TASTE, SMELL, AND HEARING 325 



592. Olfactory nerves. From the under surface of the 

 brain, about twenty nerves extend through perforations in 

 the upper part of the nose and spread out over the upper 

 one third of the surface of the nasal cavities. An odorous 

 gas entering the nose comes in contact with the ends of 

 these nerves and excites the sense of smell. An odor is 

 found only in substances which can be turned to a vapor. 



The olfactory nerves are so delicate that they can perceive the 

 presence of gases which cannot be detected in any other way. Some 

 substances excite the sense of smell when they are in such small quan- 

 tities that they are given off for years without causing a perceptible 

 lessening of the weight of the substance. 



When too much mucus covers the nerve endings, or when the sur- 

 face of epithelium is dry, no gas can reach the nerves, and then the 

 sense of smell is diminished. A cold in the head can produce either 

 condition. 



593. Use of smell. Smell is a warning against foul air 

 and decaying matter. The gases themselves are in too 

 small quantities to do harm, yet they are a sign that 

 other substances are present which can harm the body. 

 Air which has no odor is almost surely fit to be breathed. 

 Meat which has a pleasant odor is almost certainly fresh. 



Tobacco smoke and snuff are irritating to the delicate 

 nerves of smell, and partly deprive its users of nature's 

 most useful protection against foul air. 



594. The inner ear. Sound is produced by certain air 

 waves which are received by nerves in the ear. There 

 they excite impulses which the brain interprets as sound. 

 In the hard bone, which rises from the bottom of the 

 skull by each ear, is a tortuous cavity, called the laby- 

 rinth or internal ear. The center of the labyrinth is about 

 one eighth of an inch in diameter, and is called the ves- 

 tibule. From the vestibule there extends a small spiral 



