372 THE SENSES 



The afferent nerves concerned in the sensation of thirst are evidently 

 those of the throat: the ninth cranial (glosso-phargngeal) especially, but 

 also probably branches of the tenth (vagus) and the fifth (trige minus). 



Water is by far the best liquid for relieving thirst, since it is water which 

 forms the 60 or 70 per cent, of protoplasm. Warm water relieves it as 

 quickly as cold, but affects the sensation of thirst less quickly because it 

 lacks the coldness which so promptly relieves the mild inflammation 

 reflexly begun in the throat. An average person will die of thirst in 

 four or five days. 



Hunger is a condition similar to thirst save that in this case the body's 

 protoplasm lacks proteids, carbohydrates, fats, and inorganic salts instead 

 of water. 



The experience of the general sensation of hunger varies with the 

 habits of the person as regards eating. Many accustomed to omit their 

 meals or food entirely for a day or even two at a time fail to feel the 

 ordinary phenomena of the average man accustomed to go without 

 food for ten hours at the most and that in part while asleep. Normal 

 hunger exhibits a feeling of weakness plus an indescribable sensation 

 in the stomach not unpleasant at first but rapidly increasing to a " gnaw- 

 ing" pain. This is relievable temporarily by taking even quite indigesti- 

 ble matter into the stomach, or by water. After two or three days the 

 feeling in the stomach decreases and disappears and only the ever- 

 increasing feeling of weakness, in addition to the other bodily phenomena 

 of inanition, remains. 



What causes these sensations in the stomach we do not know. They 

 are very erratic, sometimes being present when the stomach is full (as 

 in cases of duodenal fistula), and on the other hand entirely absent when 

 no food has been taken for days, as is customary in gastritis, for example. 

 Here, then, as in case of thirst, we have a general sensation normally 

 referred to the organ 'where the demand is usually supplied. The 

 stomach is really no hungrier than is the kidney, yet the pain is in the 

 stomach, for here the means of relieving it are needed. In this case the 

 impulses go inward along the tenth cranial (vagus) nerve, (the chief 

 sensory nerve of the stomach), and pass to the almost universally con- 

 nected roots of this nerve in the medulla oblongata. By what periph- 

 eral organs the stimuli are started toward the brain we do not know. 

 In most normal cases distention of this hollow viscus seems to provide the 

 opposite sensation, satiety. 



NAUSEA is another general sensation referred to the stomach, but 

 about it little of a physiological nature is known. It originates from 

 a variety of causes. It comes from local irritation or over-distention of 

 the stomach (especially in children), and from stimulation in the medulla 

 or cerebellum (as for, example, in sea-sickness). It is occasioned by the 

 various central nauseants, e. g., apomorphine, and by many other drugs 

 acting locally on the stomach or centrally on the nerves. (See the 

 description of vomiting, p. 186.) 



