The muscular portions of the continence organ wrap around the anal canal most likely comparable to two tubes, one inside the other. The inner tube consists of the internal sphincter muscle as the continuation of the circular musculature of the rectum. This is smooth musculature which is – due to colonic aganglionosis – capable of permanent contraction, thus keeping the anal canal tightly closed. Pressure measurements show that it accounts for about 70% of the muscle tone at rest. Under spinal anesthesia with a corresponding elimination of the striated muscle portions, the resting pressure profile of the anal canal remains largely unchanged. This underlines the importance of the internal anal sphincter muscle as the essential continence factor. On the other hand, it makes defecation possible due to its relaxation reflex when the rectal ampulla is distended.

The striated external anal sphincter muscle encloses the internal anal sphincter muscle at the caudal end of the anal canal. Similar to the striated skeletal musculature, the external sphincter muscle is capable of a maximal short-period contraction for the purpose of deliberate action. Cranially, the striated puborectalis muscle follows, which genetically corresponds to the lowest part of the levator ani muscle. It encases dorsally the rectum like a sling, while omitting its ventral parts and inserting into the pubic bone (os pubis) with broad tendons. The contraction of this muscle leads to a diagonal sealing of the upper anal canal by a ventral shift of the back wall of the lower rectum like a crimp connection (“gross anal closure”).

The epithelial coating of the anorectum consists of three parts. The epithelium of the distal part of the anal canal, the anatomical anal canal, is especially important for the continence function. Here lies the dry, nonkeratinizing squamous epithelium of the anoderm, which due to its intensive supply with sensitive nerve endings is capable to differ between the various aggregate conditions of the colonic contents. Above the pectinate line, in the section of the surgical anal canal stemming from the endoderm, a columnar epithelium is found whose structure is similar to that of the rectal mucosa. Between these two areas lies the zone of the so-called transitional epithelium (transitional cell area), which is of individually differing width and is also supplied with sensitive nerves. The histological examination discloses a cylindrical structure. Macroscopically, the transitional cell area shows a whitish color in contrast to the reddish shimmering mucosa above, which is not innervated sensitive.