Duodenum, Jejunum and Ileum
The duodenum is the first section of the small intestine. It is located dorsally and begins on the right side at the pylorus, the exit sphincter of the stomach. The duodenum receives chyme, a mixture of partially digested stomach contents and gastric acid.
Ascending and descending limbs of the duodenum are recognised, but the division into the three sections of the small intestine are difficult to determine.The small intestine is the principal site for chemical digestion of nutrients. Although most of digestive function is provided by the enzymes produced and secreted by the pancreas, the mucosal cells of the small intestine also make a contribution. Brunner's glands are present in man within the duodenal mucosa but not in the domestic mammals. They produce bicarbonate, which neutralises the gastric acid that has passed through the pylorus.
Histologically the small intestine comprises an outer serosa (visceral peritoneum), two layers of smooth muscle (inner circular and outer longitudinal), a submucosa and a mucosa. The internal surface area of the mucosa is increased by the presence of fingerlike projections called villi and further minute projections called microvilli. This arrangement effectively increases the area for absorption of the products of digestion. In the sub-mucosal areas of the small intestine are located the intestinal crypts (crypts of Lieberkuhn) containing a variety of cells with different functions. These cells include the Paneth cells, whose function seems to be the protection of stem cells against bacterial invasion. The stem cells are capable of self-renewal of the intestinal crypt cells.
The myenteric plexus (Auerbach’s plexus) (see Section 11.4.5) lies between the muscle layers and comprises both sympathetic and parasympathetic unmyelinated fibres and cell bodies; it innervates the muscle of the intestine.
All species possess small aggregations of lymphoid tissue mainly in the distal section of the ileum but sometimes located in the distal jejunum. These are the Peyer’s patches and consist of B lymphocyte follicles, enterocytes and M cells; they function as primary lymphoid tissue entrapping foreign particles and destroying potentially pathogenic microorganisms. M cells transport antigens from the lumen of the intestine to the lymphoid tissue. Enterocytes are columnar epithelial cells that absorb nutrients, electrolytes and water.
King’s Applied Anatomy of the Abdomen and Pelvis of Domestic Mammals, First Edition. Geoff Skerritt. © 2022 John Wiley & Sons Ltd. Published 2022 by John Wiley & Sons Ltd.
Companion website: www.wiley.com/go/skerritt/abdomen
Most of the enzymes that digest nutrients in the small intestine are secreted by the pancreas and enter the duodenum via the pancreatic ducts. These are variable (see Section 7.6), but the main duct is more constant and opens into the cranial duodenum with the common bile duct. When present the accessory pancreatic duct opens into the descending duodenum.
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