THE ORGAN OF HEARING 



143 



sons to remain in places where the air is seriously vitiated, 

 without knowing it. It also explains why foul breath is 

 not smelled by the person having it. 



When air that is breathed in contains odorous material 

 it is recognized as coming from outside and the sensa- 

 tion is called smell. When the odorous material is in the 

 mouth, as when food is being eaten, and air containing 

 some of it is breathed out over the organ of smell the 

 sensation is called flavor. It is easy to show that flavor 

 is really smell and not taste, for when one has a cold in 

 the head his food, as he says, loses its taste, whereas what 

 it really loses is its flavor. The true tastes, sweet, sour, 

 salty, and bitter, are perceived readily no matter how 

 severe one's cold may be. 



Fig. 37. — Diagram of human ear. (a) External ear; (6) ear 

 canal; (c) eardrum which is set in vibration by sound waves; (d) chain 

 of bones by which vibrations of eardrum are transmitted to sensitive 

 part of ear; (e) cavity filled with liquid, within which are proto- 

 plasmic receptors which are affected by vibrations; (/) one of the 

 semicircular canals, part of the organ of equilibrium, not directly 

 concerned in hearing. The entire structure, except the external ear, 

 is imbedded in solid bone. 



The Organ of Hearing is the Ear (Fig. 37). — It 

 consists of an apparatus whereby sounds stimulate pro- 

 toplasm. Sounds are due to vibrations either of solids, 

 liquids, or gases. The majority of the sounds which 

 reach our ear are caused by vibrations in the air. These 

 vibrations strike upon a drum-head — the ear-drum — 

 and set it into exactly synchronous vibration. The 



