HUNGER and THIRST are highly unpleasant sensations which arise when there is bodily need for food and water. Both hunger and thirst should be sharply distinguished from appetite, a desire for food or drink which past experience has proved to be pleasant. Commonly the requirements of the body are provided by regular meals, tempting the appetite, and only when these fail do hunger and thirst appear. Appetite may be regarded as a primary and hunger and thirst as a secondary defence against the dangers of a serious reduction of the food and water supplies in the body.
The idea that hunger is a general sensation is based on the assumption that as the food supplies of the body decrease, nerve cells in the brain are stimulated directly by the impoverished blood, and the sensation results. The body's food supplies may be greatly reduced, however, as in starvation, without the sensation of hunger. And after eating, the hunger pangs cease long before food can be digested and absorbed into the blood stream. Further more, the pangs can be temporarily suppressed by swallowing indi gestible materials. These facts indicate that the sensation is not due directly to lack of nutriment. Again, hunger pangs are known to be intermittent, lasting between a half-minute and a minute, reappearing after the lapse of approximately another minute, and so forth; there is no evidence that the bodily supplies are inter mittently deficient. Finally, the idea that hunger is a general sensation fails to account for the common reference to the stomach region as its place of origin.
The theory that hunger is of local origin has been supported by the discovery that the empty or nearly empty stomach contracts at fairly regular intervals and that after the start of each contrac tion a hunger pang is experienced. The activity of the empty stomach occurs in cycles. It begins with occasional weak single contractions lasting about 3o seconds; they may fade out or they may become more vigorous and appear at shorter intervals, ac companied by increased tonus of the gastric muscle ; the tonus may develop into a cramp-like state. Not only are the single con tractions associated with hunger pangs, but the cramp-like state is associated with a continuous ache or gnawing sensation. After such a cycle the stomach relaxes and is inactive for a time ; but after a rest the same phenomena are repeated. Examination of the stomach by means of the X-rays has shown that the essential change underlying the hunger pang is a strong contraction of the encircling muscles of the lower third of the stomach. There is evi dence of simultaneous contractions of the small intestine, but lit , tle attention has been paid to them.
The hunger contractions occur during sleep. They are stopped by chewing and temporarily by swallowing. They cease during intense emotional states such as joy, fear and anger. They are weakened and may be completely abolished by smoking, the effects apparently varying with the "strength" of the tobacco. They may be banished for ten or 15 minutes by pulling a belt tightly about the waist. Very vigorous muscular exercise inhibits the contrac tions, but after the exercise they may be more intense than before. They persist after the stomach has been disconnected from the central nervous system and are therefore an inherent feature of that organ.
Although hunger pangs are due to strong gastric contractions, these pangs, like other sensations, are modified in their relative intensity by other conditions. Thus, the sensations of hunger may be absent in fever or during prolonged starvation, although the contractions are going on. On the other hand, the sensations may be especially strong if they receive attention.
The conditions for the occurrence of hunger contractions are not yet clearly understood. When the sugar concentration in the circulating blood is reduced about 25%, hunger contractions in crease. If the blood-sugar level is raised, they cease. They may be related, therefore, to need for the most generally useful energy yielding material in the body, viz., glucose.
Conditions which dry the mouth and throat arouse the sensa tion. Breathing hot dry air, prolonged speaking and singing, re peated chewing of desiccated food, the lessening of salivary secre tion by fear and anxiety are all attended by thirst. Also losses of fluid from the body—as in sweating, diarrhoea, haemorrhage and lactation—give rise to the same sensation. Both local and general causes of thirst have been recognized, therefore, and correspond ingly it has been explained, like hunger, as a local and as a general sensation.
Thirst is explained as a general sensation by the more concen trated state of the blood after prolonged deprivation of water. Thirst becomes prominent, however, before any change in the blood is demonstrable. Another argument for the general sensa tion is that if swallowed water is lost through a gastric fistula, it does not satisfy and drinking continues; but if the fistula is closed, a single drink soon assuages the desire for more. This is not a con clusive argument, however, because water retained in the body may moisten the mouth and thereby abolish the local unpleasant dryness.
A theory that thirst has a local origin is based on the biological consideration that it would first be manifest in air-inhabiting animals, exposed to loss of water from the body. Air when breathed moves across a region of the throat likely to be readily dried because lined with flat cells and provided with relatively few glands. A new feature in air-inhabiting animals are the sali vary glands. Interestingly enough these glands are either absent or rudimentary in mammalian forms (cetacea) which have returned to an aquatic existence. The presence or absence of salivary glands in .large groups of animals, corresponding to exposure or non-ex posure of the mouth to desiccating air, indicates that their function is to protect the mouth and throat against drying. This idea is confirmed by the reflex flow of saliva which occurs when the mouth tends to dry. Saliva consists of about 99% water. If the body is not supplied with water the blood is long maintained in an un changed state by a withdrawal of water from storage in muscles, glands and skin. The salivary glands suffer along with the other structures, but since they need water for their service they are peculiarly affected by the withdrawal. They cannot secrete ade quately, therefore the mouth dries, and the unpleasant sensation of dryness and stickiness (thirst) results.
The theory that the salivary glands act as sensitive indicators of bodily need for water because they serve to keep the mouth and throat moist and because they require water to perform that function, receives support from a number of experiments. Atro pine, which stops salivary secretion without lessening the water content of the blood, brings on marked thirst. Animals with the ducts of the salivary tied drink almost incessantly. Finally, water lack results in a gradually lessened flow of saliva under equal stimulations, and the drinking of water restores the original flow. Thus the evidence that thirst is a general sensation can be simply explained in terms of the local service of the salivary glands.
It is possible that the sensation of thirst may be stimulated in the course of the nerve paths from mouth and throat to the cere bral cortex. Certainly very slight injuries of the base of the brain below the optic thalamus cause insatiable thirst and the drinking of enormous amounts of water. This is characteristic of the dis ease diabetes insipidus. The relation of such pathological states to physiological thirst is still obscure.
BIBLIOGRAPHY.--W. B. Cannon, Bodily Changes in Pain, Hunger, Bibliography.--W. B. Cannon, Bodily Changes in Pain, Hunger, Fear and Rage; A. J. Carlson, The Control of Hunger in Health and Disease; W. B. Cannon, "The Physiological Basis of Thirst" (Proc. Roy. Soc., London, 1918, B xc., 283-30i). (W. B. C.)