1. The third and longest portion of the small intestine, about 12 feet in length in humans (by postmortem measurement), extending from an indistinct junction with the jejunum to the ileocecal opening. Overall, it is distinct from jejunum in being typically smaller in diameter with thinner walls, having smaller and less complex circular folds (plicae circulares), its mesentery having more fat and its arteries (ileal arteries) forming more tiers of arterial arcades with shorter vasa recta.

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