Gross Features of the Small Intestine and Large Intestine

SMALL INTESTINE

Overview

1. Duodenum

• Length: ~20-30 cm (the shortest, widest, and most fixed part)
• Peritoneal relations: Mostly retroperitoneal (secondarily), except the first 2.5 cm (the "cap" or bulb) which is intraperitoneal
• Divided into four parts:

• The Kerckring folds (plicae circulares) begin at the junction of the 1st and 2nd parts
• Arterial supply: Upper duodenum - branches of the celiac trunk (gastroduodenal artery); lower duodenum - superior mesenteric artery (SMA)

2. Jejunum and Ileum

• Both are intraperitoneal and suspended by the mesentery from the posterior abdominal wall
• No sharp anatomic boundary between them; by convention: proximal 2/5 = jejunum, distal 3/5 = ileum
• The jejunum begins at the ligament of Treitz (in the left upper abdomen) and the ileum terminates at the ileocecal valve (right lower quadrant)

Key Gross Differences - Jejunum vs. Ileum

• The ileocecal valve prevents reflux of colonic contents back into the small intestine (though 70-90% are functionally incompetent on contrast studies)
• Blood supply: Both jejunum and ileum are supplied by the SMA, which forms arcades within the mesentery before the vasa recta penetrate the bowel wall

LARGE INTESTINE

Fig. 4.80 - Large Intestine (Gray's Anatomy for Students)

Overview

Distinguishing Gross Features of the Large Intestine (vs. Small Intestine)

1. Larger internal diameter compared to the small intestine
2. Taeniae coli - three narrow bands of longitudinal muscle running along the wall (most visible on cecum and colon; less so on rectum); the free taenia leads directly to the base of the appendix
3. Haustra of the colon - sacculations (pouches) of the colon wall between the taeniae
4. Omental appendices (epiploic appendages) - peritoneum-covered fat tags attached to the outer surface of the colon

Parts of the Large Intestine

1. Cecum

• Lies in the right iliac fossa, inferior to the ileocecal opening
• Generally intraperitoneal (mobile, no mesentery)
• The first part of the large intestine; may descend into the true pelvis
• The appendix attaches to its posteromedial wall, just inferior to the ileal opening

2. Appendix

• Narrow, hollow, blind-ended tube with a large amount of lymphoid tissue in its walls
• Suspended from the terminal ileum by the mesoappendix (containing the appendicular vessels)
• Variable position: retrocecal (most common), pelvic/descending, subcecal, pre-ileal, post-ileal
• McBurney's point = junction of lateral and middle 1/3 of a line from the ASIS to the umbilicus (surface projection of the appendix base; tender in appendicitis)

3. Colon

Fig. 4.89 - Colon (Gray's Anatomy for Students)

• Ascending and transverse colon: Superior mesenteric artery (SMA) - via ileocolic, right colic, and middle colic arteries
• Descending and sigmoid colon: Inferior mesenteric artery (IMA) - via left colic and sigmoid arteries
• Anastomotic connection along the entire colon = marginal artery (of Drummond)

4. Rectum

• Begins at the rectosigmoid junction (level of S3 or end of sigmoid mesocolon)
• Retroperitoneal structure (no mesentery)
• Follows the sacral curvature; wider ampulla stores feces before defecation
• Blood supply: Superior rectal artery (from IMA), middle rectal artery (from internal iliac), inferior rectal artery (from internal pudendal)

5. Anal Canal

• Terminal part of the large intestine; continuation of rectum inferiorly

Summary Comparison Table: Small vs. Large Intestine

• Gray's Anatomy for Students, pp. 379-385 (Large Intestine section)
• Mulholland and Greenfield's Surgery: Scientific Principles and Practice, 7e, pp. 2362-2368 (Small Intestine Gross Anatomy and Embryology)