Anatomy of Jejunum and Ileum

Anatomy of the Small Intestine

• The jejunum and ileum are 6 meters long, with the upper 2/5 being the jejunum.
• The coils of the jejunum and ileum are freely mobile.
• The jejunum begins at the duodenojejunal junction, and the ileum ends at the ileocecal junction.
• They are attached to the posterior abdominal wall by a fan-shaped fold of peritoneum called the mesentery of the small intestine.

Distinguishing Features of Jejunum and Ileum

• The jejunum lies coiled in the upper left part of the abdominal cavity, while the ileum lies coiled in the lower right part of the abdominal cavity and in the pelvis.
• The jejunum is redder and thicker than the ileum due to the larger and more numerous plica circulares.
• The jejunal mesentery is attached to the posterior abdominal wall above and to the left of the aorta, while the ileal mesentery is attached below and to the right of the aorta.
• The jejunal mesenteric vessels form only one or two arcades with long and infrequent branching, whereas the ileal mesenteric vessels form three or four arcades with numerous and short vessels.
• Peyer's patches are present in the mucous membrane of the lower ileum.
• Fat is deposited near the root of the jejunal mesentery and is scanty near the intestinal wall, whereas in the ileal mesentery, fat is deposited throughout.

Arterial Supply and Lymph Drainage

• The superior mesenteric artery supplies the jejunum and ileum through numerous jejunal and ileal branches that anastomose with each other to form arcades.
• Lymph drainage passes to the mesenteric lymph nodes and then to the superior mesenteric nodes and finally to the celiac nodes.

Mesentery of the Small Intestine

• The mesentery of the small intestine is a double-layered fold of peritoneum that attaches the whole length of the jejunum and ileum to the oblique line across the posterior abdominal wall.
• It has a long free edge that encloses the mobile intestine and a short root that is continuous with the parietal peritoneum of the posterior abdominal wall.
• The root of the mesentery is about 15 cm long and contains the superior mesenteric vessels, crossing the horizontal part of the duodenum, aorta, inferior vena cava, ureter on the right psoas, and right testicular or ovarian vessels.

Contents of the Mesentery

• Jejunal and ileal blood vessels
• Extraperitoneal fatty tissue
• Plexus of autonomic nerves
• Large lacteal lymph vessels

Large Intestine

• Extends from the cecum in the right iliac fossa to the anus in the perineum, surrounding the centrally placed small intestine.
• Divided into the cecum, vermiform appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.

Cecum and Vermiform Appendix

• The cecum is a blind-ended sac, about 7.5 cm in length and breadth, lying in the right iliac fossa.
• The terminal part of the ileum enters the cecum at the ileocecal orifice.
• The base of the appendix is attached to the posteromedial surface of the cecum and opens into it about 2.5 cm below the ileocecal opening.
• The tip of the appendix has varying positions, including hanging down into the pelvis, coiled up behind the cecum, in front or behind the terminal part of the ileum, or projecting upward along the lateral side of the cecum.

Arterial Supply and Nerve Supply of the Appendix

• The appendicular artery, a branch of the posterior cecal artery, supplies the appendix.
• The appendix is supplied by sympathetic and parasympathetic (vagus) nerves from the superior mesenteric plexus.

Ascending Colon

• The ascending colon is about 13 cm long and extends upward from the cecum to the posteroinferior surface of the liver, where it turns to the left, forming the right colic flexure.
• Relations: anteriorly, the greater omentum, anterior abdominal wall, stomach, liver, and gallbladder; posteriorly, the second part of the duodenum, head of the pancreas, left kidney, and coils of the small intestine.

Descending Colon

• The descending colon is about 25 cm long and extends downward from the left colic flexure to the sigmoid colon.
• The peritoneum covers the front and sides and binds it to the posterior abdominal wall.
• Relations: anteriorly, the greater omentum, anterior abdominal wall, and coils of the small intestine; posteriorly, the psoas muscle, iliacus muscle, femoral nerve, lateral cutaneous nerve of the thigh, left gonadal vessels, genitofemoral nerve, iliac crest, lower part of the left kidney, and origin of the transverse abdominis.

Differences between Small and Large Intestine

• External differences:
  • Small intestine is mobile, while ascending and descending colon are fixed.
  • Caliber of small intestine is smaller than that of large intestine.
  • Small intestine has a mesentery that passes downward across the midline into the right iliac fossa.
  • Longitudinal muscle of small intestine forms a continuous layer around the gut, whereas in large intestine, it is collected into three bands (teniae coli).
  • Small intestine has no fatty tags (appendices epiploicae) attached to its wall.
• Internal differences:
  • Mucous membrane of small intestine has plicacirculares, villi, and Peyer's patches, which are absent in large intestine.

Appendicitis

• Acute inflammation of the appendix, caused by hyperplasia of lymphatic follicles in the appendix that occludes the lumen.
• Symptoms: digital pressure over the McBurney point registers maximum abdominal tenderness.
• Complications: thrombosis in the appendicular artery, ischemia, gangrene, and perforation of an inflamed appendix, leading to peritonitis.

Predisposition of the Appendix to Infection

• The appendix is prone to infection due to its long, narrow, blind-ended tube structure, which encourages stasis of large-bowel contents.
• The appendix has a large amount of lymphoid tissue in its wall.
• The lumen has a tendency to become obstructed by hardened intestinal contents (enteroliths), leading to further stagnation of its contents.