The stimulation of the gustatory end organs in the mouth, the chewing of indifferent substances, and the tasting and chewing of palatable foods would abolish the sensations of hunger to the same degree that these measures inhibit the stomach contractions. The inhibi- , tion of the stomach activity and the cessation of the hunger pains run parallel. This conclusion is based on experiments on a number of men besides Mr. V.
In the dog, food or other substances in the mouth cause inhibition of the hunger contractions of the stomach. But since these manipulations disturb the animal and induce salivation, and in many cases swallowing movements, the precise mechanism of the inhibition must remain in doubt until it can be investigated on dogs from which the cerebrum has been removed, since most of the dog's cerebral processes (pleasant or unpleasant) induce the same inhibition.
In the rabbit, the sight, smell, or taste of food, or the chewing (without swallowing) of such foods as cherries, carrots, apples, carrot leaves moistened with sugar, acid, or quinine do not inhibit the stomach contractions (Rogers). The same is also true for the guinea-pig (Dr. King). In the case of the single goat so far studied, the chewing of ordinary food (hay, oats, carrots) appeared to increase rather than decrease the hunger contractions of the rumen.
In the pigeons, Rogers encountered the same difficulties that we met in the dogs. Any disturbance of the normal pigeon inhibits the hunger contractions of the empty crop. And since it is not possible to put food or other substances in the mouth of these birds without more or less disturbance by the handling, we cannot be sure that the resulting inhibition proceeds from stimulation of nerves in the mouth. In the decerebrated bird, visual and auditory stimuli do not inhibit the crop, but handling the bird, as in feeding or placing anything in the mouth, causes inhibition. If the disturbing factors other than the mouth stimulation could be eliminated, it is likely that the mouth stimulation alone would cause little or no inhibition unless accompanied by swallowing.
In the frog, stimulation of the nerve-endings in the mouth by food substances, acids, or alkalies cause little or no inhibition of the empty stomach. This is true whether the frog is normal or decerebrated (Patterson).
It is thus evident that the marked reflex inhibition of the gastric hunger contractions from mechanical and chemical stimuli acting in the mouth of man is much less in evidence, although not entirely absent in the lower mammals, birds, and frogs. This leads us to suspect that in man and the higher animals, where this reflex is preponderant, it involves conscious cerebral processes. The question could possibly be settled by experiments on infants (normal and acephalic) and on persons in very deep sleep.