96 CONSCIOUS NERVOUS OPERATIONS 



Taste buds are affected so as to distinguish different 

 tastes only when the substances submitted to them are in 

 solution. The effect is increased by friction between the 

 mouth and the tongue. 



123. Classification of Tastes. Tastes are of four sorts : 

 (1) sweets, which are best appreciated by the tip of the 

 tongue ; (2) sour, or acid tastes, perceived best by the side 

 of the tongue ; (3) bitter tastes, most affecting the back 

 of the tongue ; (4) salts. 



It is believed that separate taste buds are provided, one 

 sort being stimulated only by bitter substances, one by 

 sweet, one by sour, and one by salt. Some substances 

 taste sweet at the tip of the tongue and bitter at the back 

 of it, because they are able to stimulate two sorts of taste 

 buds, but one kind of buds recognizes only a sweet taste, 

 the other only a bitter taste. 



124. Flavors. We are accustomed to say and to think 

 that we taste a great variety of flavors in our food ; but 

 physiologists tell us that we really taste only the four 

 flavors mentioned above, while others are recognized by 

 the sense of smell. This may be tested by holding the 

 nostrils closed by the fingers while different kinds of food 

 are eaten. An onion will not taste different from a potato, 

 though one would be known from the other by its texture. 



125. The Sense of Smell. The organ for receiving im- 

 pressions from the minute particles called odors is the mu- 

 cous membrane lining the upper part of the nasal cavity. 



126. The Olfactory Nerves are the first pair of cranial 

 nerves. They spring from the olfactory lobes, which are 

 prolongations of the hemispheres of the brain (Fig. 19, 

 p. 29) and extend forward from the base of the cere- 

 brum. Branches of the nerves of smell are distributed 

 from the olfactory bulbs, in which the olfactory lobes end, 



