right anterior, 07.00 o'clock the right posterior, and 03.00 o'clock the left lateral anal cushion [4].

There are two anal canal sphincters, the internal anal canal sphincter (IAS) as a continuation and thickening of the circular layer of rectal muscle and the external anal sphincter (EAS). The IAS is made of smooth muscle. It is an involutory muscle, while the external sphincter muscle is a voluntary muscle consisting of three layers deep, superficial, and subcutaneous [1]. Anal incontinence during rest is caused by the contraction of the IAS and anal cushion, which participate 70–80% and 20–30%, respectively. During defecation, the anal cushion will prolapse downward to protect the anal crypt, estuary of the anal gland, and anal canal skin, and return after defecation. The ability of the anal cushion to return is due to the function of the muscle of Treitz, the continuation of muscle fiber from the longitudinal muscle fiber of the rectum. The Treitz muscle consists of two parts—the submucous muscle and the Park ligament, where the last part of it is located at the bottom of the anal cushion [4].

According to Aigner, et al., (2009), there are sphincter-like structures in the vascular plexus, formed by thickened tunica media that contain 5–15 layers of smooth muscle cells located between the vascular plexus and the subepithelial space of the anal cushions in normal anorectal specimens. The role of these sphincter-like structures is to coordinate the filling and drainage of the anorectal vascular plexus. This vascular plexus is without tunica media and larger than usual, like a lacuna [5].
