Page:The Scientific Monthly vol. 3.djvu/565

Rh man with occluded esophagus, who had to be fed with stomach tube. Such facts perhaps indicate to us the biological necessity for the co-operation of appetite with hunger.

2. Appetite is sometimes regarded as a fundamental inherited reaction, and sometimes as being wholly the product of education, i. e., of the individual's experiences with foods. In contradistinction to hunger, appetite is a pleasant sensation, and is invariably associated with the taste, smell, sight or memory of palatable food. It is dependent upon, or coincident with, changes not in the muscular walls, but in the lining membrane of the mouth and stomach. The familiar "watering of the mouth" or psychic secretion of the saliva, connected with the appearance and eating of food which is enjoyed, is a good illustration of these appetite phenomena; and exactly the same thing is happening at the same moment in the stomach, though the individual is not, of course, subjectively aware of this fact.

Here, then, are two great motives having altogether different physiological basis and action (i. e., concerned with different tissues of the body), which ordinarily act together in bringing about and maintaining the desire to eat, until the amount of food taken shall have become adequate for sustenance.

The hunger motive, it seems, is extraordinarily independent of environmental and educative influences, except for certain habitual inhibitions. It can by certain means be caused to disappear instantly, but can not readily and immediately be caused to appear or increase. The strength of its contractions and pangs is, however, influenced by physical vigor and rate of metabolic activity. Moderate muscular work or exposure to cool air augments metabolism and at the same time increases hunger. The young animal, who is metabolizing (e. g., burning food as body fuel) more rapidly and therefore uses more food in proportion to its size than does the older one, also feels more keenly the hunger pang. Again, one may be much more hungry coming home at midnight after the theater (say five hours after the conclusion of the evening meal) than he is before breakfast the next morning (say twelve hours after the conclusion of the last meal); presumably because, in the latter case, metabolic activity is not yet in full swing for the day, and so the hunger contractions can not attain their maximum. And yet, pleasurable appetite sensations connected with the anticipation and appearance of a good breakfast, may lead to the taking of a hearty meal, even though the individual on arising was not aware of any special hunger pain or discomfort.

Not all food materials, however, are valuable to the body in proportion to the appeal which they make to the appetite. For example: the flavor substances in foods which stimulate the olfactory and gustatory nerves and thus give rise to appetite, are not ordinarily the