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Updated: 03/13/08 12:38 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: S. Shimoda, F. Silverstein, and D.R. Saunders
Download Chapter ♦ Download Lecture Slides & Notes
Download Chapter ♦ Download Lecture Slides & Notes
"It is a good thing for a physician to have prematurely grey hair and itching piles. The first makes
him appear to know more than he does, and the second gives him an expression
of concern which the patient interprets as being on his behalf."
A. Benson Cannon
A. Benson Cannon
A. Objectives
- Learn about the functional parts of the large intestine; the importance of the pectinate line; the myenteric plexus; the cellular population of the lamina propria; the site of interdigitation of the two major arteries; the colonic nerve supply.
- Understand colonic handling of salt and water, carbohydrates, and protein.
- Know the mechanisms of defecation, and the derangements in defecation when the cervical spinal cord is transected, or the sacral plexus is injured.
- Learn about adenocarcinoma of the large intestine, and about methods of detection of neoplastic lesions.
- Be able to describe diverticular disease, ulcerative colitis, and Crohn’s colitis, and the manifestations of common infectious organisms.
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B. Gross Anatomy
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Figure 1 |
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| Much of the colon is fixed in position, except for the sigmoid and transverse colon |
The large intestine begins at the ileocecal valve and ends at the anus. It is wider than the small intestine except the descending colon which may be the same diameter when empty. Other distinguishing features are the taenia coli which represent thickenings of longitudinal, muscle bands situated equidistant around the circumference of the colon; the tension of these bands puckers the bowel into sacculations which are called haustra and they represent the moving, segmental contractions which retard the flow of luminal contents. In addition the serosal surface of the colon is studded with appendices epiploicae (little fat-filled pouches of serosa). The colon is relatively fixed in place, in comparison to the mobile small bowel, because there is no mesentery along its descending and ascending portions and only a short one along its transverse portion. The sigmoid colon is relatively mobile because of its long mesenteric attachment: sigmoid volvulus (obstruction due to twisting) can occur causing acute illness requiring emergency surgery because the bowel is not only obstructed but its blood supply is compromised.
The colon begins at the bag-like cecum which is the widest portion of the organ. Because of its size the cecum is rarely obstructed by tumor, malignancies of this area often present as anemia secondary to occult bleeding. |
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| The appendix can vary in location |
The appendix is a worm-like structure opening on the posteromedial wall of the cecum inferior to the ileocecal valve. It is freely mobile. Its length and position are variable, but it is retrocecal or retrocolic in 2/3 of patients. The symptoms and signs of appendicitis vary according to the organ's location.
The ascending colon lies in the extreme right abdominal cavity going from the cecum towards the right lobe of the liver. It has only limited mobility since it lacks a mesentery. The colon bends at the hepatic flexure to become the horizontally disposed transverse colon which proceeds across the abdomen and then turns upward to form the splenic flexure. Both hepatic and splenic flexures are supported by peritoneal folds connecting with the diaphragm - the left and right phrenicocolic ligaments. The descending colon extends from the splenic flexure downwards along the left margin of the abdominal cavity across the iliac crest to enter the left iliac fossa. It then curves medially to end at the brim of the true pelvis where it becomes the sigmoid colon. |
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| Twisting may create sigmoid volvulus |
The sigmoid colon is mobile on its ample mesentery. Its average length and diameter varies in different patients and in different countries, apparently depending on the amount of indigestible fiber in the diet. If the indigestible residue is large, the stool is bulky and the sigmoid length and diameter is greater.
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Nerves to pelvic floor may be damaged by childbirth or surgery |
The sigmoid joins the rectum opposite the third sacral vertebra and follows the sacral convexity to the anal canal. The rectum has a complete longitudinal muscle coat but no haustra, taenia or mesentery. It is extraperitoneal in location posteriorly and partly so anteriorly (Figure 2). The large midportion of the rectum is called the ampulla. There are three variably prominent crescentic rectal mucosal folds - Houston's valves which result from the embryonic rotation of the gut and are neither physiological nor anatomical true valves. Lateral to the upper rectum, coils of ileum or sigmoid colon may occupy the pararectal fossae. Nerves from the pelvic plexus to the genitourinary organs pass forward on both sides of the rectum, between it and the levator ani muscles. Why is this anatomic fact important to the surgeon excising the rectum? Anteriorly in the female one finds the uterus, rectovaginal recess, and uterine cervix and in the male the bladder, rectovesical recess, seminal vesicles and the prostate (Figure 2).
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Figure 2 Male Pelvis – Median Section |
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The middle rectal valve is approximately 8 centimeters from the anal opening. This valve is an important anatomical landmark as it roughly marks the level of the anterior peritoneal reflection (Figure 5). An inadvertent full-thickness biopsy or perforation of the anterior rectal wall above this would probably cause contamination of the peritoneum and require surgical closure. A perforation below this valve would not connect with the peritoneal cavity and can usually be managed conservatively.
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Figure 3 |
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Figure 4 |
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| Puborectalis helps to maintain fecal continence |
The puborectalis is a horseshoe-shaped muscle which encircles the rectum posteriorly and is attached to the pubic rami anteriorly. Tonic contraction of this striated muscle creates an acute angle between the ampulla and the anal canal (Figure 3 and Figure 4). The internal sphincter muscle is a hypertrophied portion of the inner circular smooth muscle of the bowel wall in the region of the anal canal. Fibers of the outer longitudinal muscle of the rectum merge with those of the levator muscle to form the conjoined longitudinal muscle bands which insert into the perianal skin. Contraction of the levator causes shortening of these bands resulting in "puckering" of the anal canal. The striated external sphincter muscles originate at the coccyx and extend in a loose pattern around the anus to insert into the perineal body.
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| Pectinate line is an important landmark |
At the proximal part of the anal canal occur the anal crypts and papillae which alternate to form the pectinate (L. pecten, comb) or the dentate (tooth-like) line.
(Figure 5). Proximal to this line the rectum is covered with colonic mucosa with its visceral innervation while the anal canal is composed of squamous epithelium with its somatic innervation. How does this explain the painless character of mucosal biopsy and the pain associated with a thrombosis of an external hemorrhoid?
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Figure 5 The Anorectum |
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| Use the toilet test to discover hemorrhoids |
Internal hemorrhoids, a submucosal venous plexus, usually occur in three discrete areas in the proximal portion of the anal canal and are not palpable on digital examination. Because internal hemorrhoids are covered by rectal mucosa, they are insensitive; but because they are composed of venous plexuses they may enlarge under pressure and cause bleeding. (Figure 5). Sometimes, internal hemorrhoids are so large as to protrude (prolapse) through the anal canal. It may be difficult to see internal hemorrhoids when examining the patient with a sigmoidoscope. Hemorrhoids are most obvious when the patient strains while sitting on the toilet. This will cause large internal hemorrhoids to protrude from the anus making the diagnosis obvious. How would you use this fact diagnostically? An external hemorrhoid is defined as any venous dilatation below the pectinate line. These hemorrhoids are covered by squamous epithelium and may become exquisitely painful when thrombosed and swollen or after rupture into the surrounding tissue.
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| Painful anal fissures can contribute to constipation |
An anal fissure results from a split of the skin of the distal portion of the anal canal and occurs in the posterior midline 90% of the time. Usually, it is caused by the trauma of passing a large stool which splits the skin and causes a small amount of bleeding and a painful burning sensation. The great majority of fissures heal spontaneously, but on occasion the fissure becomes chronic, exposing the fibers of the internal sphincter muscle which lie at its base (Figure 5). The edema which forms as a result of this inflammation usually flows distally by the action of both gravity and the mechanical contraction of the anal sphincters. This edema is manifest clinically by the formation of a swollen external tag. In the posterior midline this tag is a sign of a proximal fissure and is then called a sentinel pile (L. pyla, a ball). Occasionally, the edema extends superiorly and forms a hypertrophied anal papilla. The hallmark of a fissure is pain. Chronicity results in scarring and contracture of the underlying internal sphincter with some resultant degree of anal stenosis. The existence of a fissure outside the posterior or anterior midline of the canal is unusual and suggests systemic disease such as Crohn's disease, syphilis or rarely leukemia.
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| Perirectal abscesses can originate in an anal crypt |
Behind and between the anal papillae exist the anal crypts which are most prevalent in the posterior portion of the anal canal and occur with greater frequency in men than in women. Draining into some anal crypts may be an anal duct from the rudimentary anal glands of Chiari. In evolutionary terms, these glands served the function of lubricating the anal canal but now they are vestigial structures which may become secondarily infected and give rise to cryptitis, abscess and fistula. (Figure 5). The two most common forms of rectal abscesses are the perianal and the ischiorectal abscesses. Decompression of such an abscess either spontaneously or by incision and drainage provides prompt relief of pain but may cause the development of a permanent fistula (pipe or tunnel) between the affected anal crypt and the external opening. The perianal abscess exists close to the anal canal, being trapped there by the bands from the conjoined longitudinal muscle fibers which insert into the skin. On the other hand, an abscess which develops in the ischiorectal fat pad may become quite large and occasionally will cross the posterior midline to affect the opposite side (horse-shoe abscess). Rarely, an abscess may occur elsewhere about the anorectum and it is named according to the space in which it occurs, e.g., pelvirectal, presacral, rectovaginal, intersphincteric abscess, etc.
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