Fig. 2.1
Arterial supply of the colon
2.2.3.1 Superior Mesenteric Artery
The superior mesenteric artery supplies the cecum, appendix, ascending colon, and two thirds of the transverse colon.
Ileocolic Artery
The ileocolic artery continues the superior mesenteric artery past the outlet of the ileal arteries. It usually divides into a superior branch for the ascending colon and an inferior branch for the cecum (colic branch) and the appendix (appendicular artery).
Right Colic Artery
The right colic artery (diameter, 2.9 ± 0.6 mm) has an inconstant origin: it arises directly from the superior mesenteric artery, the ileocolic artery, or the middle colic artery. It supplies the ascending colon and the right colonic flexure. In 70 % of individuals, however, a clearly identifiable right colic artery is not present, in which case the right colon receives its blood supply via the colic branch of the ileocolic artery and the right branch of the middle colic artery.
Middle Colic Artery
The middle colic artery (diameter, 3.3 ± 0.8 mm) is always present, arises from the initial infrapancreatic segment of the superior mesenteric artery, and passes within the transverse mesocolon right to the midline. In 50 % of individuals, before reaching the transverse colon, the arterial trunk divides upward into left and right branches to reach the transverse colon and the colonic flexures, respectively.
2.2.3.2 Inferior Mesenteric Artery
The inferior mesenteric artery (diameter, 4.4 ± 0.5 mm) supplies the left third of the transverse colon, the descending and sigmoid colons, and most of the rectum (see “Blood Supply of the Rectum and Anus” later in the chapter).
Left Colic Artery
The left colic artery (diameter, 3.1 ± 1.0 mm) arises from the left side of the inferior mesenteric artery, crosses the left kidney, and divides upward into an ascending branch that passes into the left colic flexure and a descending branch that passes into the descending and sigmoid colons. In 16 % of individuals these branches originate directly from the inferior mesenteric artery (in the absence of a left colic artery).
Sigmoid Arteries
The sigmoid arteries (diameter, 3.0 ± 0.5 mm) number between two and five. They branch from the inferior mesenteric artery and cross the left ureter and gonadal vessels, passing within the mesosigmoid to reach the sigmoid colon. Branches anastomose to the left colic artery and the superior rectal artery via primary or secondary arcades (the marginal artery of the colon). This anastomosis is also called Sudeck’s point.
2.2.3.3 Marginal Artery of the Colon
The marginal artery of the colon (Drummond’s artery) is formed by the dividing arcades of the ileocolic, right, middle, left colic, and sigmoid arteries. The artery runs parallel and adjacent to the colon within the mesentery and gives rise to the vasa recta and brevia, which directly enter the colonic wall. In addition to the anastomosis between the middle and left colic artery via the marginal artery, a large branch may be present, connecting the superior and inferior mesenteric arteries directly; this is also called the arch of Riolan (Griffith’s point).
2.2.4 Lymphatic Drainage of the Colon
Colonic lymph nodes are subdivided into four groups:
Epiploic lymph nodes on the serosal surface and within the epiploic appendices
Paracolic lymph nodes adjacent to the colonic wall
Intermediate lymph nodes along the colic blood vessels
Preterminal lymph nodes along the main trunks of the superior and inferior mesenteric arteries
Preterminal lymph nodes drain into para-aortic lymph nodes located at the origin of these visceral arteries and are referred to as the highest lymph node station of the colon.
2.2.4.1 Venous Drainage of the Colon
Venous blood from the colon is collected by branches draining into the superior mesenteric vein (ileocolic vein, right colic veins, middle colic vein) and inferior mesenteric vein (left colic vein, sigmoid veins). In most cases the right and middle colic veins are joined by the right gastroepiploic and pancreaticoduodenal veins, forming the so-called gastrocolic trunk of Henle.
2.2.5 Nerve Supply of the Colon
2.2.5.1 Sympathetic Nerves
The cecum, ascending colon, and two thirds of the transverse colon are supplied by sympathetic nerves originating from the 5th to the 12th thoracic segments. Preganglionic nerve fibers pass via the greater and lesser splanchnic nerves to the celiac and superior mesenteric plexuses, where they switch over to final neurons. Nerve fibers (postganglionic) of these neurons reach the colonic wall via the periarterial plexus along the superior mesenteric artery.
The left one third of the transverse colon, the descending colon, and sigmoid colon are supplied by sympathetic nerves from the lumbar and upper sacral spinal segments. Preganglionic nerve fibers travel via lumbar splanchnic nerves to the inferior mesenteric plexus and via sacral splanchnic nerves to the superior and inferior hypogastric plexus. Postganglionic nerve fibers enter the colonic wall via the periarterial plexus along the inferior mesenteric artery.
The sympathetic input mediates relaxation of the colonic wall and contraction of both the ileocecal valve and the vascular musculature. Afferent nerve fibers are primarily responsible for the sensation of visceral pain.
2.2.6 Parasympathetic Nerves
The cecum, ascending colon, and two thirds of the transverse colon are supplied by parasympathetic nerve fibers derived from the vagus nerve. These vagal nerve fibers travel via the celiac and superior mesenteric plexuses into the colonic wall, where they switch to intramural ganglion cells.
The left one third of the transverse colon, the descending colon, and the sigmoid colon are supplied by parasympathetic nerves originating from the second to the fourth sacral segments (the sacral parasympathetic input). The parasympathetic nerve fibers pass through the inferior and superior hypogastric plexus, via the pelvic splanchnic nerves, and reach the colonic wall, following the branches of the inferior mesenteric artery.
The parasympathetic input mediates contraction of the colonic wall musculature, relaxation of the internal anal sphincter, and secretomotor functions. Sensations of distension and pain are carried by afferent parasympathetic nerve fibers (Fig. 2.2).
Fig. 2.2
Nerve supply of the anorectum and pelvic floor. (a) Somatic and autonomic innervation of the anorectum and pelvic floor. 1 sacral nerves (a S2, b S3, c S4), 2 pudendal nerve, 3 levatory nerves, 4 inferior rectal nerves, 5 somatic innervation of the pelvic floor and external anal sphincter, 6 sympathetic trunk, 7 lumbar splanchnic nerves, 8 grey communicans nerve, 9 superior hypogastric plexus, 10 hypogastric nerves, 11 sacral splanchnic nerves, 12 inferior hypogastric plexus, 13 pelvic splanchnic nerves. (b) Somatic and autonomic innervation of the anorectum and pelvic floor in men. 1 superior hypogastric plexus, 2 hypogastric nerves, 3 sacral splanchnic nerves, 4 inferior hypogastric plexus, 5 pudendal nerve, 6 inferior rectal nerves, 7 posterior scrotal nerves, 8 dorsal nerve of the penis
2.2.6.1 Enteric Nervous System
While the connections between the central nervous system and the intestine are established by extrinsic sympathetic and parasympathetic nerves to modulate gut activities, the enteric nervous system resides within the bowel wall and is responsible for coordinating major intestinal functions such as motility and secretion. In addition to sympathetic and parasympathetic mediators, a broad spectrum of nonadrenergic, noncholinergic neurotransmitters is released by intrinsic intramural nerve cells to establish local reflex circuits, which provide control of intestinal motor functions virtually independent from higher nervous inputs.
The enteric nervous system nerves comprises ~150 million neurons (“little brain of the gut”) and is organized in different nervous networks (plexus) composed of clusters of nerve cells (enteric ganglia) and interconnecting nerve fiber strands. The major plexuses are located in the intermuscular space between the longitudinal and circular muscle layers (myenteric plexus), within the submucosa (external and internal submucosal plexuses), and within the mucosa (mucosal plexus) (Fig. 2.3).
Fig. 2.3
Topographical organisation of the enteric nervous system in the human colon. 1 plexus of the longitudinal muscle layer, 2 myenteric plexus, 3 plexus of the circular muscle layer, 4 external submucosal plexus, 5 intermediate submucosal plexus, 6 internal submucosal plexus, 7 plexus of the lamina muscularis mucosae, 8 mucosal plexus (subglandular portion), 9 mucosal plexus (periglandular portion). Ganglionated plexus appear in color, dark dots represent enteric nerve cells
In addition to the nerve plexus, both the circular and longitudinal muscle layers and the intermuscular space contain a network of interstitial cells of Cajal (ICCs). These interdigitating cells are intercalated between nerve fibers and smooth muscle cells and generate the slow-wave activity of the colonic musculature; they are also referred to as intestinal pacemaker cells. Moreover, they are actively involved in intestinal neurotransmission by mediating neuronal inputs to smooth muscle cells.
2.3 Rectum and Anus
The rectum is the final segment of the large intestine and has a twofold function:
Retention of feces and closure of the gastrointestinal tract (continence)
Controlled evacuation of feces (defecation)
The rectum is 15–19 cm long and extends from the third sacral vertebra to the perineum. It is the most dorsally located intrapelvic organ, descending along the sacrococcygeal concavity (sacral flexure) and passing through the pelvic floor at the anorectal junction (perineal flexure, anorectal angulation).
The rectum is divided into two segments:
Rectal ampulla
Anal canal
In contrast to the colon, the rectum is characterized by the following anatomic peculiarities:
Confluence of teniae coli to a continuous longitudinal smooth muscle layer
Absence of epiploic appendices
Presence of permanent semilunar transverse folds, the most constant middle fold (a Kohlrausch fold), and a superior and inferior fold (Houston’s folds)
Extraperitoneal position of the lower and dorsal parts of the organ
2.3.1 Rectal Ampulla
The rectal ampulla is the widest part of the rectum, with a perimeter varying between 8 and 16 cm. Its ventral wall is covered by visceral peritoneum reflecting on to the bladder and seminal vesicles in males (rectovesical pouch) and onto the uterus and upper posterior vaginal wall in females (rectouterine pouch, Douglas’s pouch). The rectal musculature is arranged in a folding grille-like pattern, enabling the wall to adequately adjust to the highly variable filling state.
2.3.2 Anal Canal and Anus
The anal canal (pars analis recti) is 2.5–4 cm long with a perimeter of 5–9 cm. It forms a 90–100° angle with the rectum (the anorectal angle), which is caused by the constant traction of the puborectal sling (see the section “Pelvic floor” later in the chapter). The inner lining of the anal canal varies along its course to the anus (Fig. 2.4).
Fig. 2.4
Rectum and anal canal. (a) Frontal section of the anorectum. 1 levator ani muscle (iliococcygeal muscle), 2 levator ani muscle (puborectal muscle), 3–5 external anal sphincter (deep, superficial, subcutaneous part), 6 perianal veins, 7 perianal skin, 8 anoderm, 9 anal columns and crypts, 10 conjoined longitudinal muscle (corrugator ani muscle), 11 internal anal sphincter, 12 corpus cavernosum recti, 13 anorectal junction, 14 circular rectal muscle layer, 15 longitudinal rectal muscle laycanal, 3 anal crypts, 4 anocutaneous line, 5 anorectal junction, 6 internal anal sphincter, 7 external anal sphincter (a subcutaneous part, (b) superficial part, (c) deep part), 8 puborectal muscle, 9 corpus cavernosum recti, 10 anococcygeal ligament, 11 levator ani muscle, 12 deep transverse perineal muscle, 13 prostate, 14 prerectal muscle fibres, 15 corrugator ani muscle, 16 anal canal muscle
2.3.2.1 Inner Surface of the Anal Canal
The upper part of the anal canal is covered with a pink intestinal mucosa (the colorectal zone). At the transitional zone the wet columnar epithelium gives way to the dry squamous epithelium, displaying a histological mosaic of cylindrical, cubic, and flat epithelial cells. Macroscopically, the transitional zone is characterized by 8–12 vertical anal columns (Morgagni’s columns), each of which contains a terminal branch from the superior rectal artery. The anal columns are separated by the anal sinuses, which form pocket-like mucosal folds at their lower ends, called anal valves or crypts. The row of alternating anal columns and sinuses corresponds to the dentate line (pectinate line, crypt line), which is considered to be the junction between the endodermal (cloacal) and ectodermal (proctodeal) parts of the anal canal.
Between the dentate line and the anocutaneous line, the pale anoderm (squamous zone) extends for ~1.5 cm to the anal verge. The anoderm is lined by a nonkeratinized, stratified squamous epithelium devoid of glands and hairs but richly equipped with sensory nerve endings that are highly sensitive to touch, pain, and temperature. A whitish-bluish line (lineal alba, Hilton’s line) is occasionally visible at the lower end of the anal canal, corresponding to the underlying bulge of the internal anal sphincter.
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