When a suitable cannula is put in the Stenson's duct in man so that the rate of flow of the saliva can be measured accurately, and with a balloon in the stomach to register the gastric hunger contractions, one can demonstrate that there is a rhythm of the salivary flow parallel to the gastric hunger-contraction rhythm. Each strong hunger contraction is accompanied by a brief gush of saliva from the duct. The amount of salivation varies with the individual person, and it varies somewhat from day to day in the same individual. The feebler contractions have no evident effect on the saliva flow. This saliva flow is independent of the memory, sight, or smell of palatable food during the hunger, but it is certain that these stimuli and the central processes induced by them will augment the salivation.
The increased flow of saliva that occurs simultaneously with the strong hunger contractions is probably a reflex effect from stimulation of sensory nerves in the stomach by the contractions. It is well known that strong stimulation of gastric nerves induces salivation and vomiting. Such stimulation is usually held to act on the nerves in the mucosa, while the stomach contractions stimulate the nerves in the muscularis mainly. It is probable that strong stimulation of the sensory nerves in the sub-mucosa and in the muscular layers also causes reflex salivation.
Some increased salivation is therefore a necessary effect of strong hunger, irrespective of the presence of appetite, or the memory, sight, or smell of good food. But the more copious "watering of the mouth" that accompanies the thought or sight of appetizing food is not of gastric origin.