Hunger, the sensation by which the necessity for food is made known to the system, referred to the stomach, but indicating the wants of the system at large; impelling us to supply the waste of the tissues consequent on all vital acts, and in proportion to the activity of the animal functions from exercise, etc. If the desire cannot be gratified, or if absent from disease, the phenomena of inanition or of starvation are induced, with a diminution of the bulk of nearly all the tissues and proportionate weakness. Hunger is greatest in the young and growing state, and least in old age, when the vital operations are deficient in activity. It varies with the amount of heat to be generated in the body; external cold increases hunger, while heat diminishes it; hence the voracious appetite of the arctic regions, and the general use of stimulating condiments in the tropics; it is also increased by any unusual drain upon the system, when accompanied by febrile action, as in lactation and diabetes, in the last of which especially hunger is almost insatiable.
In health, the feeling of hunger is a very good indication of the demands of the system for food, and it becomes the stimulant to mental operations, automatic in infancy, but directed by intelligence in the adult, which have for their object the gratification of the desire. Hunger depends rather upon the demand of the system for aliment than upon the state of emptiness of the stomach. The sense of hunger may be, however, immediately dependent on some condition of the stomach; it is well known that the swallowing of indigestible and non-nutritious substances will temporarily relieve it. The demands of the stomach and of the general system in this respect are probably communicated to the sensorium by the pneumogastric nerves and by the sympathetic. On the other hand, mere emptiness of the stomach does not produce hunger, as is evident from the fact that an ample supply of food passes entirely from the stomach hours before this sensation is felt, and that in disease there may be no desire for food for many days with total abstinence from it.
Moreover, hunger may be relieved by the injection of alimentary fluids into the large intestine, when the stomach cannot receive or retain food.