Anatomy and Embryology of the Colon, Rectum, and Anus
To truly understand surgical disease, one must have a comprehensive knowledge of the underlying anatomy and embryology. This chapter begins with a review of the anatomy of the rectum, anus, and pelvic floor. The anatomy of the colon is then discussed in a
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Key Concepts • The dentate line represents a true division between embryonic endoderm and ectoderm. • The location of the anterior peritoneal reflection is highly variable and can be significantly altered by disease such as rectal prolapse. • The right and left ischioanal space communicate posteriorly through the deep postanal space between the levator ani muscle and anococcygeal ligament. • The junction between the midgut (superior mesenteric artery) and the hindgut (inferior mesenteric artery) leads to a potential watershed area in the area of the splenic flexure. • There is a normal, three-stage process by which the intestinal tract rotates during development beginning with herniation of the midgut followed by return of the midgut to the abdominal cavity and ending with its fixation.
Anatomy of the Anal Canal and Pelvic Floor Textbooks of anatomy would define the “anatomic” anal canal as beginning at the dentate line and extending to the anal verge. This definition is one defined truly by the embryology and mucosal histology. However, the “surgical” anal canal, as first defined by Milligan and Morgan, [1] extends from the anorectal ring to the anal verge. The surgical definition of the anal canal takes in to account the surrounding musculature that is critical to consider during the conduct of operations from low anterior resection to anal fistulotomy. The surgical anal canal is formed by the internal anal sphincter, external anal sphincter, and puborectalis (Figure 1-1) and is easily identified on digital examination and ultrasound imaging. On average, the surgical anal canal is longer in males than in females. Intraoperative measurements of the posterior anal canal have estimated the surgical anal canal to
be 4.4 cm in men compared with 4.0 cm in women [2]. In addition, the anal canal was shown to be a unique muscular unit in that its length did not change with age. The anatomy of the anal canal has also been characterized using magnetic resonance imaging. MR imaging does not show a difference in the length of the posterior anal canal in men and women, but does show that the anterior and posterior external anal sphincter length (not including the puborectalis) is significantly shorter in women [3]. The anal canal forms proximally where the rectum passes through the pelvic hiatus and joins with the puborectalis muscle. Starting at this location, the muscular anal canal can be thought of as a “tube within a tube.” The inner tube is the visceral smooth muscle of the internal anal sphincter and longitudinal layer that is innervated by the autonomic nervous system. The outer muscular tube consists of somatic muscles including the components of the puborectalis and external anal sphincter [4]. It is the outer muscular tube that provides conscious control over continence and is strengthened during Kegal exercises. The external anal sphincter extends distal to the internal anal sphincter and the anal canal terminates at the anal verge where the superficial and subcutaneous portions of the external anal sphincter join the dermis.
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