THE LARGE INTESTINE (COLON)

DEVELOPMENT:

• Cecum, Ascending Colon, and Proximal 2/3 of Transverse Colon are midgut.
• Distal 1/3 of Transverse Colon, Splenic Flexure, Sigmoid Colon, Rectum, and Proximal Anal Canal are hindgut.
• Cloacal Membrane: At the distal end of the hindgut in the embryo.
• Allantois: Posterior part of the yolk sac. It will become the Urogenital Sinus and primitive urogenital system.
• • Invasion of the Folds:
• Tourneaux's Fold: A wedge of mesoderm that invades the hindgut region along the midsagittal plane.
• At same time, lateral Rathke's Folds invade along the frontal plane.
• These two folds come together such that the hindgut is separated from the primitive urogenital sinus.
• Perineal Body: The tissue in between the two primitive tubes formed by the Rathke's and Tourneax's Folds. It will form the future urogenital region.
• The perineal body divides two tubes, which are:
• Anorectal Canal
• Urogenital Sinus: This will be future perineum of the adult -- the region below the abdomen and superior to the pelvic bones, medial to the thighs.
• Perineal body is the common attachment site for future muscles in the region:
• Anal Sphincter.
• Muscles associated with the pelvic and urogenital diaphragms.
• In females it provides the primary support for reproductive organs.
• Proctodeum: Distal portion of hindgut, still covered by cloacal membrane. The cloacal membrane will eventually perforate, resulting in the anal opening.
• Pectinate Line: The division of hindgut (endodermal) anal canal, and ectoderm from invagination of the skin. They are both supplied by different vessels, nerves, etc.
• Upper Anal Canal, superior to pectinate line, is endodermal hindgut.
• Lower Anal Canal, inferior to pectinate line, is ectoderm.
• The Pectinate Line can be identified by looking for the anal columns, longitudinal folds of mucosa that demarcate the upper anal canal.
• Collateral Circulation: Due to the pectinate line, there are two alternative circulations in the area.
• Caval System of vessels supplies the ectodermal lower anus: Rectal Veins ------> Iliac Veins ------> Caval System
• Portal System of vessels supplies the endodermal upper anus: Superior Rectal Veins ------> Inferior Mesenteric Vein ------> Portal Vein System
• Because of the anastomosis, if there is an occlusion in one system, blood can get back to the circulation via the collateral system.

LOCATION: All four quadrants. In the nine-region system, it is located in the bottom six regions -- not the epigastric /hypochondriac regions.

EXTERNAL MORPHOLOGY:

• Order of Sections:
• Cecum / Ileocecal Junction: Intraperitoneal, for the most part.
• Vermiform Appendix: Can be intraperitoneal or retro. The appendix extends down over the pelvic brim.
• Ascending Colon: Retroperitoneal.
• Transverse colon: Intraperitoneal, covered by transverse mesocolon. Hence it is mobile.
• Descending Colon: Retroperitoneal
• Sigmoid Colon: Intraperitoneal, covered by sigmoid mesocolon. Hence it is mobile.
• Tenia Coli: Three longitudinal muscles that run the length of the large intestine.
• Rectosigmoid Junction: A complete expansion of the longitudinal muscles at the end of the colon, where it can have a muscular force.

• Sulci: Periodic indentations in the large intestine, on the external surface.
• Haustra: The "sections" of intestine created by the semilunar folds.
• Epiploic Appendices: The fatty appendages along the length of the large bowel. Their presence or absence is related to the diet of the individual.

INTERNAL MORPHOLOGY:

• There are no mucosal folding, like the small intestine.
• There are semilunar folds, the internal markings of the sulci on the outside. They are much further apart than in the jejunum.
• Diverticula: Outpocketings of the bowel, at the location of the semilunar folds. Food and popcorn can get stuck in there.

RELATIONSHIPS:

• Transverse Mesocolon: The mesentery connecting the transverse colon to the pancreas, stomach, and duodenum.

• Transverse mesocolon covers the pancreas. Hence pancreatitis can spread to the transverse colon.

• Sigmoid Mesocolon: The mesentery connecting the sigmoid colon to the posterior abdominal wall.
• Hepatic Flexure: Turning point of the ascending -> transverse colon on the right side, just inferior to the liver.
• Splenic Flexure: Turning point of the transverse ------> descending colon on the left side, just anterior to the left kidney.
• Phrenicocolic Ligament: Attaches the transverse colon to the left crus of the diaphragm, at the location of the splenic flexure.

• It is right next to the spleen.
• It inhibits the passage of fluid into the left paracolic gutter, and prevents fluid from getting into the supracolic (above mesocolon) area.

CLINICAL CONSIDERATIONS:

• Pancreatitis can spread to the transverse colon, via the transverse mesocolon.
• Diverticula can cause problems. See popcorn.
• Volvulus: is twisting of the sigmoid colon. It can lead to a strangulation of the vessels and eventual necrosis.

VASCULAR / LYMPH SUPPLY: Colic arteries have variations.

• Right Colic Artery: Comes off of the superior mesenteric artery, superior to the ileocolic artery, and supplies the ascending colon.

• It divides into the Arterial Arcades
• Middle Colic Artery: Comes off the superior mesenteric artery and supplies the Transverse Colon. It divides off right anterior to the duodenum.

• Left Colic Artery: Comes off the inferior mesenteric artery and supplies the descending colic.
• Sigmoid Arteries: Come off the inferior mesenteric and supply the sigmoid colon.

Category: Anatomy Notes