The Descending Colon

• Connects the left colic flexure to the sigmoid colon.
  
  
• About 25 cm long, and is narrower than the ascending colon.
  
  
• Ascends on the posterior abdominal wall to the iliac crest and then inclines medially on iliacus and psoas major to the pelvic brim where it becomes continuous with the sigmoid colon.
  
  
  
• Related posteriorly to the diaphragm and quadratus lumborum, but the subcostal vessels and nerve and the iliohypogastric and ilio-inguinal nerves intervene between it and quadratus.
  
  
• Beyond the femoral and genitotemoral nerves, gonadal vessels and external iliac vessels are between it and psoas major.

• Forms a sinous loop about 40 cm long
  
  
• Extends from the pelvic brim to the middle piece of the sacrum where it becomes the rectum.
  
  
• Occupies the rectovesical fossa in the male and the recto-uterine fossa in the female

• About 12.5 – 15 cm long.
  
  
• Commences in front of the middle piece of the sacrum, at the rectosigmoid junction
  
  
• Distinguished from the sigmoid colon by its lack of mobility – it lacks a mesentery.
  
  
• Its upper third is invested with peritoneum anteriorly and laterally
  
  
• its middle third  is invested with peritoneum anteriorly only
  
  
• Lower third lies below the forward reflection of peritoneum (on to uterus or bladder as the case may be.
  
  
• The lower third follows the line of curvature of the sacrum and ends below the level of the coccyx by becoming continuous with the anal canal.

Blood-supply of the large intestine

• The proximal part of the large bowel – caecum, appendix, ascending and two-thirds of the transverse colon – is supplied by the superior mesenteric artery, the artery of the midgut.
  
  
• The distal part – left third of transverse colon, descending colon, sigmoid colon and rectum – is supplied by the inferior mesenteric artery, the artery of the hindgut.
  
  
• Branches of these arteries anastomose to form the marginal artery of Drummond.

Lymphatic drainage of the large intestine

Lymph drains successively to the :

• Colic nodes (on the colonic wall)
• Paracolic nodes along the inner margin of the colon)
• Mesocolic nodes (in the mesenteries).

The lymphatics run alongside the arteries and finally join the intestinal lymph trunks which empty into cisterna chyli.

Parasympathetic:

• The proximal half of the large intestine, as far as the middle third of the transverse colon, receives its parasympathetic fibres from the posterior vagal trunk (it nerve-supply being similar to that of the small intestine.
  
  
• The distal half receives fibres from the inferior hypogastric plexus, derived from the pelvic splanchnic nerves from S2,3 and 4.
  
  
• Some of these fibres ascend in the hypogastric nerves to reach the inferior mesenteric plexus, while others ascend the parietal peritoneum to reach the colon independently.

Sympathetic:

• The larger bowel receives fibres from the plexus on its arteries
  
  
• Derived from the aortic plexus the superior and inferior hypogastric plexuses and also from medial branches of the sacral part of the sympathetic trunks.
  
  
• The preganglionic fibres originate in segments L1 and L2.