Anal Canal

The anal canal is approximately 4 cm in length extending from the top of the EAS (or anorectal ring) to the anal verge. This definition is clinical. Histologically, the anal canal mucosa extends from the anal verge to approximately 1 cm above the dentate line. The anal canal, similar to its sphincters, is related anteriorly to the perineal body and the lower posterior part of the vagina. Posterior to the anal canal is the presacral fascia, the anococcygeal ligament, the anococcygeal raphe (an extension of the iliococ-cygeus), and the posterior extension of the puborectalis and external sphincter musculature, which inserts into the coccyx.

Skin from the buttock is continuous with the anal margin and continues to the lower border of the IAS. This epithelium is keratinized stratified squamous with hair follicles, sweat glands, and sebaceous glands. Proximal to the level of the dentate line, the epithelium is nonkeratinized squamous with no dermal appendages. There is a transition zone where squamous and columnar epithelium are mixed and then the columnar epithelium of the rectum predominates. Vertical mucosal folds known as anal columns are found at the upper anal canal just above the dentate line. Anal valves connect these folds at the inferior margins. Above each valve is the anal pit or sinus, which drains on average eight anal glands.

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