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sunflower
06-12-09, 06:10 PM
ANAL CANAL

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Histologically, the rectum resembles the colon, but the crypts of Lieberkühn are deeper and number fewer per unit area. The anal canal, the constricted continuation of the rectum, is about 3 to 4 cm long. Its crypts of Lieberkühn are short and few and are no longer present in the distal half of the canal. The mucosa displays longitudinal folds, the anal columns (rectal columns of Morgagni). These meet one another to form pouch-like outpocketings, the anal valves
1. Anal Mucosa

- The epithelium of the anal mucosa is simple cuboidal from the rectum to the pectinate line (at the level of the anal valves), stratified squamous nonkeratinized from the pectinate line to the external anal orifice, and stratified squamous keratinized (epidermis) at the anus. The lamina propria, a fibroelastic con-nective tissue, houses anal glands at the rectoanal junction and circumanal glands at the distal end of the anal canal. Additionally, hair follicles and sebaceous glands are present at the anus. The muscularis mucosae is composed of an inner circular layer and an outer longitudinal layer of smooth muscle. These muscular layers do not extend beyond the pectinate line.
2. Anal Submucosa and Muscularis Externa

- The submucosa of the anal canal is composed of fibroelastic connective tissue. It houses two venous plexuses, the internal hemorrhoidal plexus, situated above the pectinate line, and the external hemorrhoidal plexus, located at the junction of the anal canal with its external orifice, the anus.

- The muscularis externa consists of an inner circular and an outer longitudinal smooth muscle layer. The inner circular layer becomes thickened as it encircles the region of the pectinate line to form the internal anal sphincter muscle. The smooth muscle cells of the outer longitudinal layer continue as a fibroelastic sheet surrounding the internal anal sphincter.

- Skeletal muscles of the floor of the pelvis form an external anal sphincter muscle that surrounds the fibroelastic sheet and the internal anal sphincter. The external sphincter is under voluntary control and exhibits a constant tonus.