376 THE ANIMALS AND MAN 



that of the lens. This must also be corrected by glasses. 

 Only a good oculist, by careful examination of the eyes, can 

 tell the kinds of glasses that are needed for the various 

 defects. 



Auditory sensations (sound and hearing) .--The ear is 

 an organ specially adapted to receive vibrations of air and 

 to transmit them to the auditory nerve. It is made up of 

 three principal parts called the outer, middle and inner 

 ears. 



The outer ear.- -The part that we commonly call the 

 ear is known as the pinna (fig. 187, M). It forms a sort of 

 funnel or trumpet for catching the sound-waves, and leads 

 into a funnel-shaped canal, the tneatus (G). The sebaceous 

 glands of this canal produce the wax of the ear. This serves 

 to keep the ear drum or tympanic membrane (T) moist and 

 to render it flexible. 



The middle ear or tympanic cavity (P) is lined with mu- 

 cous membrane and filled with air. The air is admitted to it 

 through the Eustachian tube (R) which opens into the pharynx. 

 The tympanic membrane forms the outer covering of this 

 cavity. This membrane is thrown into vibrations by the 

 sound waves that have passed through the meatus. 



On the inner surface of the cavity of the middle ear are 

 two openings which, like the one on the outer surface are 

 covered with membranes, the fenestra rotunda (r) and the 

 fenestra avails (o). There is a chain of three small bones 

 strung across the cavity from the tympanic membrane to 

 the fenestra ovalis. These bones are named from their 

 peculiar shapes, the hammer (malleus}, the anvil (incus), 

 and the stirrup (stapes). One end of the malleus rests on 

 the inner side of the tympanic membrane, the other end 

 rests upon the incus. The incus lies between the malleus 

 and the stapes and the stapes connects the inner end of the 

 incus with the fenestra ovalis. Vibrations of the tympanic 

 membrane, due to air pressure on its outer surface, are 



