1. Caphalic phase:
Stimulation by the sight, taste, odour and more thought of food results in a psychic secretion of gastric juice even before the food enters the stomach.
This cephalic phase of gastric secretion is of considerable importance in the dog but may not be so important in man.
It is partly under the control of the nervous system by way of the secretory fibres in the vagus nerves.
2. Gastric phase:
The presence of food in the stomach is itself capable of provoking the continued secretion of gastric juice; this is, so called, the gastric phase of gastric secretion.
Certain articles of food especially meat, and soups made from meat, are particularly active as secretagogues, as are also some of the products of protein digestion.
These materials exert their action indirectly by causing the liberation of gastrin from the mucosa of the pyloric antrum into the blood stream which carries it to the glands. Distension of the antrum also releases gastrin. Gastrin is believed to be a hormone of gastric-secretion.
Histamines are also powerful stimulant of gastric secretion in addition to their well known effects on blood pressure.
3. Intestinal phase:
When the products of gastric digestion leave the stomach and enter the duodenum, they have a stimulating effect on gastric secretion.
The mechanism of this action is not yet clear but it is probably due to substances present in the foods. These are absorbed and, perhaps, stimulate nervous structures.