<P> The small intestine develops from the midgut of the primitive gut tube . By the fifth week of embryological life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the primary intestinal loop . The loop grows so fast in length that it outgrows the abdomen and protrudes through the umbilicus . By week 10, the loop retracts back into the abdomen . Between weeks six and ten the small intestine rotates anticlockwise, as viewed from the front of the embryo . It rotates a further 180 degrees after it has moved back into the abdomen . This process creates the twisted shape of the large intestine . </P> <P> Food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter . </P> <P> The small intestine is where most chemical digestion takes place . Many of the digestive enzymes that act in the small intestine are secreted by the pancreas and liver and enter the small intestine via the pancreatic duct . Pancreatic enzymes and bile from the gallbladder enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients . Secretin, another hormone produced in the small intestine, causes additional effects on the pancreas, where it promotes the release of bicarbonate into the duodenum in order to neutralize the potentially harmful acid coming from the stomach . </P> <P> The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrates: </P>

In which layer of the intestine does most chemical digestion take place