Descending mesocolon

The descending mesocolon is a fold of the peritoneum that suspends the descending colon from the dorsal abdominal wall. It forms part of the dorsal mesentery and is continuous cranially with the transverse mesocolon and caudally with the mesorectum.

Carnivores (Dog and Cat)

• The descending mesocolon is relatively short and simple.

• It is attached to the dorsal abdominal wall along the left side of the abdominal cavity.

• The descending colon runs caudally from the transverse colon to the rectum along the left flank.

• The caudal mesenteric artery and its branches (notably the left colic artery) run within the mesocolon to supply the descending colon.

• It is easily mobilized during surgical procedures due to its limited length and mobility.

Ruminants (Cattle, Sheep, Goats)

• The descending mesocolon is longer and more developed.

• Due to the complexity of the ruminant gastrointestinal tract, it is involved in suspending a more extended portion of the descending colon.

• It also forms part of the coiled arrangement of the large intestine in ruminants, especially in sheep and goats where the colon may coil in a spiral loop (ansa spiralis coli).

• In large ruminants like cattle, the descending colon is relatively straight, and the mesocolon anchors it to the left side of the dorsal abdominal wall.

• It is commonly accessed during colotomy or resection surgeries for obstructions or intussusceptions.

Equine (Horse)

• The descending colon in the horse is often referred to as the small colon.

• The mesocolon in this species is extensively developed and elongated, allowing the small colon to have a great deal of mobility and to form loops.

• It is attached dorsally and is often involved in impaction colic, a common issue in horses.

• The mesocolon contains prominent lymph nodes and vascular structures (such as the branches of the caudal mesenteric artery), which are of clinical importance.

Swine (Pig)

• The descending colon is part of a spiral colon, and the mesocolon is part of the central mesentery of the coiled large intestine.

• The descending mesocolon in pigs is intertwined with the mesenteries of the ascending colon loops.

• It allows for considerable fixity and helps stabilize the coiled colon complex.

• Less often accessed surgically due to the intraperitoneal complexity of the porcine gut.

Vascular and Nervous Supply

• Arterial Supply: Primarily by the left colic artery, a branch of the caudal mesenteric artery.

• Venous Drainage: Corresponds to arterial supply, with the left colic vein draining into the caudal mesenteric vein, which then joins the portal venous system.

• Lymphatics: Drain into the mesocolic lymph nodes located within the descending mesocolon.

• Innervation: Via the pelvic plexus and hypogastric nerves, carrying both sympathetic and parasympathetic fibers.