Duodenal cap
Encyclopedia
The duodenal cap or duodenal ampulla is the very first part of the duodenum
Duodenum
The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...

 which is slightly dilated.

It is the part which is intraperitoneal and is about 2 cm long.
It is mobile and has a mesentery. It is also smooth walled.
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