The arrangement of the muscle at the ileo-caeco-rectal junction of the domestic duck (Anas platyrhynchos) and the presence of anatomical sphincters.

The arrangement of the muscle at the ileo-caeco-rectal junction of the domestic duck (Anas platyrhynchos) was studied using light microscopy, three-dimensional reconstruction models and scanning and transmission electron microscopy. The junction consists of a papilla-like protrusion of the ileum into the rectum, the 'ileal papilla', lateral to which are the openings of the large right and left caeca. The ileal papilla consists entirely of circular muscle which is not divided into an inner electron-dense portion and an outer electron-lucent portion as is the rest of the ileum. The longitudinal muscle layer does not enter the ileal papilla. The circular muscle at the junction forms three thickenings or sphincters which are continuous with one another: an ileal sphincter in the ileal papilla, and right and left caecal sphincters around the orifices of the caeca. The majority of nerve bundles are found in the circular muscle layer and consist of axons with small granular vesicles, axons with small agranular vesicles and axons with many large granular vesicles. The density of innervation, including the total number of axon profiles and the number of vesiculated axon profiles, is significantly higher in the circular muscle of the sphincters than in the ileum, caecum and rectum 5 mm from the junction.