When food reaches the small intestine, the pancreas secretes pancreatic juices. When there is no food in the small intestine, the pancreas does not secrete its juices. The economy of this process puzzled researchers who wondered what the mechanism for this control might be. In 1902, William Bayliss and Ernest Starling, two British physiologists, conducted experiments to find the answer. They reasoned that the same mechanism that initiated gastric juices when food first enters the mouth might be the same mechanism for releasing the flow of pancreatic juices.
These researchers made an extract from the lining of the small intestine and injected it into an experimental animal. The extract caused the animal to secrete large amounts of pancreatic juice. They concluded that the extract from the intestinal lining must have some substance responsible for the flow, which they named secretin. The experiment gave the first real proof for the existence of hormones, substances secreted by one group of cells that travel around the body which target other groups of cells.
Insulin is another important hormone secreted by a group of cells within the pancreas called the islets of Langerhans, which are part of the endocrine system rather than the digestive system. Insulin released into the bloodstream targets liver and muscle cells, and allows them to take excess sugar from the blood and store it in the form of glycogen. When the pancreas does not produce sufficient insulin to store dietary sugar, the blood and urine levels of sugar reach dangerous levels. Diabetes mellitus is the resultant disease. Mild cases can be controlled by a properly regulated diet, but severe cases require the regular injection of insulin.
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