THE SMALL INTESTINE (JEJUNUM / ILEUM)

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.

Category: Anatomy Notes