Colon, rectum and anus

Clinical anatomy

The colon is a capacious tubular conduit that extends from the ileocaecal junction to the anus and frames the small intestine. It is approximately 150 cm long, which is about 25% of the length of the small intestine. Its diameter ranges from 2.5 cm (sigmoid colon) to 7.5 cm (caecum), and significant augmentation may occur with distension.

The anatomical characteristics that differentiate the small intestine from the large intestine are used clinically when assessing imaging or when evaluating anatomy during an operation. In addition to possessing a larger calibre and restricted mobility, the large bowel is distinguished by taeniae coli, haustra, plicae semilunares and appendices epiploicae. The three taeniae coli are bands of longitudinal smooth muscle that start at the base of the appendix and merge at the level of the rectosigmoid junction. They are located in fairly constant positions beneath the serosal surface of the colon, except in the transverse colon. They are found on the anti-mesenteric aspect of the colon, directly opposite the mesentery (taenia libera), posterolaterally (taenia omentalis) and posteromedially (taenia mesocolica) midway between the taenia libera and the mesentery. They contract longitudinally, forming haustra, outpouchings of the colon that result in the formation of circumferential folds of the bowel wall, plicae semilunares. Haustrations may be absent in the caecum and relatively sparse in the ascending and proximal transverse colon; they become more pronounced beyond the middle of the transverse colon. Appendices epiploicae are fatty tags that arise from the serosa of the colon ( Fig. 60.1 ).

The large bowel is divided into the caecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal (see Fig. 60.1 ). The terminal ileum enters the caecum at its posteromedial aspect; its angulated entry is maintained by superior and inferior ileocaecal folds. The ileocaecal valve consists of two semilunar mucosal lips that fuse, regulating ileal emptying by preventing backward reflux of colonic contents. Clinically, a competent ileocaecal valve can lead to a closed loop type of large bowel obstruction if there is an obstructing distal lesion.

The appendix is a blind-ended tubular structure arising from the posteromedial caecum approximately 2 cm below the ileocaecal junction. It is 2–20 cm in length and has an average diameter of 5 mm. Its position varies: in order of frequency, the appendix may be retrocaecal, retrocolic, pelvic (descending), subcaecal or ileocaecal. The mesentery of the appendix is attached to the caecum and the proximal appendix, and contains an appendicular artery, a branch of the ileocolic artery ( Video 60.1 ).

The rectum is a wide, easily distensible reservoir that lacks haustra, appendices epiploicae and taeniae. It exhibits three curvatures that correspond to three intraluminal folds: the left superior, right middle and left inferior folds, known collectively as the valves of Houston ( Fig. 60.2 ). They are encountered during endoscopic evaluation of the rectum and are absent after surgical mobilization of the rectum. The preoperative endoscopic measurement of a rectal lesion from an anal landmark (for example, the anal verge) is often found to be even higher after the rectum is elongated by operative mobilization. The rectum is surrounded by perirectal fat containing terminal branches of the inferior mesenteric artery and lymph nodes, and is enveloped by the fascia propria (see Fig. 68.2 ).

The anal canal may be defined anatomically or surgically (see Fig. 60.2 ). The anatomical canal extends from the anal verge to the dentate line, which marks the point of transition from visceral afferent to somatic afferent innervation. In the mid-anal canal, 6–10 vertical mucosal folds form the anal (haemorrhoidal) columns. They frequently contain a terminal branch of the superior rectal artery and vein, supplemented to a variable degree by middle and inferior rectal vessels; dilated submucosal veins in the upper anal canal form an internal rectal (haemorrhoidal) venous plexus ( Fig. 60.3 ). The submucosal vessels are most prominent in the left lateral, right posterior and right anterior quadrants of the wall of the canal (at approximately 3, 7 and 9 o'clock when viewed in the lithotomy position); here, the subepithelial tissues are expanded into three ‘anal cushions’. Although variable in number and position, the cushions help to seal the anal canal and contribute to the maintenance of continence to flatus and fluid. They are important in the pathogenesis of haemorrhoids.