| The small intestine |
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The small intestine of an average sized horse (450 kg) is about 20 meter long. It is anatomically divided into 3 segments: Duodenum - the first 1-1.5 meters. The bile duct and pancreatic duct empty in the duodenum. Jejunum - the middle part of the intestine (> 80%). Contains large amounts of digestive glands in the wall which secrete enzymes and buffer. Large absorptive surfaces for nutrients including vitamins and minerals. Ileum - the final 1-2 meters of the small intestine. Structure and function much like jejunum. The horse has no gall bladder, and the bile, which is produced by the liver, is secreted continuously into the small intestine. Bile contents (phospholipids and bile salts) help in the emulsifiation of fat necessary for its digestion. The pancreas secretes a bicarbonate rich fluid which neutralises the acidic stomach contents, and enzymes which hydrolyse proteins (trypsin), fats (lipase) and starch (amylase). The gastric contents are neutralised within a few meters of their emergence into the small intestine. Enzymes secreted by the glands of the intestinal wall continue to digest the end products from the pancreatic enzyme reactions into small components which can be absorbed by the cells of the intestinal epithelium. The transport velocity through the small intestine has been estimated to approximately 20 cm per minute, thus giving the gut about 1.5 hours to digest the feed and extract nutrients. In the last part of the ileum the digesta accumulates before it is squirted under pressure into the large intestine at the junction between the colon and the caecum. Emptying of the ileum appears to take place 3-6 times per hour, and delivers 0.2 to 1.5 litres of intestinal contents to the hind guts at a time. The digestive secretions increase the water content of the intestinal contents to make it like a thick soup which can be propelled through the long and curving intestine. At the terminal ileum the water content will still be 96% (4% dry matter) on a roughage diet and in the range 96 to 90% on diets with increasing contents of grains and commercial concentrates. |