Large Intestine

• The colon


  The distal part of the gastrointestinal tract, extending from the caecum to the anal canal

  
• Anatomical parts of the colon

  • Ascending (caecum, vermiform appendix)
  • Transverse
  • Descending
  • Sigmoid

  
• The colon averages 150cm in length
• How the large intestine can be distinguished from the small intestine

  • Taeniae coli (three thickened bands of longitudinal muscle)
  • Sacculations of its walls between the taeniae, called haustra
  • Appendices epiploicae (small pouches of omentum filled with fat)
  • Much greater caliber

  
• Taeniae coli

  Thickened bands of smooth muscle representing most of the longitudinal coat
• Formation of taeniae coli

  1. Begin at the base of the appendix as the thick longitudinal layer of the appendix splits to form three bands

  2. Run the length of the large intestine, merging again at the rectosigmoid junction into a continuous longitudinal layer around the rectum

  
• Haustra


  Multiple pouches in the wall of the large intestine formed where the longitudinal muscle layer is deficient
• Appendices epiploicae


  Fat-filled pockets of peritoneum projecting from the visceral peritoneum on the surface of the large intestine (except the rectum)

  
• Cecum


  The sac-like first part of the large intestine, continuous with the ascending colon, located in the right lower quadrant

  
• Cecum


  • 5 to 7 cm in length
  • Located in the iliac fossa, inferior to the ascending colon
  • Ileum opens into its superior part at the ileocaecal junction
  • Vermiform appendix opens into its medial aspect about 2.5 cm inferior to the ileocaecal junction
  • Intraperitoneal

  
• Ileocaecal orifice

  Where the ileum enters the cecum obliquely, partially invaginating into it and forming superior and inferior lips (frenula) of the ileocaecal valve

  
• Vermiform appendix

  A narrow, worm-shaped blind tube joining the cecum about 2.5 cm inferior to the ileocaecal junction
• Vermiform appendix

  • Variable in length, averaging 8 cm
  • Relatively longer in infants and children than in adults
  • Has its own short triangular mesentery (mesoappendix)
  • Position is variable: retrocaecal, retrocolic (65%), pelvic (31%), subcaecal (2.3%), rarely anterior or posterior to the terminal ileum
  • Base is fairly constant, usually at McBurney's point (junction of lateral and middle 1/3 of line joining ASIS and umbilicus)
  • Three taeniae coli of the cecum converge at the base and form a complete outer longitudinal coat

  
• Ascending colon

  Located in the right paracolic gutter, covered by peritoneum on front and sides, posterior surface on iliacus, quadratus lumborum, and transversus abdominis muscles, crossed by lateral cutaneous nerve of thigh, ilioinguinal nerve, and iliohypogastric nerve

  
• Right colic flexure

  Junction of ascending and transverse colon, located anterior to lower right kidney and inferior to right liver lobe
• Transverse colon


  Longest (20 inch/50 cm) and most mobile part of large intestine, stretching from right to left colic flexures, creating a dependent loop in front of small intestine

  
• The lowest point of the transverse colon loop generally reaches the level of the umbilicus, but may occasionally extend into the pelvis

  
• Differences between right 2/3 and left 1/3 of transverse colon

  • Development (right 2/3 from midgut, left 1/3 from hindgut)
  • Arterial supply (right 2/3 from middle colic artery of superior mesenteric, left 1/3 from left colic artery of inferior mesenteric)
  • Nerve supply (right 2/3 by vagus, left 1/3 by pelvic splanchnic)

  
• Left colic flexure


  Junction of transverse and descending colon, located anterior to left kidney and inferior to spleen
• Descending colon


  Longer (25 cm), narrower, and more deeply located than ascending colon, going from left colic flexure to pelvic brim where it becomes sigmoid colon, covered by peritoneum on front and sides

  
• Course of descending colon


  • Proximal part descends vertically, entering in front of quadratus lumborum, transversus abdominis, and iliacus muscles, and the iliohypogastric, ilioinguinal, and lateral cutaneous nerves of the thigh
  • Distal part turns medially from left iliac fossa to front of left external iliac vessels, entering in front of femoral nerve, psoas major, testicular vessels, genitofemoral nerve, and left external iliac vein

  
• Sigmoid (pelvic) mesocolon


  Large peritoneal fold suspending the sigmoid colon from the pelvic wall, with an inverted V-shaped root connecting to the external iliac artery and pelvic sacrum

  
• The intersigmoid recess is a pocket-like expansion of the peritoneal cavity entering upward posterior to the root of the sigmoid mesocolon, with the left ureter located behind it

  
• The inferior mesenteric artery splits near the apex of the inverted V-shaped root of the sigmoid mesocolon
• Sigmoid (pelvic) mesocolon


  The sigmoid colon is suspended from the pelvic wall by a large peritoneal fold termed sigmoid mesocolon

  
• Sigmoid mesocolon


  1. Left limb connected on the external iliac artery

  2. Right limb connected on the pelvic outermost layer of the sacrum

  3. Meeting point of 2 limbs is termed apex
• Intersigmoid recess


  Pocket-like expansion of the peritoneal cavity that enters upward posterior to the root of the mesocolon
• Midgut development

  1. Returns to abdomen

  2. Undergoes rotation
• Midgut undergoes rotation

  6th week of embryonic development
• Structures derived from the ventral mesogastrium

  • Lesser omentum
  • Falciform ligament
  • Hepatic ligaments
• Cranial limb of the midgut


  Includes the duodenum (proximal part), jejunum, and part of the ileum
• Anal columns


  Separated from one another by anal sinuses
• Vertical part of the duodenum

  • Not crossed by the root of the mesentery
  • Lies anterior to the root of the mesentery
• Ileocolic artery


  The inferior most branch of the superior mesenteric artery
• Ileocolic artery


  • Supplies the ascending colon, appendix, cecum, and ileum
• Greater omentum


  Forms from the ventral mesentery
• Mesoduodenum


  Formed by the dorsal mesentery
• Mesocolon


  Forms from the dorsal mesentery
• Mesentery


  Also forms from the dorsal mesentery
• Mesoduodenum


  Part of the dorsal mesentery that supports the duodenum, attaching the proximal part of the duodenum to the posterior abdominal wall