250 THE SPECIAL SENSES 



tongue around the large papillae found there. The dis- 

 solved substances come into contact with the exposed 

 portions of the taste buds and cause nervous impulses 

 .to start inward along the cranial nerves of taste. These 



Taste bud 



Circumvallate 

 papillae 



FIG. 134. A Circumvallate papilla. (After Testat.) 



impulses eventually reach the centres in the cortex of 

 the brain and give rise to sensations of taste. 



Kinds of taste. Careful experiments made by dry- 

 ing the tongue and applying to its surface solutions 

 of different substances, have shown that there are really 

 four different tastes, each of which probably has a dif- 

 ferent kind of end organ. These tastes are sweet, bitter, 

 sour or acid, and salty. The tip of the tongue is more 

 sensitive to sweetness, the sides to acidity, and the back 

 to bitterness. Sweetness is the most powerful of the 

 tastes and is able to overcome the others. For this 

 reason, we take bitter medicine in a sweet syrup and 

 put sugar into lemonade. 



THE SENSE OF SMELL 



Structure. The sense of smell is closely associated 

 with that of taste. The surface which contains the cells 

 .sensitive to odors, the olfactory cells, is situated in the 



