THE EAR. 233 



because it has many winding passages in it. The nerves 

 of hearing are the two auditory nerves. One runs to each 

 ear from the brain, and its fibres end in the labyrinth, 

 in connection with peculiar very small organs which are 

 easily excited by slight shaking, and then excite the 

 fibres of the auditory nerve. Everything that gives out 

 sound shakes or vibrates, and sets the air all round it 

 shaking. The use of the outer ear and middle ear is to 

 take up the vibrations of the air and pass them on to the 

 organs on the ends of the nerve-fibres in the inner ear. 



13. The External Ear consists of the expansion (M, 

 Fig. 58) seen on the exterior of the head, called the con- 

 cha (shell), and a passage leading in from it, the external 

 auditory meatus, G. This passage is closed at its inner end 

 by the tympanic, or drum, membrane, T. It is lined by a 

 prolongation of the skin, through which numerous small 

 glands, secreting the wax of the ear. open. 



14. The Tympanum, or drum-chamber of the ear (Fig. 

 59 and P, Fig. 58), is a small cavity in one of the bones 

 on the side of the skull. It is closed externally by the 

 drum-membrane. From its inner side the Eustachian 

 tube (R, Fig. 58) proceeds and opens into the pharynx 

 (g, Fig. 30). This tube allows air from the throat to 

 enter the tympanum, and serves to keep equal the pres- 

 sure of the air on each side of the drum-membrane. 

 Three small bones (Fig. 59) stretch across the tympanic 

 cavity from the drum-membrane to the labyrinth; they 



labyrinth so named? What are the auditory nerves? What is at- 

 tached to ends of their fibres in the ear? How used in helping us to 

 hear ? Use of outer and middle ear ? 



13. Of what does the external ear consist? What is found at the 

 inner end of its passage ? How is the passage lined ? 



14. Describe the tympanum. What is the Eustachian tube? Its 



