||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (April 2009)|
|The cecal fossa. The ileum and cecum are drawn backward and upward.|
|Gray's||subject #248 1171|
|Nerve||celiac ganglia, vagus |
The ileum // 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.
The ileum follows the duodenum and jejunum and is separated from the cecum by the ileocecal valve (ICV). In humans, the ileum is about 2–4 m long, and the pH is usually between 7 and 8 (neutral or slightly alkaline).
The function of the ileum is mainly to absorb vitamin B12 and bile salts and whatever products of digestion that were not absorbed by the jejunum. The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi on its surface. In turn, the epithelial cells that line these villi possess even larger numbers of microvilli. Therefore the ileum has an extremely large surface area both for the adsorption (attachment) of enzyme molecules and for the absorption of products of digestion. The DNES (diffuse neuroendocrine system) cells of the ileum secrete various hormones (gastrin, secretin, cholecystokinin) into the blood. Cells in the lining of the ileum secrete the protease and carbohydrase enzymes responsible for the final stages of protein and carbohydrate digestion into the lumen of the intestine. These enzymes are present in the cytoplasm of the epithelial cells.
The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver. Lacteals are small lymph vessels, and are present in villi. They absorb fatty acid and glycerol, the products of fat digestion. Layers of circular and longitudinal smooth muscle enable the digested food to be pushed along the ileum by waves of muscle contractions called peristalsis. The undigested food (waste and water) are sent to the colon.
Differences between jejunum and ileum 
There is no line of demarcation between the jejunum and the ileum. There are, however, subtle differences between the two.
- The ileum has more fat inside the mesentery than the jejunum
- The ileum has a mesenteric and an anti-mesenteric vascular supply, while the jejunum has only a mesenteric vasculature
- While the length of the intestinal tract contains lymphoid tissue, only the ileum has abundant Peyer's patches, unencapsulated lymphoid nodules that contain large numbers of lymphocytes and other cells of the immune system.
In the fetus the ileum is connected to the navel by the vitelline duct. In roughly 2% of humans, this duct fails to close during the first seven weeks after birth, causing a condition called Meckel's diverticulum.Empty citation (help)
Veterinary anatomy 
The ileum is the short terminal part of the small intestine and forms the connection to the large intestine. It is suspended by the caudal part of the mesentery (mesoileum) and is attached, in addition, to the cecum by the ileocecal fold. The ileum terminates at the cecocolic junction of the large intestine forming the ileal orifice. In the dog the ileal orifice is located at the level of the first or second lumbar vertebra, in the ox in the level of the fourth lumbar vertebrae, in the sheep and goat at the level of the caudal point of the costal arch. By active muscular contraction of the ileum, and closure of the ileal opening as a result of engorgement, the ileum prevents the backflow of ingesta and the equalization of pressure between jejunum and the base of the cecum. Disturbance of this sensitive balance is not uncommon and is one of the causes of colic in horses. During any intestinal surgery, for instance, during appendectomy, distal 2 feet of ileum should be checked for the presence of Meckel's diverticulum.
Additional images 
Goblet cell in ileum
- Physiology at MCG 6/6ch2/s6ch2_30
- Guillaume, Jean; Praxis Publishing, Sadasivam Kaushik, Pierre Bergot, Robert Metailler (2001). Nutrition and Feeding of Fish and Crustaceans. Springer. p. 31. ISBN 1-85233-241-7, 9781852332419 Check
|isbn=value (help). Retrieved 2009-01-09.
- 9-Nickel,R., Shummer,A., Seiferle,E. (1979) The viscera of the domestic mammals, 2nd edn. Springer-Verlag, New York, USA.
- SUNY Labs 37:11-0101 - "Abdominal Cavity: The Jejunum and the Ileum"
- SUNY Anatomy Image 7787
- SUNY Anatomy Image 8755
- BU Histology Learning System: 12001oca
- Ileal Villi at endoatlas.com
- Ileum Microscopic Cross Section at nhmccd.edu
- Ileum 20x at deltagen.com
- Peyer's Patches at thehealthnews.org