294 STUDIES IN PHYSIOLOGY 



nervous system impulses that result in sensations of pain. 

 We should regard painful sensations, like danger signals, 

 as of use to us in preventing permanent injury to the body. 

 Sensations of Hunger and Thirst. If food is withheld from 

 our bodies for a time, we become conscious of a sensation of 

 hunger. We can get rid of this feeling, temporarily at least, 

 by swallowing pebbles or any other indigestible substances. 

 We therefore infer that hunger in its early stages is largely 

 due to the condition of the mucous membrane of the 

 stomach. But, since hunger likewise disappears if food 

 is injected directly into the blood, we conclude that this is 

 a general sensation belonging to the whole body. Thirst, 

 too, while it can be relieved for a time by moistening the 

 mucous membrane of the throat, soon reappears unless water 

 is swallowed and absorbed by the blood, or unless water is 

 injected directly into the blood vessels. Hence we conclude 

 that sensations of hunger and thirst, and the feeling of satis- 

 faction that follows the taking of food, are common sensations 

 which acquaint us with the general condition of the whole body, 

 and especially of the blood. 



3. THE SENSE OF TASTE 



Papillae of the Tongue. While studying the tongue, we 

 called attention to certain elevations called papillae. These 

 differ from those of the dermis in the fact that the mucous 

 membrane follows the outline of the papillae on the tongue, 

 and so the latter project from the surface. On the tongue 

 there are three kinds of papillae. The Jil'i-form (Latin 

 filum = a thread) are long and slender, and are found in 

 great numbers along the sides and at the tip of the tongue. 

 These regions we found (p. 292) to be most sensitive to tac- 

 tile impressions, and so we believe that the filiform papillae 

 send to the brain impulses that result in sensations of touch. 



A second class of papillae are the fun'gi-form (Latin fun- 

 gus = a mushroom). In shape they resemble a mushroom 



