Corpus: Anus

The anus is the opening at the end of the digestive tract responsible for the excretion of stool (defecation).

2.1. Continence organ

The anus is controlled by two ring-shaped muscles:

• The internal anal sphincter, which is controlled involuntarily by the autonomic nervous system.
• The external anal sphincter, which is under voluntary control.

Together with the levator ani muscle, these muscles form the rectal closing mechanism, allowing control over defecation. This functional system, along with the venous plexus of the anus (referred to as the anal vascular cushion), is collectively known as the continence organ.

2.2. Anal canal

The anal canal is the terminal section of the intestine, shaped by the sphincter muscles. It is approximately 2 to 4 cm long and lies within the pelvic floor, lacking a peritoneal covering. When closed, it appears as a slit running lengthwise from front to back.

The junction between the rectum and the anal canal is marked by the dentate line (or anorectal junction). At this point, the columnar epithelium of the rectal mucosa transitions into non-keratinized squamous epithelium found in the anal canal. Below this line, there are 8 to 10 vertical folds called anal columns (Morgagni's columns), which are raised by the internal anal sphincter muscle. In this region, a vascular structure called the anal vascular cushion (fed by the superior rectal artery) is located.

At the base of the anal columns, small flap-like structures called anal valves are found. These mark the openings of the anal crypts, which contain the anal glands. Below the anal valves begins the area where the anal mucosa transitions into the skin. This region, known as the anal pecten or zona alba (due to its pale color), ends at the anocutaneous line, which is also the lower boundary of the anal canal.

2.3. Topography

The anus is located dorsally in connection with the coccyx via the anococcygeal ligament. Ventrally, it is closely associated with the urethra in men and the vagina in women, separated from these structures by connective tissue and muscle.

The most important diseases of the anus include

• Perianal thrombosis
• Perianal abscess
• Perianal fistulas
• Skin tags (mariscae)
• Anal fissures
• Anal cancer
• Haemorrhoids