Chapter 36: The rectum and anal canal
At the level of the middle of the sacrum, the sigmoid colon loses its mesentery and gradually becomes the rectum, which, at the upper limit of the pelvic diaphragm, ends in the anal canal (fig. 36-1). The rectum, about 15 cm long, widens below as the ampulla, which is very distensible. Although variable in shape, the rectum follows the sacrococcygeal curve. At the anorectal junction, the gut curves backward and its concavity is held by the puborectal sling, which can be palpated per anum. The rectum presents three or more lateral curvatures, which correspond to transverse rectal folds in the interior of the gut. The rectum has neither mesentery nor haustra, and it has an almost complete outer longitudinal muscular coat rather than teniae.
In the upper third of the rectum, its front and sides are covered by peritoneum; in its middle third, the front only; its lower third is devoid of peritoneum. The rectovesical and recto-uterine pouches descend to within about 7 to 8 cm and 5 to 6 cm, respectively, of the anus (see figs. 31-9 and 35-4). Below the pouches, condensations of parietal pelvic fascia are found, and the rectum is surrounded by visceral pelvic fascia from the superior fascia of the pelvic diaphragm (see fig. 38-3).
Anteriorly, the rectum is related in the male to coils of small intestine in the rectovesical pouch above and to the back of the bladder, prostate, seminal vesicles, and ductus deferentes below; in the female, it is related to coils of small intestine in the recto-uterine pouch above and to the back of the vagina below. Laterally, the rectum is related to the ileum or sigmoid colon.
Posteriorly, the rectum is related to the sacrum, coccyx, and pelvic diaphragm.
Anatomically, the anal canal extends from the level of the upper aspect of the pelvic diaphragm to the anus. In surgical usage, however, the anal canal is frequently limited to that part of the intestine below the pectinate line; this part differs from the part above the pectinate line in several respects, including innervation, venous and lymphatic drainage, and possibly lining epithelium. The anal canal (fig. 36-1) is about 3 cm in length. The congenital anomaly in which the anal canal fails to communicate with the exterior is known as imperforate anus.
Surrounded by the levatores ani, the anal canal passes through the pelvic diaphragm (see fig. 38-3), and the anorectaljunction is held forward by the puborectal sling (see fig. 37-1). Below the pelvic diaphragm, the anal canal is surrounded by the sphincter ani externus, and the ischiorectal fossae are situated laterally (see fig. 38-3).
The external sphincter (figs. 36-1 and 38-3) is usually described in three parts. * The subcutaneous part surrounds the lowermost portion of the canal. The superficial part, situated above the subcutaneous division, is attached to the perineal body and coccyx. The deep part, more or less continuous with the superficial division, surrounds the uppermost portion of the canal and is associated with the puborectalis posteriorly. The sphincter ani externus is supplied by the inferior rectal nerves and by a perineal branch of the fourth sacral nerve (S.N.4). The muscle is in variable tonic contraction during waking hours, and it can be contracted voluntarily. The sphincter ani internus is the thick lower end of the inner circular layer of the gut.
Several vertical mucosal folds, the anal (formerly called rectal) columns, are usually visible in the upper half of the canal (fig. 36-1). The columns are vascular, and enlargement of their venous plexus results in internal hemorrhoids. The anal columns are united below by anal valves, which bound anal sinuses. The lower limit of the anal valves is the pectinate line, below which is a zone termed the pecten. The interval between the internal and external sphincters may be marked by a white line. The pecten merges with the skin of the anus. An ischiorectal abscess may drain through afistula in ano into the anal canal.
The interior of the lower portion of the intestine can be inspected by electrically lit instruments (proctoscopy, sigmoidoscopy, and colonoscopy). Digital examination per rectum is an important clinical procedure, e.g., in assessment of the prostate. The anal sphincters can be felt by an index finger inserted into the anal canal and rectum. The following structures are palpable per rectum:
1. Anteriorly in the male (see fig. 33-1) - rectovesical pouch, full bladder, seminal vesicles, displaced or enlarged ductus deferentes, membranous part of urethra when catheterized, and bulbo-urethral glands
2. Anteriorly in the female-vagina, cervix, ostium uteri, body of uterus when retroverted, recto-uterine fossa, and, pathologically, broad ligaments, uterine tubes, and ovaries
3. Laterally - ischial tuberosity and spine and sacrotuberous ligament
4. Posteriorly-pelvic surface of sacrum and coccyx.
The rectum and anal canal are supplied by the superior rectal artery (the continuation of the inferior mesenteric artery), with assistance from the middle and inferior rectal arteries, and by the median sacral artery. The submucosal venous plexus above the pectinate line drains into the superior rectal veins (portal system), which may become varicose, resulting in internal hemorrhoids or "piles." The submucosal plexus below the pectinate line drains into the inferior rectal veins, which may become varicose, resulting in external hemorrhoids or piles. The unions of the superior with the middle and inferior rectal veins are important portal-systemic anastomoses.
Innervation (fig. 36-2).
Parasympathetic fibers supply the smooth muscle, including the internal sphincter. Sympathetic fibers are mainly vasomotor. Somatic motor fibers supply the external sphincter. Sensory fibers are concerned with the reflex control of the sphincters and with pain. The anal canal is very sensitive below the pectinate line, so that external hemorrhoids may be very painful.
The rectosigmoid junction does not possess an anatomical sphincter but does contract; therefore the sigmoid colon usually contains feces and the rectum is empty. Fecal continence depends on colonic control and also on the reflex control of the sphincter ani internus and externus. When a sensation of fullness arrives, the abdominal muscles can be contracted, thereby increasing intra-abdominal pressure. The puborectal sling and the sphincters relax, and the rectal musculature contracts. The colon and rectum then descend, the rectum becomes elongated, feces are discharged, and the anal canal is closed by its sphincters.
36-1 Where does the rectum begin and end?
36-2 What is imperforate anus?
36-3 Where is the pectinate line?
36-4 What are hemorrhoids?
Figure 36-1 Median section of the rectum and anal canal. The anterior aspect is on the left side of the drawing. (Based on Morgan and Thompson and on Walls.)
Figure 36-2 Nerve supply of the rectum and anal canal. Motor fibers are shown in red, parasympathetic fibers as interrupted lines, and sensory fibers in blue. The fibers in the pelvic splanchnic nerves reach the intestine by way of plexuses.
* Although it is customary to consider the external sphincter in three parts, it has also been regarded as one muscular mass (S. F. Ayoub. Acta Anal., 105:25-36, 1979). Moreover, the proposal that the puborectalis is a part of the external sphincter (A. Shafik, Invest. Urol., 13: 175-182, 1975) rather than of the levator ani has been disputed.