The duodenum's the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine aren't as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum. In mammals the duodenum may be the principal site for iron absorption. The duodenum precedes the jejunum and ileum and's the shortest part of the small intestine, where most chemical digestion takes place. The name duodenum'… (More on Duodenum)