Anal Canal The anal canal begins at the level of the sharp turn in the direction of the distal colon and is 2 to 3 cm in length. At the anorectal junction, the mucosa changes to stratified squamous epithelium (the pectinate line), which continues until the termination of the anus at the anal verge, where there is a transition to perianal skin with typical skin appendages. It is surrounded by a thickened ring of circular muscle fibers that are a continuation of the circular muscle of the rectum, the internal anal sphincter. Its lower part is surrounded by bundles of striated muscle fibers, the external anal sphincter (28).
Fecal continence is provided primarily by the puborectalis muscle and the internal and external anal sphincters. The puborectalis surrounds the anal hiatus in the pelvic diaphragm and interdigitates posterior to the rectum to form a rectal sling. The external anal sphincter surrounds the terminal anal canal below the level of the levator ani. A growing body of literature suggests that both external anal sphincter and levator ani muscles are important for fecal continence and that multiple injuries contribute to pelvic ﬂoor dysfunction (29,30) One such MRI study showed that major levator ani muscle injuries were observed in 19.1% women who delivered vaginally with external anal sphincter injuries, 3.5% who delivered vaginally without external anal sphincter injury, and none of the women who delivered by cesarean section before labor. Among women with external anal sphincter injuries, those with major levator ani muscle injuries trended more toward fecal incontinence (29). Further studies suggest that thickening of the internal anal sphincter occurs with aging, and that thinning of the external anal sphincter and a corresponding drop in squeeze pressure correlated with fecal incontinence, but not aging (31).
The anatomic proximity of the lower gastrointestinal tract to the lower genital tract is particularly important during surgery of the vulva and vagina. Lack of attention to this proximity during repair of vaginal lacerations or episiotomies can lead to damage of the rectum and fistula formation or injury to the external anal sphincter, resulting in fecal incontinence. Because of the avascular nature of the rectovaginal space, it is relatively easy to dissect the rectum from the vagina in the midline, which is routinely done in the repair of rectoceles.
Blood Supply The vascular supply to the rectum and anal canal is from the superior, middle, and inferior rectal arteries. The venous drainage is a complex submucosal plexus of vessels that, under conditions of increased intra-abdominal pressure (pregnancy, pelvic mass, chronic constipation, ascites), may dilate and become symptomatic with rectal bleeding or pain as hemorrhoids.
Innervation The nerve supply to the anal canal is from the middle rectal plexus, the inferior mesenteric plexus, and the pudendal nerve.
The Genital Tract and Its Relations
The genital tract is situated at the bottom of the intra-abdominal cavity and is related to the intraperitoneal cavity and its contents, the retroperitoneal spaces, and the pelvic ﬂoor. Accessing it through the abdominal wall or the perineum requires a thorough knowledge of the anatomy of these areas and their relationships.
Figure 5.17 The abdominal wall muscles.
Table 5.7 Muscles Contributing to the Structure and Strength of the Anterolateral Abdominal Wall
A. Above the arcuate line
B. Below the arcuate line
Ext. abdominal oblique aperneurosis
Int. abdominal oblique aponeurosis
Figure 5.18 A transverse section of the rectus abdominis. The aponeurosis of the external and internal oblique and the transversus abdominis from the rectus abdominis. A: Above the arcuate line. B: Below the arcuate line.