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. In mammals the duodenum may be the principal site for iron absorption. The duodenum precedes the jejunum and ileum and is the shortest part of the small intestine, where most chemical digestion takes place. The name duodenum is from the Latin duodenum digitorum, or "twelve fingers' breadth". In humans, the duodenum is a hollow jointed tube about 25-38cm (10-15 inches) long connecting the stomach to the jejunum. It begins with the duodenal bulb and ends at the ligament of Treitz. Anatomically, this small organ is divided into four segments: the superior, descending, horizontal and ascending duodenum. It is a C-shaped organ, inner lining of which is made of crypts. These crypts are responsible for increasing the surface area of the intestinal membrane and thereby ensure better digestion.