DEVELOPMENT: Small intestine develops as a herniation into the umbilical region.

• Bowel spins 90 counterclockwise during growth, so that the distal end is to the left of the proximal end.
• Then, in the Return Phase, there is a 180 rotation, which places the cecum just inferior, to the liver. Then the Cecum usually descends somewhat, but in some people and it doesn't, and is thus termed a sub hepatic cecum.
• Fixation occurs lastly: Organs become retroperitoneal secondarily. They start with peritoneum surrounding them, then they implant on the posterior wall, then they lose their peritoneum.
• At this point, what was once visceral peritoneum is now parietal.
• This secondary fixation occurs with all retroperitoneal organs except the rectum, which never has peritoneum in the first place.

LOCATION: It occupies most of the left upper quadrant and right lower quadrant of the abdomen.

· Jejunum Mostly in the umbilical region.

EXTERNAL MORPHOLOGY:

· 18-20 feet in length, but the mesentery holding it is only 4 feet long because it is scrunched up.

· Jejunum Proximal to the Ileum.

· THE Mesentery is the peritoneum surrounding the small intestine.

INTERNAL MORPHOLOGY:

• Jejunum has many circular folds on the inside lining, in the mucosa.
• It has a thicker wall.
• The Ileum is smoother and has solitary lymph follicles (little spots) on inside lining.
• It has a thinner wall.

CLINICAL CONSIDERATIONS:

o Meckel's Diverticulum: A portion of the bowel along the Ileum that may be left over from development.

§ Rule of Twos: In 2% of population, 2 feet from the distal end of the Ileum, and 2 inches long.

§ It creates a pouch which can collect unwanted waste and materials.

VASCULAR / LYMPH SUPPLY:

• Arteriae Rectae come off the superior mesenteric artery and supply the Jejunum, throughout the Mesentery. They run perpendicular to the superior mesenteric artery.
• Arterial Arcades have a more web-like pattern coming off the Superior Mesenteric Artery, and supply the Ileum.

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.

Abdominal Aorta:

· Enters the Aortic Hiatus between the right crus and left crus of the diaphragm at the level of T12.

· Extends retroperitoneally along the anterior surface of the vertebrae (slightly to the left), until the level of L4.

· Bifurcation of the Abdominal Aorta: It bifurcates at L4, into the Left Common Iliac and Right Common Iliac Arteries.

· Relations:

§ Goes posterior to the Uncinate Process and Body of the pancreas.

§ Goes posterior to the horizontal (third portion of) duodenum.

§ Goes posterior to the Left Renal Vein.

§ The left renal vein passes over (anterior to) the Aorta.

§ The left renal vein passes under (posterior to) the superior mesenteric artery.

§ The Inferior Vena Cava is to the right and slightly more anterior than the abdominal aorta.

§ At the bifurcation, the inferior vena cava passes posterior to the Aorta.

· Principle Branches:

§ Celiac Trunk

§ Superior Mesenteric Artery

§ Inferior Mesenteric Artery

§ Renal Arteries

§ Gonadal Arteries -- gonadal arteries pass to a region in the upper abdomen, not lower.

Celiac Trunk:

· Located just inferior to Aortic Hiatus.

· Branches:

§ Splenic

· Splenic ------> Left Gastroepiploic

§ Common Hepatic

· Common Hepatic ------> Proper Hepatic ------> Gastroduodenal ------> Right Gastroepiploic.

§ Right Gastroepiploic ------> Gastroduodenal Arteries

· Right Gastroepiploic ------> Superior Pancreaticoduodenal Arteries

§ Left Gastric

· Clinical:

§ If the left gastric is occluded, blood can be rerouted through the right gastric.

§ With gastroeosphageal cancer, the left gastric can be ligated, and the right gastric will still supply blood.

Superior Mesenteric Artery:

· Branches:

§ SMA ------> Inferior Pancreaticoduodenal Arteries

§ SMA ------> Middle Colic ------> (transverse colon)

§ SMA ------> Right Colic ------> (ascending colon)

§ SMA ------> Ileocolic ------> Ileal and Colic

· Marginal Artery: Comes off the Left Colic Artery and can supply the medial aspect of the large intestine in the absence of a middle colic.

· Inferior Mesenteric Artery:

§ IMA ------> Left Colic

§ IMA ------> Sigmoid Artery

§ IMA ------> Rectosigmoid

§ IMA ------> Superior Rectal

Pancreaticoduodenal Arcade: An alternative route for blood flow through the branches of the celiac, if there should be an occlusion in the celiac trunk.

§ Superior Pancreaticoduodenal Arteries come from the Hepatic branch of the Celiac.

§ Inferior Pancreaticoduodenal Arteries come from the SMA.

Lumbar Arteries: Supply the posterior abdominal wall.

§ 1st - 4th Lumbar Arteries come off of the Aortic Trunk directly.

§ 5th Lumbar Artery comes off of the Median Sacral Artery, below the bifurcation of the Aorta.