The duodenal cap or duodenal ampulla is the very first part of the 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.
This definition incorporates text from the wikipedia website - Wikipedia: The free encyclopedia. (2004, July 22). FL: Wikimedia Foundation, Inc. Retrieved August 10, 2004, from http://www.wikipedia.org