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 os 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.