The arrangement of the muscle layers at the equine ileocaecal junction.

The muscle layers of the ileocaecal junctions of horses (n = 18), plains zebras (Equus burchelli antiquorum) (n = 3), mountain zebras (Equus zebra zebra) (n = 2), and a donkey were examined macroscopically and microscopically. The muscular tunic of the ileocaecal papilla was made up of 3 layers: an innermost circular layer which was a continuation of the circular muscle of the ileum, a central longitudinal layer which was a continuation of one part of the longitudinal layer of the ileum and an outermost layer, composed of two collateral semicircular lips, formed by the circular layer of the caecum. The other part of the longitudinal ileal muscle layer contributed to the dorsal caecal taenia. The circular and semicircular muscle layers contributed by the ileum and caecum respectively represented what could be termed as a M. sphincter ilei, which did not exist as a separate annular muscle confined only to the papilla as the name would suggest. The amount of intermuscular connective tissue increased in both these layers towards the tip of the papilla, while at the same time the layers decreased in thickness. This finding supports the view that their sphincteroid action would be less efficient and that an additional closing mechanism of vascular origin may be required at the ileocaecal papilla of the horse.