Emotion and gastrointestinal motility (psychic tone)

Bayliss (1915) described saliva as the first fluid met with, produced even before food enters the mouth. He said this psychical secretion is caused by reflexes through sight and smell, so that the mouth waters, and that taste causes renewed secretion.⁴ Cannon (1915) stressed what this early saliva does. He said saliva matters because food is tasted only when dissolved, and that the saliva which waters the mouth ensures that dry but soluble food dissolves even when a large amount is taken.³ Cannon also noted the importance of an initial psychic secretion of gastric juice.

Cannon (1915) began from the point that digestive secretions are of little worth unless food is carried onward through the canal.¹ Studying these mechanical aspects, he inferred that there is probably a psychic tone, or psychic contraction, of the gastro-intestinal muscles, just as there is a psychic secretion. His evidence came from the vagus nerve supply to the stomach. If the nerves were cut immediately before an animal took food, the usual gastric contractions, seen by Roentgen rays, did not occur.² He contrasted this with cutting the nerves after food had been eaten with relish.

Bayliss (1915) argued that gastric secretion from meat extracts is chemical, not nervous. He reported that meat juice or Liebig's extract caused secretion, and that such substances still worked after the vagus nerves were divided. From this he concluded that the mechanism is chemical and not nervous.⁵ For the pancreas, he said that acid gastric contents reaching the duodenum give rise to secretin. Whether the vagus takes any part in the normal process he called doubtful.⁶ He allowed that, if it does, appetite might produce psychical secretion there too.

Pavlov (1927) described how psychic secretion led him away from subjective inquiry. While studying the digestive glands, he had to inquire into the so-called psychic secretion of some of them.⁷ The result, he wrote, was an unqualified conviction of the futility of subjective methods.⁸ He then began to record the external stimuli acting on the animal when its reflex reaction appeared, in this case the secretion of saliva. His account treats the same phenomenon that Bayliss and Cannon discuss as a matter for objective measurement.