Figure 10-2. Diagram of the anal canal. Note the following structures: ampulla of the rectum (AR), puborectalis muscle (PR), anal columns (AC), anal verge (AV), pectinate line (PL; thin dotted line), internal anal sphincter (IS), and external anal sphincter (ES). Thick dotted line marks the anorectal junction. (Adapted with permission from Ernest WA: NMS Anatomy, 2nd ed. Media, PA, Harwal, 1990, p 225.)
III. ANAL CANAL (Figure 10-2) is divided into the upper and lower anal canal by the pectinate line. It is surrounded by the internal anal sphincter, which is a continuation of smooth muscle from the rectum, with involuntary control via autonomic innervation, and the external anal sphincter, which consists of striated muscle under voluntary control via the pudendal nerve.
A. The upper anal canal extends from the anorectal junction (perineal flexure) to the pectinate line. The mucosa of the upper anal canal extends into longitudinal folds called the anal columns, or columns of Morgagni. The base of the anal columns defines the pectinate line. At the base of the anal columns are folds of tissue called the anal valves. Behind the anal valves are small, blind pouches called the anal sinuses into which anal glands open.
B. The lower anal canal extends from the pectinate line to the anal verge (the point at which perianal skin begins).
1. Internal hemorrhoids are varicosities of the superior rectal veins. They are located above the pectinate line and are covered by rectal mucosa. Clinical findings include: bleeding, mucous discharge, prolapse, and pruritus, but no pain.
2. External hemorrhoids are varicosities of the inferior rectal veins. They are located below the pectinate line, near the anal verge, and are covered by skin. Clinical findings include: bleeding, swelling, and pain.
IV. DEFECATION REFLEX. When feces are present, sensory impulses from pressure-sensi-tive receptors within the ampulla of the rectum travel to sacral spinal cord levels. Motor impulses travel with the pelvic splanchnic nerves (parasympathetics; S2—4), which increase peristalsis and relax the internal anal sphincter. If the external anal sphincter and puborectalis muscle also are relaxed, defecation takes place with the help of contraction of the anterior abdominal wall muscles and closure of the glottis. If the external anal sphincter and puborectalis muscle are voluntarily contracted via the pudendal nerve, defecation is delayed, and the feces move back into the sigmoid colon for storage. The hypogastric plexus and lumbar splanchnic nerves (sympathetics) decrease peristalsis and maintain tone of the internal anal sphincter.
V. RADIOLOGY. Radiologic features of Hirschsprung disease, diverticulosis, FAP, and ulcerative colitis (Figure 10-3)
Figure 10-3. (A) Hirschsprung disease. Lateral radiograph of the colon after a barium enema in ^ a 3-year-old girl with aganglionic megacolon. The upper segment (*) of normal colon is distended with fecal material. The distal segment f *) of the colon is narrow; this is the portion of colon where the myenteric plexus of ganglion cells is absent. (B) Diverticulosis. Barium radiograph of the sigmoid colon showing numerous small outpouchings, or diverticula (arrows), from the colonic lumen. These diverticula are filled with barium and fecal material. (C) Familial adenomatous polyposis. This radiograph, taken after a barium enema, shows numerous adenomatous polyps carpeting the mucosal surface of the descending and rectosigmoid portions of the colon. (D) Ulcerative colitis. This barium radiograph shows ulcerative colitis involving the rectum and extending proximally to the ascending colon. Note the smooth border of the colon due to the lack of haustra. (A reprinted with permission from Behrman RE, Kliegman RM, Arvin AM: Nelson's Textbook of Pediatrics, 15th ed. Philadelphia, WB Saunders, 1996; Sand D reprinted with permission from Rosenbaum HD, Hildner JH: Basic Clinical Diagnostic Radiology, pp 241, 243. Baltimore, University Park Press, 1984; C reprinted with permission from Eisenberg RL: Gastrointestinal Radiology: A Pattern Approach. Philadelphia, JB Lippincott, 1987, p 355.)
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