Anal region Figs 251 and 172

The anal region contains the anal canal and ischiorectal fossae.

• Anal sphincter: comprises external and internal sphincter components. The internal anal sphincter is a continuation of the inner circular smooth muscle of the rectum. The external anal sphincter is a skeletal muscular tube which, at its rectal end, blends with puborectalis to form an area of palpable thickening termed the anorectal ring. The competence of the latter is fundamental to anal continence.

• Ischiorectal fossae: lie on either side of the anal canal. The medial and lateral walls of the ischiorectal fossa are the levator ani and anal canal and the obturator internus, respectively. The fossae are filled with fat. The anococcygeal body separates the fossae posteriorly; however, infection in one fossa can spread anteriorly to the contralateral fossa forming a horseshoe abscess. The pudendal (Alcock's) canal is a sheath in the lateral wall of the ischiorectal fossa. It conveys the pudendal nerve and internal pudendal vessels from the lesser sciatic notch to the deep perineal pouch (see below). The inferior rectal branches of the pudendal nerve and internal pudendal vessels course transversely across the fossa to reach the anus.

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