This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
These sensations regulate the taking of food. Local conditions play a part in their production, but general states of the body are also concerned.
Hunger in its first stages is due to a condition of the gastric mucous membrane which comes on when the stomach has been empty some time; it may then be temporarily stilled by filling the stomach with indigestible substances. But soon the feeling comes back intensified and can only be allayed by the ingestion of nutritive materials; provided these are absorbed and reach the blood their mode of entry is unessential; hunger may be stayed by injections Of food into the intestine as completely as by filling the stomach with it.
Similarly, thirst may be temporarily relieved by moistening the throat without swallowing, but then soon returns; while it may be permanently relieved by water injections into the veins, without wetting the throat at all.
Both sensations depend in part on local conditions of sensory nerves, but may be more powerfully excited by poverty of the blood in foods or water; this deficiency directly stimulates the hunger and thirst centres of the brain.