This section is from the book "The Control Of Hunger In Health And Disease", by Anton Julius Carlson. Also available from Amazon: The Control of Hunger in Health and Disease.
The most direct and at the same time the least objectionable method of attack on this problem is the section of the extrinsic nerves to the stomach, although this operation abolishes not only all direct influences from the brain of a motor or inhibitory type, but also the central reflexes (motor or inhibitory) that may be called into action through the sensory nerves in the stomach. The splanchnic nerves were sectioned through a median incision. The vagi nerves were sectioned 2 to 3 cm. above the diaphragm, thus leaving the fibers to the esophagus, the heart, and the lungs intact.
Observations were made on five dogs with complete sections of the splanchnic nerves on both sides. The longest period of observation after the splanchnic section was two months. Observations were in some cases begun two hours after the operation. When the records of these five dogs are viewed as a whole, it is clear the complete section of the splanchnic nerves in dogs increases the gastric tonus and augments the gastric hunger contractions. The hunger contractions become more rapid and continuous, that is, there is less evidence of the periodic groups with intervening periods of relative quiescence. It is not uncommon to observe contractions at the rate of about two per minute during an entire observation period of 2 to 4 hours. The section of the splanchnic nerves does not abolish the periodicity completely, however. It seems to be a question of relative degree of gastric tonus. If for any reason the tonus of the empty stomach is relatively low on any day, the hunger contractions are less frequent, and there is greater evidence of periods of relative quiescence. We desire to emphasize the fact that this conclusion is based on the observations as a whole. Even the dogs with the splanchnic nerves sectioned showed on some days no greater tonus of the empty stomach and no greater rate and persistence of the gastric hunger contractions than does the dog with these nerves intact. And occasionally a dog with the splanchnic nerves intact exhibits as great a degree of gastric tonus and rate* and persistence of the gastric hunger contractions as the maximum observed in dogs with the splanchnic nerves cut. This is to be expected, as by section of these nerves one eliminates only one (and in the normal animal probably one of the least important) of the factors in the motor activity of the empty stomach. The conditions that affect the stomach through the blood and through the vagi are still subject to practically the same variations as in the animal with the splanchnic nerves intact.
After complete section of the splanchnic nerves the psychic or reflex inhibition of the gastric hunger contractions is greatly diminished. The stimuli that cause anger, fear, pain, joy, or pleasure no longer lead to complete cessation of the hunger contractions.
The maximum effect is a slight and transitory weakening of the contractions. It is therefore evident that the inhibitory fibers in the splanchnic nerves (and possibly also the secretory fibers of the adrenals) constitute the main efferent path in this type of inhibition. The slight degree of inhibition usually in evidence after section of the splanchnic nerves must be due to some central inhibition of the vagus tonus or to action of the few inhibitory fibers in the vagi.
Particular attention was given to the effect of seeing and smelling food on the hunger contractions in these dogs with section of the splanchnic nerves, in order to determine whether these stimuli augment the tonus of the vagi and thus increase the hunger contractions. The results were negative. Even with the greater part of the extrinsic inhibitory fibers to the stomach eliminated, the sight, smell, and taste of food not only fail to inhibit or augment the gastric hunger contractions, but, so far as these stimuli affect the stomach at all, it is in the direction of inhibition of the hunger movements. The apparent increase in the intensity of the hunger pangs in man on seeing or smelling palatable food must therefore be essentially a central phenomenon of "facilitation" or Bahnung.