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I. ANATOMY & FUNCTION
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The anatomy of the anus and rectum dictates the clinical evaluation and treatment of patients with anorectal disorders (Figure 35–1). From external to internal, the surface anatomy of the anorectum is composed of gluteal skin, anoderm, the anal transitional zone (ATZ), and proximally the rectal mucosa. The gluteal skin includes hair, sebaceous glands, and sweat glands.
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The anoderm begins at the anal verge and ends at the dentate line. Unlike the gluteal skin, the anoderm is devoid of hair and sweat glands. Surgical excision of too much anoderm can result in anal stenosis. The ATZ lies between the squamous anoderm and the rectal mucosa. In this zone, squamous, cuboidal, transitional, and columnar epithelium exist with longitudinal ridges called the columns of Morgagni. Between the columns of Morgagni are anal crypts with associated anal glands that open into their bases. Clinically, the ATZ is important for two main reasons. First, the ATZ is the crossover from somatic to visceral innervation and for lymphatic drainage from the inguinal to the pelvic nodes. Lymphatics from the anal canal above the dentate line drain via the superior rectal lymphatics to the inferior mesenteric lymph nodes and laterally to the internal iliac nodes. Below the dentate line, drainage occurs to the inguinal lymph nodes but can occur to the inferior or superior rectal lymph nodes. Second, the anal glands in the crypts are the site of anorectal abscesses and anal fistulas. Anatomically, the anal glands in the crypts extend to a variable depth, resulting in abscesses when these glands become blocked.
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Proximal to the ATZ is the rectal mucosa. Above the dentate line and underlying the rectal mucosa are the vessels that, when abnormally engorged, manifest as internal hemorrhoids. The rectum extends 12-15 cm proximal to the dentate line. The rectum has three curves that create folds called the valves of Houston. The puborectalis and levator muscles provide support of the rectum and are referred to as the pelvic floor. In addition, the rectum is fixed posteriorly by presacral (Waldeyer) fascia, laterally by the lateral ligaments, and anteriorly by Denonvilliers fascia. The peritoneum covers the upper two-thirds of the rectum anteriorly and the upper third laterally, and the lower third of the rectum is extraperitoneal.
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Blood Supply & Lymphatics
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The arterial supply of the anorectum is via the superior, middle, and inferior rectal arteries. The superior rectal artery is the terminal branch of the inferior mesenteric artery and descends in the mesorectum. It supplies the upper and middle rectum. The middle rectal arteries arise from the internal iliac arteries and enter the rectum anterolaterally at the level of the pelvic floor musculature. They supply the lower two-thirds of the rectum. ...