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Horse · Splanchnology (organs)

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Model: "Heart (horse) Plastinated" by the Veterinary Faculty, University of Sarajevo (UNSA-VF on Sketchfab), CC BY 4.0

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Large colon

Colon ascendens

The horse's ascending colon, folded into a double horseshoe of four parts and three flexures. Only its right half is fixed; the left half is free and can displace or twist, and the pelvic flexure is the narrowest point and the commonest site of impaction.

Laterality:: Unpaired

Draft, not yet reviewed by a veterinarian. Content may contain errors. It is published so the community can review it.

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3D model of this piece: "Equine Colon" by OVCanatomy, Ontario Veterinary College (Sketchfab), CC BY 4.0

Overview

The large colon (colon ascendens) is, with the caecum, the horse's great fermentation chamber. It forms two stacked U-shaped loops, one ventral and one dorsal, that fill much of the abdomen. The ventral parts have four taeniae and sacculations; the pelvic flexure and left dorsal colon have a single taenia and a smooth wall; the right dorsal colon has three taeniae. The diameter changes greatly: it narrows sharply at the pelvic flexure and widens in the right dorsal colon.

Parts

Right ventral colon: leaves the caecocolic orifice and runs cranially along the right abdominal floor.
Sternal flexure: crosses from right to left on the floor, caudal to the xiphoid cartilage.
Left ventral colon: runs caudally along the left floor.
Pelvic flexure: at the pelvic inlet, often slightly to the left; it is the narrowest segment.
Left dorsal colon: runs back cranially, dorsal to the left ventral colon and joined to it by a short mesocolon.
Diaphragmatic flexure: against the diaphragm and the liver.
Right dorsal colon: runs caudally on the right and widens into an ampulla before continuing as the transverse colon, cranial to the cranial mesenteric artery.

Clinical relevance

Pelvic flexure impaction: the most common cause of impaction colic; the firm mass is felt on rectal palpation. It is treated with fluids by tube and intravenously and, if it does not resolve, by pelvic flexure enterotomy.
Left dorsal displacement (nephrosplenic entrapment): the left colon rises between the spleen and the body wall and hangs over the nephrosplenic ligament. It is diagnosed by rectal palpation and ultrasound (the left kidney can no longer be seen) and corrected with phenylephrine and exercise, by rolling the anaesthetised horse or surgically.
Large colon volvulus: typical of postpartum broodmares; very severe pain and rapid deterioration. It is a surgical emergency.
Enteroliths and sand: enteroliths form mainly in the right dorsal and transverse colon and cause obstruction when they move into the small colon; sand accumulates in the ventral colon.
Colitis: Salmonella, clostridia and Neorickettsia risticii cause severe acute diarrhoea; non-steroidal anti-inflammatory drugs such as phenylbutazone can cause ulcerative right dorsal colitis.