Within the anal canal, three highly vascularized submucosal arteriovenous plexuses, termed anal cushions, aid complete closure of the canal and fecal continence when apposed. Increasing uterine size, excessive straining, and hard stool create increased pressure that ultimately leads to degeneration and subsequent laxity of the cushion's supportive connective tissue base. These cushions then protrude into and downward through the anal canal. This leads to venous engorgement within the cushionsnow termed hemorrhoids. Venous stasis results in inflammation, erosion of the cushion's epithelium, and then bleeding.
internus m.
Pudendal n.
External hemorrhoids are those that arise distal to the pectinate line. They are covered by stratiied squamous epithelium and receive sensory innevation from the inferior rectal nerve. Accordingly, pain and a palpable mass are typical complaints. Following resolution, a hemorrhoidal tag may remain and is composed of redundant anal skin and ibrotic tissue. In contrast, internal hemorrhoids are those that form above the pectinate line and are covered by insensitive anorectal mucosa. hese may prolapse or bleed but rarely become painful unless they undergo thrombosis or necrosis.
Anal Sphincter Complex. Two sphincters surround the anal canal to provide fecal continence-the external and internal anal sphincters. Both lie near the vagina and may be torn during vaginal delivery. he internal anal sphincter (lAS) is a distal continuation of the rectal circular smooth muscle layer. It receives predominantly parasympathetic ibers, which pass through the pelvic splanchnic nerves. long its length, this sphincter is supplied by the superior, middle, and inferior rectal arteries. he lAS contributes the bulk of anal canal resting pressure for fecal continence and relaxes prior to defecation. The lAS measures 3 to 4 cm in length, and at its distal margin, it overlaps the external sphincter for 1 to 2 cm (DeLancey, 1997). he distal site at which this overlap ends, called the intersphincteric groove, is palpable on digital examination.
In contrast, the external anal sphincter (EAS) is a striated muscle ring that anteriorly attaches to the perineal body and posteriorly connects to the coccyx via the anococygealiligament. The EAS maintains a constant resting contraction to aid continence, provides additional squeeze pressure when continence is threatened, yet relaxes for defecation. he external sphincter receives
Posterior labial n.
Ischiocavernosus m.
Bulbospongiosus m.
perineal m.
Perineal n.
Pudendal n.
sphincter m.
Levator ani mm.
blood supply from the inferior rectal artery, which is a branch of the internal pudendal artery. Somatic motor ibers from the inferior rectal branch of the pudendal nerve supply innervation. Clinically, the AS and EAS may be involved in third-and fourthdegree lacerations during vaginal delivery, and reunion of these rings is integral to defect repair (Chap. 27, p. 532).