Most of the mechanism of the secondary phase of gastric secretion still remains a fascinating mystery. One knows the site of origin of the secretory stimulus and something of its transmission to the secreting cells, but that is nearly all. Yet this phase of secretion is of great importance not only to the physiologist, but to the surgeon, for it plays an important part in the results obtained following modern surgical procedures on the stomach. There are three stages of gastric secretion, the primary, the secondary and the intestinal. The primary is also known as the vagal, the psychic or the cephalic. Its purpose is to stimulate the flow of gastric juices so that they can act on the food and by digestion produce chemical substances that initiate and carry on the second phase. The primary phase follows a psychic or sensory stimulus, such as the thought of food or