The duodenum is the first part of the small intestine and is much shorter compared to the longest portion, the jejunum. The duodenum is much more straight than the jejunum as well. The mesentery of the duodenum (the duomesentery) contains one of the tails of the pancreas. It starts just after the pyloric sphincter and ends where the small intestine begins to beome wiggly and compacted (where the jejunum begins)

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In this picture the person's outermost finger is touching the opening in the stomach (towards the top right of the picture). The duodenum is the length of the small intestine clearly visible from the end of the stomach (the pyloric sphincter) towards the left of the picture
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In this cartoon, the duodenum is clearly visible and labeled 1 and the pancreas lining its underside is attached and labeled 2:

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