238 SMELL AND TASTE. 



20. Smell. The organ of smell, or the olfactory organ, 

 consists of the mucous membrane lining the upper por- 

 tions of the two nostril-cavities. Part of it is shown at 

 o and/, Fig. 42. The nerves of smell are the two olfac- 

 tory nerves, one of which runs from each nostril-chamber 

 to the brain. 



21. Odorous Substances frequently act powerfully when 

 present in very small quantity. A grain or two of musk 

 kept in a room will give the air in it an odor for years, 

 and yet at the end will hardly have diminished in weight, 

 so infinitesimal is the quantity given off from it to the 

 air and able to excite the sense of smell. 



22. Taste. The organ of taste is the mucous mem- 

 brane on the upper side of the tongue, and the under 

 side of the soft palate (p. 101), The mucous membrane of 

 the tongue presents innumerable elevations or papillae. 

 Some are organs of touch, for the tongue has the sense 

 of touch as well as of taste. Others contain the endings 

 of nerve-fibres which, when excited, stimulate the taste- 

 centres in the brain and cause sensations of taste. 



Many so-called tastes (flavors) are really smells ; 

 particles of substances which are being eaten reach the 

 nose through the pharynx (see Fig. 42), and arouse smell- 

 sensations which, because they accompany the presence of 

 objects in the mouth, we take for tastes. Such is the case 

 with most spices; when the nasal chambers are blocked 

 during a cold in the head (p. 154), or closed by holding 



20. Of what does the olfactory organ consist? 



21. Illustrate the efficiency, so far as producing smell-sensations is 

 concerned, of a very small quantity of an odorous substance. 



22. What is the organ of taste ? What is found on the mucous 

 membrane of the tongue ? What are the uses of its papillae ? 



What are many so called tastes ? Illustrate. 



