Endodermal ,longest part of alimentary canal present in the abdominal cavity, supported by a peritoneal membrane called mesentery .Wall of jejunum and ileum has circular or spiral internal fold called fold of kerckring or valvulae conniventes.Also numerous figer-like projection called villi project from the wall of lumen,increasing internal surface area about ten time.The distal end of ileum leads into the large intestine by ileo-caecal valve in man but in rabbit sacculus rotundus and ileo-coecal valve both are present.
It is approximately 3 metres in human.It is divisible into three parts duodenum, jejunum and ileum.
Parts of small intestine
25cm.Long.Forming U-shaped loop before leading to jejunum ,pancreas lies in the loop.
Jejunum (middle part):
About 1m long and about 4cm wide.Wall is thicker and more vascular. Villi thicker and tongue-like.Plicae best developed. Peyer's patches are lacking.
About 2m long and about 3-5cm wide. Wall is thinner and less vascular.Villi thinner and finger-like. Plicae less developed.Peyer's patches are present.
Serosa is the outer most covering. Muscular layer is generalized with involutionary, unstriated muscles.The cells present in the epithelial lining are simple columnar epithelial cells,which are brush-bordered.i.e.provided with villi and microvilli to increase the surface area.The folds present are longitudinal and are called folds of kerckring or valvulae canniventes. Goblet cells secrete mucous.Peyer's patches are the oval ,rounded masses of lymphatic tissue present in between lamina propria adn epithelial lining.They produce lymphocytes. Brunner's glands or Duodenal glands are the multicellular mucous glands present in the submucosa of duodenum only.They secrete mucous.In addition there are also found granular arogyrophil cell.
(4)Glands of small intestine:
Various glands found in small intestine.Each gland has three types of cells.
(i)Undifferentiated epithelial cell
(ii)Zymogenic cell(Paneth cell)
and (iii)Argentaffin (Enterochromaffin cell).
Function of small intestine
Digestion and absorption of food.