Anatomy of anorectum
It is angulated at junction with the rectum because the pull of the sling-like action of puborectalis muscle forms anorectal angle (Fig. 1.2). It lies 2–3 cm in front of and slightly below the tip of the coccyx, which is opposite the apex of the prostate in males. The anal canal is attached posteriorly to the coccyx by the anococcygeal ligament. The anus is surrounded laterally and posteriorly by loose adipose tissue within the ischioanal fossae, a potential pathway for the spread of perianal sepsis from one side to the other. Inferior rectal vessels and nerves cross it to reach the anal canal. Anteriorly, the perineal body separates the anal canal from the membranous urethra and penile bulb in males or from the lower vagina in females.
The anal complex has two sphincters: internal and the external. The internal anal sphincter is a smooth muscle, involuntary in nature, and is formed by the condensation of circular muscle of the rectum, while the external anal sphincter is voluntary in nature and is formed by the striated skeletal muscle (Felt-Bersma et al. 1989). The external sphincter has subcutaneous, superficial, and deep parts (Fig. 1.1). It covers the entire length of internal sphincter tube. It ends little below it as subcutaneous portion. The intersphincteric groove between lower edges of internal and external sphincter can be palpated more so when anus is stretched. This groove is used to enter the intersphincteric space in surgical procedures like internal sphincterotomy for anal fissure and ligation of intersphincteric tract (LIFT) for fistulae in ano. Endosonographically external and internal anal sphincters measure 6–8 and 2–3 mm, respectively. Internal sphincter appears uniformly hypoechogenic, while external anal sphincter and puborectalis are predominantly hyperechogenic. The external and internal sphincters and puborectalis are important muscles to maintain automatic continence and prevent fecal leakage at the time of threatened incontinence. The conjoined longitudinal muscle is formed by continuation of longitudinal layer of rectum along with some fibers of levator ani at the level of anorectal angle. It continues down between the internal and external sphincters binding them together and traverses the subcutaneous part of external sphincter as a corrugator cutis ani to get inserted into perianal skin. This muscle is supposed to act as skeletal support that attaches the anorectum to the pelvis. The levator ani consisting of iliococcygeus, pubococcygeus, and puborectalis forms the pelvic floor and is important for gross fecal continence. External and internal sphincters control gas and liquid contents. Puborectalis is a U-shaped sling which slings the anorectal angle to pubis and forms anorectal angle.
1.2.1 Inner Lining
The partition line between upper endodermal and lower ectodermal part (proctodeum) of anal canal is called dentate or pectinate line. Failure of breakdown of separating membrane between the two parts results in an imperforate anus. The upper part is lined by columnar cells, while the lower part is lined by squamous epithelium which is thin, pale, and smooth devoid of hair and glands. Above the dentate line, it is innervated by sympathetic and parasympathetic system, while the distal part is innervated by somatic nerves, and any inflammatory process or procedure in this part of anal canal is painful. The blood supply and venous drainage above the dentate line come from and go to superior and middle hemorrhoidal vessels and drain to the portal system, while the part below the dentate line drains into systemic circulation through inferior hemorrhoidal vessels, thereby making this area site for portosystemic shunts and cavernoma. Lymphatic above and below dentate line also drains to different groups of lymph nodes (inferior mesenteric and groin). Anal valves are located at dentate line. These valves are remnants of proctodeal membrane. Above each valve, there is an opening of anal glands known as anal crypts or sinus. Anal glands are 3–12 in number located in the submucosa, internal sphincter, or intersphincteric space. More than one gland may open into the same crypt. Half of the crypts have no communication with the glands. Obstruction of ducts of these glands causes stasis, infection, perianal sepsis, and fistula formation.
There are 8–14 longitudinal folds known as columns of Morgagni. These are present cranially on the dentate line. Anal papillae are present at the lower end of these columns. A strip of 0.5–1 cm of mucosa consisting of several layers of cuboidal cells above the dentate line in the area of column of Morgagni has a deep purple color because of internal hemorrhoidal plexus and is known as anal transition zone. Above this zone, epithelium changes to single layer of columnar cells (pink color). Just below the dentate line, the anal canal is lined by modified squamous epithelium which is thus as pale in color. This white color makes the reference point for taking a purse string suture in stapled hemorrhoidopexy.