When we bite off a piece of roti and mutton, chew it and then swallow it, there is some gastric juice already present in the stomach. Sometimes even the sight of food causes the release of gastric juice. When the bite is in the oral cavity, the message is conveyed to the brain in the form of nerve impulses. From the brain, the message is passed to the walls of the stomach for the secretion of some gastric juice. When food touches the walls of the stomach, more gastric juice is secreted. If we eat only a bite of bread, which has little protein in it, the stomach does not secrete more gastric juice.

When we eat mutton, it results in the release of abundant gastric juice. The reason is that when food reaches the stomach the already present gastric juice begins the digestion of any proteins present in it. The huge protein molecules are broken down into peptides. These peptides stimulate some cells of the stomach walls to release a hormone called gastrin. This hormone enters the blood and is distributed to all parts of the body, including the stomach. Here it has a specific effect and stimulates the cells of gastric glands to secrete more gastric juice.