ileum ileum the ileum (/ˈɪliəm/) is the final 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 posterior intestine or distal intestine may be used instead of ileum. its main function is to absorb vitamin b12, bile salts, and whatever products of digestion that were not absorbed by the jejunum. ileumsmall intestinethe cecal fossa. the ileum and cecum are drawn backward and upward.detailsprecursormidgutarteryileal arteriesveinileal veinsnerveceliac ganglia, vagusidentifierslatinileummeshd007082ta98a05.6.04.001ta22959fma7208anatomical terminology major parts of thegastrointestinal tract upper gastrointestinal tract mouth esophagus stomach duodenum lower gastrointestinal tract small intestine jejunum ileum large intestine sigmoid colon rectum anus