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.