Handbook of Colorectal Surgery David E Beck
INDEX
×
Chapter Notes

Save Clear


1Basic Principles and Skills

Anatomy1

Knowledge of intra-abdominal anatomy is essential to understand and treat intestinal diseases. This chapter briefly summarizes anatomic features and principles that are important to the colorectal surgeon. Discussions in greater depth are available in comprehensive anatomy and colon and rectal surgery books [1–6]. Although study and experience increases a surgeon's knowledge of expected anatomic findings, it must be remembered that variability is the rule in human anatomy. The abdominal cavity contains many structures (Figure 1.1). The abdominal portion of the intestinal tract starts with the esophagus which connects to the stomach. Structures of major importance to the colorectal surgeon include the small bowel, colon, rectum, and anus.
 
MACROSCOPIC ANATOMY
 
Small bowel
The small bowel starts at the stomach and connects to the large bowel or colon at the ileocecal valve. The small bowel has three parts, the duodenum, jejunum, and ileum. The proximal portion, the duodenum is approximately 25 cm in length. The bile duct from the liver passes through the head of the pancreas and enters into the second part of the duodenum. The duodenum ends at a fibrous band called the ligament of Treitz. Two-fifths of the remaining small bowel is called the jejunum and the distal three-fifths is called the ileum.
The total length of the small bowel varies from 250 to 800 cm with a mean of 500 cm. It is folded in a variable fashion to accommodate the length of bowel in the abdomen. The major function of the small bowel is digestion and absorption of fluid and nutrients. The diameter of the small bowel is greatest at the duodenum and gradually narrows to the ileum.
 
Colon
The colon (large intestine) starts from the cecum (usually located in the right lower quadrant) and continues through all portions of the abdomen to the colorectal junction in the pelvis. The colon is about 1.5 m long and classically has been divided into segments based on the vascular supply and location of each segment within the abdomen, as shown in Figure 1.2 [1]. The cecum, right colon (supplied by the right and ileocolic artery), and left colon (supplied by the left colic artery) are usually retroperitoneal and fixed. The transverse colon (supplied by the middle colic artery) and sigmoid colon (supplied by branches 4of the inferior mesenteric artery) are intraperitoneal and relatively mobile.
zoom view
Figure 1.1: Normal gastrointestinal anatomy (mid-coronal view).
The colon contains two flexures (or bends) in the right upper quadrant (hepatic) and left upper quadrant (splenic). When the colon is not unduly distended with feces, its diameter is largest at the cecum and gradually narrows to the distal sigmoid colon (the narrowest part of the colon).
The external wall of the colon is unique because of the presence of several appendages (taeniae, omentum, appendices epiploicae, and diverticula). The outer longitudinal muscle is thickened into three longitudinal bands called taeniae. These average 8 mm in width and are named in reference to their relationship to the bowel mesentery or omentum. Thus there is a taenia mesocolica (associated with the mesentery), a taenia omentalis (associated with the omentum), and 5a taenia libera (not related to either the mesentery or omentum).
zoom view
Figure 1.2: Topographic anatomy of the colon.
The three taeniae meet at the appendiceal orifice and continue to the colorectal junction, where they expand to form a solid layer. Intermittent contractions of the inner circular muscle result in formation of semicircular folds called haustra, which are thought to aid in mixing the stool. The haustra are visible on the exterior surface of the colon.
The omentum is a sheet of fat and fibrous tissue that is well vascularized.
It starts at the greater curvature of the stomach, attaches to the transverse colon at the taenia omentalis, and extends into the abdomen. It doubles back on itself and attaches again to the colon, dividing the abdomen into several spaces (Figure 1.3). This arrangement allows it to be detached from the colon with minimal dissection in an almost bloodless plane.
zoom view
Figure 1.3: Sagittal section of the abdomen, demonstrating attachments of the greater omentum.
It has been theorized that the omentum functions 6to localize inflammatory processes and to assist in healing. This is supported by clinical experience and the frequent finding of omental adhesions to other portions of the bowel. Because of its multiple important functions, I prefer to preserve the omentum in operations on patients in whom neoplastic lesions are not present. This is easily accomplished by elevating the omentum superiorly and dividing the thin avascular tissues that attach the omentum to the colon. With care, the omentum can be detached intact.
The appendices epiploicae are subserous pockets of fat that occur in two rows on the right and the sigmoid colon and in a single row on the transverse colon. Their only recognized role is to act as a storage site for fat cells. Many adults also have colonic diverticula (mucosal herniations) located adjacent to the taeniae (see Chapter 13).
The colon connects to the small bowel at the ileocecal valve. Although lacking an anatomic sphincter, this functional valve is responsible for several physiologic actions: it allows the digested contents of the small bowel to pass into the cecum at a controlled rate and acts as a relative barrier to prevent the large number of bacteria (concentration of 1010) in the colon from moving to the distal small intestine (concentration of 103). Approximately 15% of patients have an incompetent ileocecal valve as demonstrated on barium enema studies.
 
Rectum
The rectum (Figure 1.4) is 12–15 cm long and can be divided into thirds based on its peritoneal relations. The upper third is intraperitoneal and covered anteriorly and laterally by peritoneum. At its middle portion, the rectum passes through the peritoneal floor and is covered by peritoneum on the anterior surface.
zoom view
Figure 1.4: Rectum and anus (coronal section).
The lower third is extraperitoneal 7as it travels through the levators to the anus. The lower rectum is enveloped by visceral pelvic fascia. Anteriorly, Denonvilliers’ fascia (Figure 1.5) separates the rectum from the seminal vesicles, prostate, and bladder trigone in males and the posterior vaginal wall in women. Posteriorly, Waldeyer's fascia separates the rectum from the presacral venous plexus.
The rectum can be differentiated from the colon by its lack of a posterior mesentery, sacculations, and appendices epiploicae. The outer longitudinal muscle layer of the rectum diffuses to form a solid, thick layer. Thus there are no taeniae or diverticula. The rectum is also larger in diameter than the sigmoid colon.
The inner rectum contains three indentations or valves of Houston. These are composed of circular muscle only. The superior valve is located 4 cm below the rectosigmoid junction on the left side; the middle valve is located at the peritoneal reflection on the right side; the inferior valve is located 2–3 cm above the dentate line on the left side. These valves aid the surgeon in localizing lesions with respect to the peritoneal location.
 
Anus
The anal canal starts at the anorectal junction located at the palpable upper edge of the anal sphincter mechanism (junction of the puborec-talis and the anal sphincter). The anal canal ends at the intersphincteric groove (approximately 2 cm distal to the dentate line). The anal margin is that portion of the perineum from the intersphincteric groove to approximately 5 cm out from the dentate line (Figure 1.4). The surface of the anal margin is skin that lacks appendages such as hair follicles. The surface outside the anal margin is referred to as perianal skin and contains all appendages of skin elsewhere.
zoom view
Figure 1.5: Sagittal section of the pelvis demonstrating anterior and posterior rectal fascia. (a) Male, (b) female.
8
The complex musculature of the anal canal can be thought of as composed of two tubes: the outer tube is funnel shaped, composed of skeletal muscle, and innervated by somatic nerves; the upper portion of this funnel is formed by the levator ani muscles. This sheet of muscle originates from the pelvic side wall (laterally), the sacrum (posteriorly), and the pubis (anteriorly) to the upper anus. Fibers of the levators can be grouped into three sections: the puborectalis (inner), pubococcygeus, and ileococcygeus muscles (posterolateral).
The lower portion of this outer cylinder of muscle is composed of the external anal sphincter. Although this voluntary muscle has been divided into three portions, clinically and physiologically it acts as a unit. Contraction of this muscle and the puborectalis produces the anal squeeze examined during the digital examination described in Chapters 2 and 3.
The inner tube of the anal canal is composed of visceral smooth muscle that is controlled by autonomic nerves. At the anus the inner circular muscle of the rectum thickens to become the internal anal sphincter. The longitudinal muscles of the rectum pass through the internal sphincter and attach to the perianal skin. The inner muscles of the anus are controlled by branches of the inferior rectal nerve and the perineal branch of the fourth sacral nerve. The internal anal sphincter is normally contracted and provides the resting anal tone felt during a digital anal examination. At rest, the lateral walls of the anal canal are opposed to form an anteroposterior slit [7].
The pelvic musculature and its attachments divide the pelvis into several spaces; these are described in Chapter 17.
 
VASCULAR ANATOMY
The colon receives its blood supply from branches of two major vessels, the superior and inferior mesenteric arteries (Figure 1.6). The superior mesenteric artery (SMA) originates on the anterior surface of the aorta, at the level of the first lumbar vertebrae, 1.25 cm caudal to the celiac artery, superior to the duodenum and pancreas [1]. Its first major branch is the middle colic artery. The middle colic artery divides close to its origin into an ascending and descending branch. After further branching it connects to the marginal artery and supplies the transverse colon. Distal to the marginal artery, end vessels travel in the mesentery to connect the marginal artery to the bowel (Figure 1.6).
The inferior mesenteric artery (IMA) originates 2–3 cm caudal to the SMA (inferior to the duodenum and pancreas). Its first major branch is the left colic artery. The left colic artery usually divides into two branches within 4–5 cm of its origin. This area is important in colonic operations. The next branches off the IMA are three to six sigmoid arteries. As branches of the artery approach the bowel, they communicate with the marginal artery.
The IMA continues to the upper rectum, where it becomes the superior hemorrhoidal artery. As it courses distally it splits into multiple branches that enter the rectum laterally.9
zoom view
Figure 1.6: Arterial supply to the colon.
The venous drainage of the colon (Figure 1.7) goes to the portal system and tends to follow the arterial system. The ileocolic vein attaches to the superior mesenteric vein (SMV) approximately 3 cm before the SMV joins to the splenic vein (inferior to the pancreas). The left colic vein enters the inferior mesenteric vein (IMV) at the level of the IMA origin. The IMV travels to the left of the IMA and continues to enter the splenic vein beneath the pancreas.
The lymphatic drainage of the colon follows the arterial supply. Major lymphatic chains are located along and named after the major named veins. The lymph nodes along these chains are important in colorectal cancer recurrence and prognosis.
The upper rectum receives blood from branches of the IMA. At the upper rectum this vessel is called the superior hemorrhoidal (rectal) artery. As it continues down the rectum the vessel splits, and branches move laterally and communicate with branches of the middle hemorrhoidal arteries. The distal rectum and anus are supplied by branches of middle and inferior hemorrhoidal arteries. As these vessels approach the internal iliac arteries and the bowel, they split into multiple communicating vessels.
The anus receives blood from two sources: branches of the lower hemorrhoidal plexus (inferior hemorrhoidal arteries) communicate with the middle hemorrhoidal arteries (as described previously) and with branches from the pudendal arteries. The pudendal arteries branch from the internal iliac arteries. Venous and lymphatic drainage goes to both mesenteric and systemic veins.10
zoom view
Figure 1.7: Venous drainage of the colon.
 
NERVOUS SUPPLY
The colon and rectum are richly innervated by multiple nerves whose function is poorly understood. The majority of efferent fibers to the intestine originate in the hypothalamus. The parasympathetic efferent fibers exit the central nervous system in two areas (cranial and sacral). The foregut is supplied via the vagus nerves and the hindgut fibers exit the sacral cord via the dorsal columns at sacral roots 2 through 4. Fibers from S3 and S4 are called the nervi erigentes [8]. After exiting the spinal cord, the fibers pass through a sacral plexus and then join with the hypogastric nerves (sympathetic nerves) to form the pelvic plexus. Parasympathetic nerves then pass upward in the inferior mesenteric plexus to be distributed to the superior hemorrhoidal artery and left colonic arteries. Other sacral fibers (S2–4) supply fibers to the levators, then enter the perineum via Alcock's canal as the pudendal nerve. At the anus the pudendal nerve becomes the hemorrhoidal nerve, perianal and dorsal penile nerve, or clitoral nerve.
The sympathetic efferent nerves exit the spinal cord at the thoracic and lumbar segments. The fibers pass through the splanchnic nerves to the mesenteric ganglia. Fibers then travel along the superior and inferior mesenteric arteries to reach the intestine. Additional fibers pass through the inferior hypogastric (pelvic) plexus, as previously described, to supply the rectum.
Afferent fibers from the intestine carry sensations of stretch, distention, and pain (anoxia or chemical damage) to the brain. The intestines are 11also affected by intrinsic innervation via the enteric plexus. These nerve cells and fibers are grouped into the myenteric (Auerbach) plexus and the submucosal (Meissner) plexus.
 
BOWEL WALL
The colon wall is composed of several layers (Figure 1.8). The innermost layer is the mucosa, a single layer of columnar cells with a cuticular border; it contains tubular pits and goblet cells. The submucosa is the strength layer of the bowel; this layer also contains blood vessels, lymphatics, Meissner's plexus, and solitary lymphatic nodules. There are two muscular layers: the inner layer, composed of muscle cells oriented in a circular fashion, and the outer layer of muscle, oriented in a linear fashion. In three areas the muscle fibers are thickened and fused to form the taeniae. The outermost layer is the serosa, which is composed of fibrous tissue.
The rectum contains layers similar to those of the colon, with two exceptions. The upper rectum contains a serosal covering on the anterior and lateral surface; however, this is lost as the rectum becomes extraperitoneal. The outer longitudinal muscle layer is thickened and diffused to form a solid sheet. The inner muscles are circular and, as described earlier, form three semicircular valves. The inner and outer muscles contribute fibers to the formation of the internal anal sphincter, as described previously.
The lining of the anus is composed of a transitional zone, where the mucosa changes from a columnar cell layer to a squamous cell layer at the dentate line. The area distal to the anal canal is lined by modified squamous epithelium without hair or glands [7].
zoom view
Figure 1.8: Bowel wall anatomy.
12
Further caudally, the lining changes to squamous epithelium, with hair and glands at the anal verge.
The submucosa of the anal canal contains three bundles of vascular sinusoids, called hemorrhoidal tissue [9]. (For additional discussion of these structures, see Chapter 16.)
 
References
  1. Gray H, Goss CM. Anatomy of the Human Body. Philadelphia: Lea & Febiger  1974;1233.
  1. Hollinshead WH. Anatomy for Surgeons: The thorax, abdomen, and pelvis. New York: Harper & Row  1971;676–718.
  1. Goligher JC. Surgery of the Anus, Rectum, and Colon 5th ed. London: Baillière Tindall,  1984;1–47.
  1. Netter F. Ciba Collection of Medical Illustrations, vol 3, part 11. Summit, NJ: Ciba-Geigy Corp  1962.
  1. Gordon PH, Nivatvongs S (Eds). Principles and Practice of Surgery for the Colon, Rectum, and Anus. St. Louis: Quality Medical Publishing  1992;3–38.
  1. Jorge JMN, Habr-Gama A. Anatomy and embryology of the colon, rectum, and anus. In: Wolff BG, Fleshman JW. Beck DE, Pemberton JH, Wexner SD (Eds). ASCRS Textbook of Colorectal Surgery. Springer-Verlag,  New York 2007;1–22.
  1. Phillips SF, Edwards DAW. Some aspects of anal continence and defecation. Gut 1965;6:396–406.
  1. Pemberton JH. Anatomy and physiology of the anus and rectum. In: Beck DE, Wexner SD (Eds). Fundamentals of Anorectal Surgery. New York: McGraw-Hill  1992;1–24.
  1. Corman ML. Colon and Rectal Surgery 3rd ed. Philadelphia: JB Lippincott  1984;1–48.